View source for Astronomy:Kepler space telescope

{{Short description|NASA satellite for exoplanetology (2009–2018)}}
{{Infobox spaceflight
| name                  = Kepler
| image                 = Kepler Space Telescope spacecraft model 2.png
| image_caption         = Artist's impression of the Kepler telescope
| image_alt             = Kepler in orbit
| image_size            = 300px
| mission_type          = [[Astronomy:Space telescope|Space telescope]]
| operator              = [[Organization:NASA|NASA]]{{\}}LASP
| COSPAR_ID             = 2009-011A
| SATCAT                = 34380
| website               = {{url|http://www.nasa.gov/kepler}}
| mission_duration      = Planned: 3.5 years <br/> Final: 9 years, 7 months, 23 days

| manufacturer          = Ball Aerospace & Technologies
| launch_mass           = {{convert|1052.4|kg|lb|abbr=on}}<ref name="PressKit200902">{{cite web |url=http://www.nasa.gov/pdf/314125main_Kepler_presskit_2-19_smfile.pdf |title=''Kepler'': NASA's First Mission Capable of Finding Earth-Size Planets |publisher=NASA |date=February 2009 |access-date=March 13, 2015}}</ref>
| dry_mass              = {{convert|1040.7|kg|lb|abbr=on}}<ref name="PressKit200902"/>
| payload_mass          = {{convert|478|kg|lb|abbr=on}}<ref name="PressKit200902"/>
| dimensions            = {{convert|4.7|x|2.7|m|ft|abbr=on}}<ref name="PressKit200902"/>
| power                 = 1100&nbsp;watts<ref name="PressKit200902"/>

| launch_date           = {{start-date|March 7, 2009, 03:49:57}}&nbsp;UTC<ref name=kasc/>
| launch_rocket         = [[Engineering:Delta II|Delta II]] (7925-10L)
| launch_site           = {{wipe|Cape Canaveral Air Force Station}} SLC-17B
| launch_contractor     = [[Company:United Launch Alliance|United Launch Alliance]]
| entered_service       = May 12, 2009, 09:01&nbsp;UTC

| disposal_type         = <!--Whether the spacecraft was deorbited, decommissioned, placed in a graveyard orbit, etc-->
| deactivated           = {{Start date|2018|11|15}}
| destroyed             = <!--when craft was destroyed (if other than by re-entry)-->
| last_contact          = <!--when last signal received if not decommissioned-->

| orbit_reference       = [[Astronomy:Heliocentric orbit|Heliocentric]]
| orbit_regime          = [[Astronomy:Earth|Earth]]-trailing
| orbit_semimajor       = 1.0133 [[Astronomy:Astronomical unit|AU]]
| orbit_eccentricity    = 0.036116
| orbit_periapsis       = 0.97671 AU
| orbit_apoapsis        = 1.0499 AU
| orbit_inclination     = 0.4474 degrees
| orbit_period          = 372.57 days
| orbit_arg_periapsis   = 294.04 degrees
| orbit_mean_anomaly    = 311.67 degrees
| orbit_mean_motion     = 0.96626 deg/day
| orbit_epoch           = {{nowrap|January 1, 2018 ([[Astronomy:J2000|J2000]]: 2458119.5)<ref name="horizons">{{cite web |url=http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=-227 |title=Kepler (spacecraft) |work=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons On-Line Ephemeris System]] |publisher=NASA/JPL |date=January 6, 2018 |access-date=January 6, 2018}}</ref>}}
| apsis                 = helion

| instrument_type       = [[Astronomy:Telescope|telescope]]
| telescope_type        = [[Astronomy:Schmidt camera|Schmidt]]
| telescope_diameter    = {{convert|0.95|m|ft|abbr=on}}
| telescope_area        = {{convert|0.708|m2|sqft|abbr=on}}{{efn-ua|Aperture of 0.95 m yields a light-gathering area of Pi×(0.95/2)<sup>2</sup> {{=}} 0.708 m<sup>2</sup>; the 42 CCDs each sized 0.050&nbsp;m&nbsp;× 0.025m yields a total sensor area of 0.0525&nbsp;m<sup>2</sup>:<ref name="various-64714">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/spacecraft/index.html |title=Kepler Spacecraft and Instrument |publisher=NASA |date=June 26, 2013 |access-date=January 18, 2014 |archive-url=https://web.archive.org/web/20140119031411/http://www.nasa.gov/mission_pages/kepler/spacecraft/index.html |archive-date=January 19, 2014 |url-status=live }}</ref>}}
| telescope_wavelength  = 430–890 nm<ref name="horizons"/>

| trans_bandwidth       = [[Physics:X band|X band]] up: 7.8 bit/s – 2 kbit/s<ref name="horizons"/> <br /> X band down: 10 bit/s – 16 kbit/s<ref name="horizons"/> <br /> [[Astronomy:Ka band|K<sub>a</sub> band]] down: Up to 4.3 Mbit/s<ref name="horizons"/>

| insignia              = Kepler logo.svg

| programme             = '''Discovery program'''
| previous_mission      = ''[[Engineering:Dawn (spacecraft)|Dawn]]''
| next_mission          = ''GRAIL''
}}

The '''Kepler space telescope''' is a defunct [[Astronomy:Space telescope|space telescope]] launched by [[Organization:NASA|NASA]] in 2009<ref name="KeplerLaunch">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/launch/index.html |title=Kepler Launch |publisher=[[Organization:NASA|NASA]] |access-date=September 18, 2009}}</ref> to discover Earth-sized [[Astronomy:Exoplanet|planets orbiting other stars]].<ref name="QuickGuide">{{cite web |url=http://kepler.nasa.gov/Mission/QuickGuide/ |archive-url=https://web.archive.org/web/20110520143321/http://kepler.nasa.gov/Mission/QuickGuide/ |url-status=dead |archive-date=2011-05-20 |title=Kepler: About the Mission |publisher=NASA{{\}}Ames Research Center |date=2013 |access-date=April 11, 2016}}</ref><ref name="Earth-size">{{cite web |url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=52 |archive-url=https://web.archive.org/web/20110810200834/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=52 |url-status=dead |archive-date=August 10, 2011 |title=Statement from the Kepler Science Council |publisher=NASA{{\}}Ames Research Center |last1=Dunham |first1=Edward W. |last2=Gautier |first2=Thomas N. |last3=Borucki |first3=William J. |date=August 2, 2010 |access-date=April 14, 2016}}</ref> Named after astronomer [[Biography:Johannes Kepler|Johannes Kepler]],<ref>{{cite news |last=DeVore |first=Edna |title=Closing in on Extrasolar Earths |url=http://www.space.com/searchforlife/080619-seti-extrasolar-earths.html |work=Space.com |date=June 9, 2008 |access-date=March 14, 2009}}</ref> the spacecraft was launched into an Earth-trailing [[Astronomy:Heliocentric orbit|heliocentric orbit]].  The principal investigator was [[Biography:William J. Borucki|William J. Borucki]]. After nine and a half years of operation, the telescope's [[Engineering:Reaction control system|reaction control system]] fuel was depleted, and [[Organization:NASA|NASA]] announced its retirement on October 30, 2018.<ref name="NASA-20181030" /><ref name="NYT-20181030" />

Designed to survey a portion of Earth's region of the [[Astronomy:Milky Way|Milky Way]] to discover [[Astronomy:Terrestrial planet|Earth-size]] [[Astronomy:Exoplanet|exoplanet]]s in or near [[Astronomy:Habitable zone|habitable zone]]s and estimate how many of the billions of stars in the Milky Way have such planets,<ref name="QuickGuide" /><ref name="NYT-20140512">{{cite news |last=Overbye |first=Dennis |title=Finder of New Worlds |url=https://www.nytimes.com/2014/05/13/science/finder-of-new-worlds.html |date=May 12, 2013 |work={{wipe|The New York Times}} |access-date=May 13, 2014}}</ref><ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next |url=https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |date=January 6, 2015 |work={{wipe|The New York Times}} |access-date=January 6, 2015}}</ref> Kepler's sole scientific instrument is a [[Physics:Photometer|photometer]] that continually monitored the brightness of approximately 150,000 [[Astronomy:Main sequence star|main sequence star]]s in a fixed field of view.<ref name=Borucki2010-FR>{{cite journal |title=Kepler Planet-Detection Mission: Introduction and First Results |journal=Science |first1=William J. |last1=Borucki |first2=David |last2=Koch |first3=Gibor |last3=Basri |display-authors=0 |volume=327 |issue=5968 |pages=977–980 |date=February 2010 |doi=10.1126/science.1185402 |pmid=20056856 |bibcode=2010Sci...327..977B|s2cid=22858074 |url=https://authors.library.caltech.edu/17670/2/2.pdf }}</ref> These data were transmitted to [[Astronomy:Earth|Earth]], then analyzed to detect periodic dimming caused by exoplanets that [[Astronomy:Astronomical transit|cross in front]] of their host star. Only planets whose orbits are seen edge-on from Earth could be detected. Kepler observed 530,506 stars and detected 2,778 confirmed planets as of June 16, 2023.<ref>{{Cite web |title=Exoplanet and Candidate Statistics |url=https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html |access-date=2023-06-16 |website=exoplanetarchive.ipac.caltech.edu}}</ref><ref name="NYT-20181031" />

==History==
===Pre-launch development===
The Kepler space telescope was part of NASA's [[Astronomy:Discovery Program|Discovery Program]] of relatively low-cost science missions. The telescope's construction and initial operation were managed by NASA's [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]], with Ball Aerospace responsible for developing the Kepler flight system.

In January 2006, the project's launch was delayed eight months because of budget cuts and consolidation at NASA.<ref name="MissionHistory"/> It was delayed again by four months in March 2006 due to fiscal problems.<ref name="MissionHistory"/> At this time, the high-gain antenna was changed from a design using a [[Engineering:Gimbal|gimbal]] to one fixed to the frame of the spacecraft to reduce cost and complexity, at the cost of one observation day per month.

===Post launch===
The [[Organization:Ames Research Center|Ames Research Center]] was responsible for the ground system development, mission operations since December 2009, and scientific data analysis. The initial planned lifetime was 3.5 years,<ref Name="BBC7March">{{cite news |title=Nasa launches Earth hunter probe |work=BBC News |date=March 7, 2009 |url=http://news.bbc.co.uk/2/hi/science/nature/7926277.stm |access-date=March 14, 2009}}</ref> but greater-than-expected [[Noisy data|noise in the data]], from both the stars and the spacecraft, meant additional time was needed to fulfill all mission goals. Initially, in 2012, the mission was expected to be extended until 2016,<ref name="Space.com2016">[http://www.space.com/15160-alien-planet-kepler-mission-2016.html "NASA Extends Planet-Hunting Kepler Mission Through 2016"]. Space.com. April 4, 2012. Retrieved May 2, 2012.</ref> but on July 14, 2012, one of the spacecraft's four [[Engineering:Reaction wheel|reaction wheel]]s used for pointing the spacecraft stopped turning, and completing the mission would only be possible if all other reaction wheels remained reliable.<ref>{{cite news |first=Stephen |last=Clark |title=Kepler's exoplanet survey jeopardized by two issues |publisher=Spaceflight Now |date=October 16, 2012 |url=http://spaceflightnow.com/news/n1210/16kepler/ |access-date=October 17, 2012}}</ref> Then, on May 11, 2013, a second reaction wheel failed, disabling the collection of science data<ref name=prs>[http://www.nasa.gov/mission_pages/kepler/news/keplerm-20130521.html NASA – Kepler Mission Manager Update (May 21, 2013)]</ref> and threatening the continuation of the mission.<ref name="nytimes.com">{{cite web |url=https://www.nytimes.com/2013/05/16/science/space/equipment-failure-may-cut-kepler-mission-short.html |title=Equipment Failure May Cut Kepler Mission Short |work={{wipe|The New York Times}} |date=May 15, 2013 |access-date=May 15, 2013}}</ref>

On August 15, 2013, NASA announced that they had given up trying to fix the two failed reaction wheels. This meant the current mission needed to be modified, but it did not necessarily mean the end of planet hunting. [[Organization:NASA|NASA]] had asked the space science community to propose alternative mission plans "potentially including an exoplanet search, using the remaining two good reaction wheels and thrusters".<ref name="NASA-20130815">{{cite web |url=http://www.nasa.gov/content/nasa-ends-attempts-to-fully-recover-kepler-spacecraft-potential-new-missions-considered/ |title=NASA Ends Attempts to Fully Recover Kepler Spacecraft, Potential New Missions Considered |date=August 15, 2013 |access-date=August 15, 2013}}</ref><ref name="NYT-20130815">{{cite news |last=Overbye |first=Dennis |title=NASA's Kepler Mended, but May Never Fully Recover |url=https://www.nytimes.com/2013/08/16/science/space/nasas-kepler-mended-but-may-never-fully-recover.html |date=August 15, 2013 |work={{wipe|The New York Times}} |access-date=August 15, 2013}}</ref><ref name="SP-20130815">{{cite news |last=Wall |first=Mike |title=Planet-Hunting Days of NASA's Kepler Spacecraft Likely Over |url=http://www.space.com/22387-nasa-planet-hunting-kepler-spacecraft-problems.html |date=August 15, 2013 |work=Space.com |access-date=August 15, 2013}}</ref><ref>{{cite news |url=https://www.bbc.co.uk/news/science-environment-23724344 |title=Kepler: NASA retires prolific telescope from planet-hunting duties |work=BBC News |date=August 16, 2013}}</ref> On November 18, 2013, the K2 "Second Light" proposal was reported. This would include utilizing the disabled Kepler in a way that could detect habitable planets around smaller, dimmer [[Astronomy:Red dwarf|red dwarf]]s.<ref name="NYT-20131118">{{cite news |last=Overbye |first=Dennis |title=New Plan for a Disabled Kepler |url=https://www.nytimes.com/2013/11/19/science/space/new-plan-for-a-disabled-kepler.html |date=November 18, 2013 |work={{wipe|The New York Times}} |access-date=November 18, 2013}}</ref><ref name="NASA-20131125" /><ref name="NASA-20131211a" /><ref name="NASA-20131211b" /> On May 16, 2014, NASA announced the approval of the K2 extension.<ref name="NASA-20140516">{{cite web |url=http://www.nasa.gov/content/ames/kepler-mission-manager-update-k2-has-been-approved/ |title=Kepler Mission Manager Update: K2 Has Been Approved! |date=May 16, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=May 17, 2014 |others=NASA Official: Brian Dunbar; Image credit(s): NASA Ames/W. Stenzel |first=Charlie |last=Sobeck |editor1-first=Michele |editor1-last=Johnson |archive-url=https://web.archive.org/web/20140517191944/http://www.nasa.gov/content/ames/kepler-mission-manager-update-k2-has-been-approved/ |archive-date=May 17, 2014 |url-status=live }}</ref>

By January 2015, Kepler and its follow-up observations had found 1,013 confirmed exoplanets in about 440 star systems, along with a further 3,199 unconfirmed planet candidates.{{efn-ua|This does not include Kepler candidates without a KOI designation, such as circumbinary planets, or candidates found in the Planet Hunters project.}}<ref name=wall-2013>{{cite news |url=http://www.space.com/21570-nasa-kepler-alien-planet-candidates.html |title=Ailing NASA Telescope Spots 503 New Alien Planet Candidates |work=Space.com |date=June 14, 2013 |agency=TechMediaNetwork |access-date=June 15, 2013 |first=Mike |last=Wall}}</ref><ref name="NASA-ExoplanetArch">{{cite web |title=NASA's Exoplanet Archive KOI table |url=http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nph-exotbls?dataset=cumulative |publisher=NASA |access-date=February 28, 2014 |archive-url=https://archive.today/20140226203336/http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nph-exotbls?dataset=cumulative |archive-date=February 26, 2014 |url-status=dead }}</ref> Four planets have been confirmed through Kepler's K2 mission.<ref name="Crossfield2015">{{cite journal |title=A nearby M star with three transiting super-Earths discovered by K2 |arxiv=1501.03798 |first1=Ian J. M. |last1=Crossfield |first2=Erik |last2=Petigura |first3=Joshua |last3=Schlieder |first4=Andrew W. |last4=Howard |first5=B. J. |last5=Fulton |first6=Kimberly M. |last6=Aller |first7=David R. |last7=Ciardi |first8=Sebastien |last8=Lepine |first9=Thomas |last9=Barclay |first10=Imke |last10=de Pater |first11=Katherine |last11=de Kleer |first12=Elisa V. |last12=Quintana |first13=Jessie L. |last13=Christiansen |first14=Eddie |last14=Schlafly |first15=Lisa |last15=Kaltenegger |first16=Justin R. |last16=Crepp |first17=Thomas |last17=Henning |first18=Christian |last18=Obermeier |first19=Niall |last19=Deacon |first20=Brad M. S. |last20=Hansen |first21=Michael C. |last21=Liu |first22=Tom |last22=Greene |first23=Steve B. |last23=Howell |first24=Travis |last24=Barman |first25=Christoph |last25=Mordasini |display-authors=5 |date=January 2015 |doi=10.1088/0004-637X/804/1/10 |volume=804 |issue=1 |journal=The Astrophysical Journal |page=10 |bibcode=2015ApJ...804...10C|s2cid=14204860 }}</ref> In November 2013, astronomers estimated, based on Kepler space mission data, that there could be as many as 40 billion [[Astronomy:Terrestrial planet|rocky]] Earth-size exoplanets orbiting in the [[Astronomy:Habitable zone|habitable zone]]s of Sun-like stars and [[Astronomy:Red dwarf|red dwarf]]s within the [[Astronomy:Milky Way|Milky Way]].<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis |title=Far-Off Planets Like the Earth Dot the Galaxy |url=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=November 4, 2013 |work={{wipe|The New York Times}} |access-date=November 5, 2013}}</ref><ref name="PNAS-20131031">{{cite journal |last1=Petigura |first1=Erik A. |last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W. |title=Prevalence of Earth-size planets orbiting Sun-like stars |date=October 31, 2013 |journal=Proceedings of the National Academy of Sciences of the United States |doi=10.1073/pnas.1319909110 |arxiv=1311.6806 |bibcode=2013PNAS..11019273P |volume=110 |issue=48 |pages=19273–19278 |pmid=24191033 |pmc=3845182|doi-access=free }}</ref><ref name="Space-20130107">{{cite news |title=17 Billion Earth-Size Alien Planets Inhabit Milky Way |url=http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html |date=January 7, 2013 |work=Space.com |access-date=January 8, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20141006095334/http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html |archive-date=October 6, 2014 }}</ref> It is estimated that 11 billion of these planets may be orbiting Sun-like stars.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=November 4, 2013 |work={{wipe|Los Angeles Times}} |access-date=November 5, 2013}}</ref> The nearest such planet may be {{convert|12|ly|pc|order=flip|lk=on}} away, according to the scientists.<ref name="NYT-20131104" /><ref name="PNAS-20131031" />

On January 6, 2015, NASA announced the 1,000th confirmed exoplanet discovered by the Kepler space telescope. Four of the newly confirmed exoplanets were found to orbit within [[Astronomy:Habitable zone|habitable zone]]s of their related [[Astronomy:Star|star]]s: three of the four, [[Astronomy:Kepler-438b|Kepler-438b]], [[Astronomy:Kepler-442b|Kepler-442b]] and [[Astronomy:Kepler-452b|Kepler-452b]], are almost Earth-size and likely rocky; the fourth, [[Astronomy:Kepler-440b|Kepler-440b]], is a [[Astronomy:Super-Earth|super-Earth]].<ref name="NASA-20150106">{{cite web |last1=Clavin |first1=Whitney |last2=Chou |first2=Felicia |last3=Johnson |first3=Michele |title=NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones |url=http://www.jpl.nasa.gov/news/news.php?release=2015-003 |date=January 6, 2015 |publisher=[[Organization:NASA|NASA]] |access-date=January 6, 2015}}</ref> On May 10, 2016, NASA verified 1,284 new exoplanets found by Kepler, the single largest finding of planets to date.<ref name="NASA-20160510a">{{cite web |url=http://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered |title=NASA's Kepler Mission Announces Largest Collection of Planets Ever Discovered |publisher=[[Organization:NASA|NASA]] |date=May 10, 2016 |access-date=May 10, 2016}}</ref><ref name="NASA-20160510b">{{cite web |url=http://www.nasa.gov/feature/ames/kepler/briefingmaterials160510 |title=Briefing materials: 1,284 Newly Validated Kepler Planets |publisher=[[Organization:NASA|NASA]] |date=May 10, 2016 |access-date=May 10, 2016}}</ref><ref name="NYT-20160510db">{{cite news |url=https://www.nytimes.com/2016/05/11/science/kepler-planets-nasa.html |title=Kepler Finds 1,284 New Planets |work={{wipe|The New York Times}} |last=Overbay |first=Dennis |date=May 10, 2016 |access-date=May 11, 2016}}</ref>

Kepler data has also helped scientists observe and understand [[Astronomy:Supernova|supernova]]e; measurements were collected every half-hour so the light curves were especially useful for studying these types of astronomical events.<ref name="super">{{cite journal |title=Kepler clue to supernova puzzle |journal=Nature |date=January 16, 2014 |publisher=[[Company:Nature Publishing Group|Nature Publishing Group]] |issn=1476-4687 |oclc=01586310 |doi=10.1038/505274a |pmid=24429610 |pages=274–275 |volume=505 |issue=7483 |first=Ron |last=Cowen |bibcode=2014Natur.505..274C |doi-access=free }}</ref>

On October 30, 2018, after the spacecraft ran out of fuel, NASA announced that the telescope would be retired.<ref>{{cite web |title=NASA Retires Kepler Space Telescope, Passes Planet-Hunting Torch |url=https://www.nasa.gov/press-release/nasa-retires-kepler-space-telescope-passes-planet-hunting-torch |publisher=NASA|date=October 30, 2018 }}</ref> The telescope was shut down the same day, bringing an end to its nine-year service. Kepler observed 530,506 stars and discovered 2,662 exoplanets over its lifetime.<ref name="NYT-20181031">{{cite news |last1=Overbye |first1=Dennis |title=Kepler, the Little NASA Spacecraft That Could, No Longer Can |url=https://www.nytimes.com/2018/10/30/science/nasa-kepler-exoplanet.html |newspaper=The New York Times |access-date=October 31, 2018|date=October 30, 2018 }}</ref> A newer NASA mission, TESS, launched in 2018, is continuing the search for exoplanets.<ref name="NASA-20180917b">{{cite news |last1=Wiessinger |first1=Scott |last2=Lepsch |first2=Aaron E. |last3=Kazmierczak |first3=Jeanette |last4=Reddy |first4=Francis |last5=Boyd |first5=Padi |title=NASA's TESS Releases First Science Image |url=https://svs.gsfc.nasa.gov/13069 |date=September 17, 2018 |work=[[Organization:NASA|NASA]] |access-date=October 31, 2018 }}</ref>

==Spacecraft design==
[[File:Kepler in Astrotech's Hazardous Processing Facility (KSC-2009-1645).jpg|thumb|Kepler in Astrotech's Hazardous Processing Facility|alt=|left]]
[[File:Kepler Space Telescope.stl|alt=Interactive 3D model of Kepler|thumb|Interactive 3D model of Kepler]]

The telescope has a mass of {{convert|1039|kg|lb}} and contains a [[Astronomy:Schmidt camera|Schmidt camera]] with a {{convert|0.95|m|in|1|adj=on|sp=us}} front corrector plate (lens) feeding a {{convert|1.4|m|in|0|adj=on|sp=us}} [[Physics:Primary mirror|primary mirror]]—at the time of its launch this was the largest mirror on any telescope outside Earth orbit,<ref>{{cite web |last=Atkins |first=William |title=Exoplanet Search Begins with French Launch of Corot Telescope Satellite |url=http://www.itwire.com/content/view/8299/1066/ |publisher=iTWire |date=December 28, 2008 |access-date=May 6, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081204114533/http://www.itwire.com/content/view/8299/1066/ |archive-date=December 4, 2008 }}</ref> though the [[Astronomy:Herschel Space Observatory|Herschel Space Observatory]] took this title a few months later. Its telescope has a 115 deg<sup>2</sup> (about 12-degree diameter) [[Physics:Field of view|field of view]] (FoV), roughly equivalent to the size of one's fist held at arm's length. Of this, 105 deg<sup>2</sup> is of science quality, with less than 11% [[Vignetting|vignetting]]. The photometer has a [[Soft focus|soft focus]] to provide excellent [[Astronomy:Photometry|photometry]], rather than sharp images. The mission goal was a combined differential photometric precision (CDPP) of 20 ppm for a ''m''(V)=12 Sun-like star for a 6.5-hour integration, though the observations fell short of this objective (see mission status).

===Camera===
[[File:Keplerspacecraft-FocalPlane-cutout.svg|thumb|left|Kepler's image sensor array. The array is curved to account for [[Physics:Petzval field curvature|Petzval field curvature]].]]

The focal plane of the spacecraft's camera is made out of forty-two {{convert|50|xx|25|mm|in|0|abbr=on}} [[Engineering:Charge-coupled device|CCDs]] at 2200×1024 [[Pixel|pixel]]s each, possessing a total resolution of 94.6 megapixels,<ref name="Caldwell2010" /><ref>{{cite web |url=http://www.nasa.gov/mission_pages/kepler/spacecraft/index.html |title=Kepler: Spacecraft and Instrument |publisher=NASA |editor-first=Michele |editor-last=Johnson |date=July 30, 2015 |access-date=December 11, 2016}}</ref> which at the time made it the largest camera system launched into space.<ref name="BBC7March" /> The array was cooled by heat pipes connected to an external radiator.<ref name=presskit>{{cite web |url=http://www.nasa.gov/pdf/314125main_Kepler_presskit_2-19_smfile.pdf |title=Kepler: NASA's First Mission Capable of Finding Earth-Size Planets |publisher=[[Organization:NASA|NASA]] |date=February 2009 |access-date=March 14, 2009}}</ref> The CCDs were read out every 6.5 seconds (to limit saturation) and co-added on board for 58.89 seconds for short cadence targets, and 1765.5 seconds (29.4 minutes) for long cadence targets.<ref name="Kepler/K2 Data Products">{{cite web |url=https://keplerscience.arc.nasa.gov/data-products.html |archive-url=https://web.archive.org/web/20160107031435/http://keplerscience.arc.nasa.gov/data-products.html |url-status=dead |archive-date=January 7, 2016 |title=Kepler and K2 data products |publisher=NASA |editor-first=Geert |editor-last=Barentsen |date=August 16, 2017 |access-date=August 24, 2017}}</ref> Due to the larger bandwidth requirements for the former, these were limited in number to 512 compared to 170,000 for long cadence. However, even though at launch Kepler had the highest data rate of any NASA mission, {{Citation needed|date=October 2015}} the 29-minute sums of all 95 million pixels constituted more data than could be stored and sent back to Earth. Therefore, the science team pre-selected the relevant pixels associated with each star of interest, amounting to about 6 percent of the pixels (5.4 megapixels). The data from these pixels was then requantized, compressed and stored, along with other auxiliary data, in the on-board 16 gigabyte solid-state recorder. Data that was stored and downlinked includes science stars, p-mode stars, smear, black level, background and full field-of-view images.<ref name="presskit"/><ref>{{cite web |title=PyKE Primer – 2. Data Resources |url=http://keplergo.arc.nasa.gov/PyKEprimerResources.shtml |archive-url=https://web.archive.org/web/20130217132822/http://keplergo.arc.nasa.gov/PyKEprimerResources.shtml |url-status=dead |archive-date=February 17, 2013 |publisher=[[Organization:NASA|NASA]] |access-date=March 12, 2014}}</ref>

===Primary mirror===
[[File:comparison optical telescope primary mirrors.svg|thumb|Comparison of primary mirror sizes for the Kepler telescope and other notable optical telescopes.]]

The Kepler primary mirror is {{convert|1.4|m|ft|sp=us}} in diameter. Manufactured by glass maker [[Company:Corning Inc.|Corning]] using [[Chemistry:Ultra low expansion glass|ultra-low expansion (ULE) glass]], the mirror is specifically designed to have a mass only 14% that of a solid mirror of the same size.<ref>{{cite web |title=Kepler Primary Mirror |url=http://www.nasa.gov/mission_pages/kepler/multimedia/images/PMA_backside_inspection_1_at_SOC.html |publisher=NASA |access-date=April 5, 2013}}</ref><ref>{{cite web |title=Corning To Build Primary Mirror For Kepler Photometer |url=http://www.spacedaily.com/news/extrasolar-03q.html |access-date=April 5, 2013}}</ref> To produce a space telescope system with sufficient sensitivity to detect relatively small planets, as they pass in front of stars, a very high reflectance coating on the primary mirror was required. Using ion assisted evaporation, Surface Optics Corp. applied a protective nine-layer silver coating to enhance reflection and a dielectric interference coating to minimize the formation of color centers and atmospheric moisture absorption.<ref>{{cite journal |last1=Fulton L. |first1=Michael |last2=Dummer |first2=Richard S. |date=2011 |title=Advanced Large Area Deposition Technology for Astronomical and Space Applications |journal=Vacuum & Coating Technology |issue=December 2011 |pages=43–47 |url=http://e-ditionsbyfry.com/Olive/ODE/VTC/Default.aspx?href=VTC/2011/12/01 |access-date=April 6, 2013 |archive-url=https://web.archive.org/web/20130512233653/http://e-ditionsbyfry.com/Olive/ODE/VTC/Default.aspx?href=VTC%2F2011%2F12%2F01 |archive-date=May 12, 2013 |url-status=dead |df=mdy-all }}</ref><ref>{{cite press release |title=Ball Aerospace Completes Primary Mirror and Detector Array Assembly Milestones for Kepler Mission |url=http://www.spaceref.com/news/viewpr.html?pid=23610 |archive-url=https://archive.today/20130616050826/http://www.spaceref.com/news/viewpr.html?pid=23610 |url-status=dead |archive-date=June 16, 2013 |date=September 25, 2007 |publisher=Ball Aerospace and Technologies |work=SpaceRef.com |access-date=April 6, 2013 }}</ref>

===Photometric performance===
In terms of photometric performance, Kepler worked well, much better than any Earth-bound telescope, but short of design goals. The objective was a combined differential photometric precision (CDPP) of 20 parts per million (PPM) on a magnitude&nbsp;12 star for a 6.5-hour integration. This estimate was developed allowing 10&nbsp;ppm for stellar variability, roughly the value for the Sun. The obtained accuracy for this observation has a wide range, depending on the star and position on the focal plane, with a median of 29&nbsp;ppm. Most of the additional noise appears to be due to a larger-than-expected variability in the stars themselves (19.5&nbsp;ppm as opposed to the assumed 10.0&nbsp;ppm), with the rest due to instrumental noise sources slightly larger than predicted.<ref>{{cite journal |last1=Gilliland |first1=Ronald L. |date=2011 |title=Kepler Mission Stellar and Instrument Noise Properties |arxiv=1107.5207 |display-authors=0 |doi=10.1088/0067-0049/197/1/6 |volume=197 |issue=1 |journal=The Astrophysical Journal Supplement Series |page=6 |bibcode=2011ApJS..197....6G|s2cid=118626534 }}</ref><ref name="Caldwell2010">{{cite journal |first1=Douglas A. |last1=Caldwell |first2=Jeffrey E. |last2=van Cleve |first3=Jon M. |last3=Jenkins |first4=Vic S. |last4=Argabright |first5=Jeffery J. |last5=Kolodziejczak |first6=Edward W. |last6=Dunham |first7=John C. |last7=Geary |first8=Peter |last8=Tenenbaum |first9=Hema |last9=Chandrasekaran |first10=Jie |last10=Li |first11=Hayley |last11=Wu |first12=Jason |last12=von Wilpert |display-authors=5 |url=http://kepler.nasa.gov/Science/ForScientists/papersAndDocumentation/SOCpapers/spie_2010_instrument_char_web_version.pdf |archive-url=https://web.archive.org/web/20110721122315/http://kepler.nasa.gov/Science/ForScientists/papersAndDocumentation/SOCpapers/spie_2010_instrument_char_web_version.pdf |url-status=dead |archive-date=2011-07-21 |title=Kepler Instrument Performance: An In-Flight Update |journal=[[Physics:Proceedings of SPIE|Proceedings of SPIE]] |series=Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave |publisher=International Society for Optics and Photonics |editor1-last=Oschmann |editor1-first=Jacobus M. Jr. |editor2-last=Clampin |editor2-first=Mark C. |editor3-last=MacEwen |editor3-first=Howard A. |volume=7731 |at=773117 |date=July 2010 |doi=10.1117/12.856638 |bibcode=2010SPIE.7731E..17C|s2cid=121398671 }}</ref>

Because decrease in brightness from an Earth-size planet transiting a Sun-like star is so small, only 80&nbsp;ppm, the increased noise means each individual transit is only a 2.7&nbsp;σ event, instead of the intended 4&nbsp;σ. This, in turn, means more transits must be observed to be sure of a detection. Scientific estimates indicated that a mission lasting 7 to 8 years, as opposed to the originally planned 3.5 years, would be needed to find all transiting Earth-sized planets.<ref name="SandT">{{cite web |url=http://www.skyandtelescope.com/news/126242378.html |title=Kepler's Dilemma: Not Enough Time |publisher=Sky and Telescope |first=Kelly |last=Beatty |date=September 2011 |access-date=August 2, 2011 |archive-date=October 22, 2013 |archive-url=https://web.archive.org/web/20131022190523/http://www.skyandtelescope.com/news/126242378.html |url-status=dead }}</ref> On April 4, 2012, the Kepler mission was approved for extension through the fiscal year 2016,<ref name=Space.com2016/><ref name="mission extension">{{cite web |url=http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=199 |archive-url=https://web.archive.org/web/20120707131538/http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=199 |url-status=dead |archive-date=July 7, 2012 |title=NASA Approves Kepler Mission Extension |publisher=NASA |date=April 4, 2012}}</ref> but this also depended on all remaining reaction wheels staying healthy, which turned out not to be the case (see Reaction wheel issues below).

===Orbit and orientation===
[[Image:LombergA1024.jpg|thumb|left|Kepler's search volume, in the context of the Milky Way]]
[[File:Kepler-earthdirection 2009-2019.png|thumb|The motion of Kepler relative to Earth, slowly drifting away from Earth in a similar orbit, looking like a spiral over time]]
Kepler [[Astronomy:Heliocentric orbit|orbits the Sun]],<ref name="keplerlaunch1"/><ref>{{cite web |last1=Koch |first1=David |last2=Gould |first2=Alan |title=Kepler Mission: Launch Vehicle and Orbit |url=http://kepler.nasa.gov/sci/design/orbit.html |archive-url=https://web.archive.org/web/20070622161529/http://kepler.nasa.gov/sci/design/orbit.html |archive-date=June 22, 2007 |publisher=[[Organization:NASA|NASA]] |date=March 2009 |url-status=dead |access-date=March 14, 2009}}</ref> which avoids Earth [[Astronomy:Occultation|occultation]]s, stray light, and [[Physics:Gravity|gravitational]] perturbations and [[Physics:Torque|torque]]s inherent in an Earth orbit.

NASA has characterized Kepler's orbit as "Earth-trailing".<ref>{{cite web |url=http://www.nasa.gov/mission_pages/kepler/spacecraft/index-mission.html |title=Kepler: Spacecraft and Instrument |publisher=NASA |access-date=December 21, 2011}}</ref> With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). {{As of|2018|05|01|df=US}}, the distance to Kepler from Earth was about {{convert|0.917|AU|e6km|0|abbr=unit}}.<ref name="horizons"/> This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.

Until 2013 the [[Physics:Photometer|photometer]] pointed to a field in the [[Astronomy:Celestial coordinate system|northern]] [[Astronomy:Constellation|constellation]]s of [[Astronomy:Cygnus (constellation)|Cygnus]], [[Astronomy:Lyra|Lyra]] and [[Astronomy:Draco (constellation)|Draco]], which is well out of the [[Astronomy:Ecliptic|ecliptic]] plane, so that sunlight never enters the photometer as the spacecraft orbits.<ref name="presskit"/> This is also the direction of the Solar System's motion around the center of the galaxy. Thus, the stars which Kepler observed are roughly the same distance from the [[Galactic Center]] as the [[Astronomy:Solar System|Solar System]], and also close to the [[Astronomy:Galactic plane|galactic plane]]. This fact is important if position in the galaxy is related to habitability, as suggested by the [[Astronomy:Rare Earth hypothesis|Rare Earth hypothesis]].

Orientation is three-axis stabilized by sensing rotations using fine-guidance sensors located on the instrument focal plane (instead of rate sensing gyroscopes, e.g. as used on [[Astronomy:Hubble Space Telescope|Hubble]]).<ref>[https://www.nasa.gov/pdf/314125main_Kepler_presskit_2-19_smfile.pdf Kepler press kit]</ref> and using [[Engineering:Reaction wheel|reaction wheel]]s and [[Chemistry:Hydrazine#Rocket fuel|hydrazine]] thrusters<ref>{{Cite web|url=https://astrobiology.nasa.gov/news/the-kepler-space-telescope-mission-is-ending-but-its-legacy-will-keep-growing/|title = NASA Astrobiology}}</ref> to control the orientation.

{{multiple image
| align = left
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| header = Animation of Kepler{{'s}} trajectory
| image1 = Animation of Kepler trajectory.gif
| caption1 = Relative to the Sun
| image2 = Animation of Kepler trajectory around Earth.gif
| caption2 = Relative to Earth
| image3 = Animation of Kepler's trajectory relative to Sun and Earth.gif
| caption3 = Relative to Sun and Earth
| footer = {{legend2|magenta|Kepler}}{{·}}{{legend2|RoyalBlue|[[Astronomy:Earth|Earth]]}}{{·}}{{legend2|Yellow|[[Astronomy:Sun|Sun]]}}
}}
{{clear}}

===Operations===
[[Image:Kepler orbit.png|thumb|Kepler's orbit. The telescope's solar array was adjusted at solstices and equinoxes.]]

Kepler was operated out of Boulder, Colorado, by the Laboratory for Atmospheric and Space Physics (LASP) under contract to Ball Aerospace & Technologies. The spacecraft's solar array was rotated to face the Sun at the [[Astronomy:Solstice|solstice]]s and [[Astronomy:Equinox|equinox]]es, so as to optimize the amount of sunlight falling on the solar array and to keep the heat radiator pointing towards deep space.<ref name=presskit/> Together, LASP and Ball Aerospace control the spacecraft from a mission operations center located on the research campus of the [[Organization:University of Colorado Boulder|University of Colorado]]. LASP performs essential mission planning and the initial collection and distribution of the science data. The mission's initial life-cycle cost was estimated at US$600 million, including funding for 3.5 years of operation.<ref name=presskit/> In 2012, NASA announced that the Kepler mission would be funded until 2016 at a cost of about $20 million per year.<ref name=Space.com2016/>

====Communications====
NASA contacted the spacecraft using the [[Physics:X band|X band]] communication link twice a week for command and status updates. Scientific data are downloaded once a month using the [[Astronomy:Ka band|K<sub>a</sub> band]] link at a maximum data transfer rate of approximately 550&nbsp;kB/s. The high gain antenna is not steerable so data collection is interrupted for a day to reorient the whole spacecraft and the high gain antenna for communications to  Earth.<ref name=KPK>[https://www.nasa.gov/pdf/314125main_Kepler_presskit_2-19_smfile.pdf ''Kepler press kit'']</ref>{{rp|16}}

The Kepler space telescope conducted its own partial analysis on board and only transmitted scientific data deemed necessary to the mission in order to conserve bandwidth.<ref>{{cite news |last=Ng |first=Jansen |url=http://www.dailytech.com/Kepler+Mission+Sets+Out+to+Find+Planets+Using+CCD+Cameras/article14421.htm |title=Kepler Mission Sets Out to Find Planets Using CCD Cameras |work=DailyTech |date=March 8, 2009 |access-date=March 14, 2009 |archive-url=https://web.archive.org/web/20090310010146/http://www.dailytech.com/Kepler+Mission+Sets+Out+to+Find+Planets+Using+CCD+Cameras/article14421.htm |archive-date=March 10, 2009 |url-status=dead  }}</ref>

====Data management====
Science data telemetry collected during mission operations at LASP is sent for processing to the Kepler Data Management Center (DMC) which is located at the [[Organization:Space Telescope Science Institute|Space Telescope Science Institute]] on the campus of [[Organization:Johns Hopkins University|Johns Hopkins University]] in Baltimore, Maryland. The science data telemetry is decoded and processed into uncalibrated [[FITS]]-format science data products by the DMC, which are then passed along to the Science Operations Center (SOC) at NASA Ames Research Center, for calibration and final processing. The SOC at NASA Ames Research Center (ARC) develops and operates the tools needed to process scientific data for use by the ''Kepler'' Science Office (SO). Accordingly, the SOC develops the pipeline data processing software based on scientific algorithms developed jointly by the SO and SOC. During operations, the SOC:<ref>{{cite web |url=http://archive.stsci.edu/kepler/manuals/KSCI-19081-002-KDPH.pdf |title=Kepler Data Processing Handbook (KSCI-19081-002) |date=January 25, 2017 |first=Jon M. |last=Jenkins |publisher=NASA}}</ref>
#Receives uncalibrated pixel data from the DMC
#Applies the analysis algorithms to produce calibrated pixels and light curves for each star
#Performs transit searches for detection of planets (threshold-crossing events, or TCEs)
#Performs data validation of candidate planets by evaluating various data products for consistency as a way to eliminate false positive detections
The SOC also evaluates the photometric performance on an ongoing basis and provides the performance metrics to the SO and Mission Management Office. Finally, the SOC develops and maintains the project's scientific databases, including catalogs and processed data. The SOC finally returns calibrated data products and scientific results back to the DMC for long-term archiving, and distribution to astronomers around the world through the Multimission Archive at STScI (MAST).

====Reaction wheel failures====
On July 14, 2012, one of the four [[Engineering:Reaction wheel|reaction wheel]]s used for fine pointing of the spacecraft failed.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20122407.html |title=Kepler Mission Manager Update |date=July 24, 2012 |first=Roger |last=Hunter |publisher=NASA}}</ref> While Kepler requires only three reaction wheels to accurately aim the telescope, another failure would leave the spacecraft unable to aim at its original field.<ref>{{cite magazine |title=Kepler glitch may lower odds of finding Earth's twin |date=July 24, 2012 |first=Maggie |last=McKee |magazine=New Scientist |url=https://www.newscientist.com/article/dn22096-kepler-glitch-may-lower-odds-of-finding-earths-twin.html}}</ref>

After showing some problems in January 2013, a second reaction wheel failed on May 11, 2013, ending Kepler's primary mission. The spacecraft was put into safe mode, then from June to August 2013 a series of engineering tests were done to try to recover either failed wheel. By August 15, 2013, it was decided that the wheels were unrecoverable,<ref name="NASA-20130815" /><ref name="NYT-20130815" /><ref name="SP-20130815" /> and an engineering report was ordered to assess the spacecraft's remaining capabilities.<ref name="NASA-20130815" />

This effort ultimately led to the "K2" follow-on mission observing different fields near the ecliptic.

===Operational timeline===
[[Image:Ignition of Kepler's Delta II 7925-10L.jpg|thumb|left|Kepler's launch on March 7, 2009]]
[[Image:Keplerspacecraft-20110215.jpg|thumb|Interior illustration of Kepler]]
[[Image:Keplerpacecraft.019e.jpg|thumb|A 2004 illustration of Kepler]]

In January 2006, the project's launch was delayed eight months because of budget cuts and consolidation at NASA.<ref name="MissionHistory">{{cite web |last=Borucki |first=W. J. |date=May 22, 2010 |url=http://kepler.nasa.gov/Mission/QuickGuide/history/ |title=Brief History of the Kepler Mission |publisher=[[Organization:NASA|NASA]] |access-date=April 23, 2011 |archive-url=https://web.archive.org/web/20110721123028/http://kepler.nasa.gov/Mission/QuickGuide/history/ |archive-date=July 21, 2011 |url-status=dead  }}</ref> It was delayed again by four months in March 2006 due to fiscal problems.<ref name="MissionHistory"/> At this time, the high-gain antenna was changed from a gimballed design to one fixed to the frame of the spacecraft to reduce cost and complexity, at the cost of one observation day per month.

The Kepler observatory was launched on March 7, 2009, at 03:49:57&nbsp;UTC aboard a [[Engineering:Delta II|Delta II]] rocket from {{wipe|Cape Canaveral Air Force Station}}, Florida.<ref name="kasc">{{cite web |title=KASC Scientific Webpage |url=http://astro.phys.au.dk/KASC/ |work=Kepler Asteroseismic Science Consortium |publisher=Aarhus University |date=March 14, 2009 |access-date=March 14, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20120505003051/http://astro.phys.au.dk/KASC/ |archive-date=May 5, 2012 }}</ref><ref name="KeplerLaunch"/> The launch was a success and all three stages were completed by 04:55&nbsp;UTC. The cover of the telescope was jettisoned on April 7, 2009, and the [[Astronomy:First light|first light]] images were taken on the next day.<ref>{{cite web |last=DeVore |first=Edna |url=http://www.space.com/searchforlife/090409-kepler-dust-cover.html |title=Planet-Hunting Kepler Telescope Lifts Its Lid |date=April 9, 2009 |work=Space.com |access-date=April 14, 2009}}</ref><ref>{{cite web |url=http://www.nasa.gov/home/hqnews/2009/apr/HQ_09-085_Kepler_First_Light.html |title=NASA's Kepler Captures First Views of Planet-Hunting Territory |date=April 16, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=April 16, 2009}}</ref>

On April 20, 2009, it was announced that the Kepler science team had concluded that further refinement of the focus would dramatically increase the scientific return.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20090420.html |title=04.20.09 – Kepler Mission Manager Update |date=April 20, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=April 20, 2009}}</ref> On April 23, 2009, it was announced that the focus had been successfully optimized by moving the primary mirror 40&nbsp;micrometers (1.6&nbsp;thousandths of an inch) towards the focal plane and tilting the primary mirror 0.0072 degree.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20090424.html |title=04.23.09 – Kepler Mission Manager Update |date=April 23, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=April 27, 2009}}</ref>

On May 13, 2009, at 00:01 UTC, Kepler successfully completed its commissioning phase and began its search for planets around other stars.<ref name="Update20090514">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20090514.html |title=05.14.09 – Kepler Mission Manager Update |date=May 14, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=May 16, 2009}}</ref><ref>{{cite web |title=Let the Planet Hunt Begin |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-200905013.html |publisher=[[Organization:NASA|NASA]] |date=May 13, 2009 |access-date=May 13, 2009}}</ref>

On June 19, 2009, the spacecraft successfully sent its first science data to Earth. It was discovered that Kepler had entered [[Engineering:Safe mode (spacecraft)|safe mode]] on June 15. A second safe mode event occurred on July 2. In both cases the event was triggered by a ''processor reset''. The spacecraft resumed normal operation on July 3 and the science data that had been collected since June 19 was downlinked that day.<ref name="Update20090707">{{cite web |url=http://kepler.nasa.gov/Mission/mmupdates/missionManagerArchive/ |archive-url=https://web.archive.org/web/20100528141716/http://kepler.nasa.gov/Mission/mmupdates/missionManagerArchive/ |url-status=dead |archive-date=May 28, 2010 |title=2009 July 7 Mission Manager Update |date=July 7, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=April 23, 2011}}</ref> On October 14, 2009, the cause of these safing events was determined to be a [[Physics:Low voltage|low voltage]] power supply that provides power to the [[Astronomy:RAD750|RAD750]] processor.<ref name="Update20091014">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20091014.html |title=Kepler Mission Manager Update |date=October 14, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=October 18, 2009}}</ref> On January 12, 2010, one portion of the focal plane transmitted anomalous data, suggesting a problem with focal plane MOD-3 module, covering two out of Kepler's 42 [[Engineering:Charge-coupled device|CCDs]]. {{As of|2010|10}}, the module was described as "failed", but the coverage still exceeded the science goals.<ref>{{cite web |title=Kepler outlook positive; Followup Observing Program in full swing |url=http://kepler.nasa.gov/news/mmu/index.cfm?FuseAction=ShowNews&NewsID=55 |archive-url=https://web.archive.org/web/20110721122210/http://kepler.nasa.gov/news/mmu/index.cfm?FuseAction=ShowNews&NewsID=55 |url-status=dead |archive-date=July 21, 2011 |date=August 23, 2010 |access-date=April 23, 2011}}</ref>

Kepler downlinked roughly twelve [[Gigabyte|gigabyte]]s of data<ref name="Update20090923">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20090923.html |title=Kepler Mission Manager Update |date=September 23, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=September 25, 2009}}</ref> about once per month.<ref name="Update20091105">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20091105.html |title=Kepler Mission Manager Update |date=November 5, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=November 8, 2009}}</ref>

==Field of view==
[[File:Kepler FOV hiRes.jpg|thumb|Diagram of Kepler's investigated area with celestial coordinates]]
Kepler has a fixed [[Physics:Field of view|field of view]] (FOV) against the sky. The diagram to the right shows the celestial coordinates and where the detector fields are located, along with the locations of a few bright stars with celestial north at the top left corner. The mission website has a calculator<ref>[https://web.archive.org/web/20040725111550/http://www.kepler.nasa.gov/cgi-bin/ra2pix.pl mission website calculator]</ref> that will determine if a given object falls in the FOV, and if so, where it will appear in the photo detector output data stream. Data on exoplanet candidates is submitted to the [[Engineering:Kepler Follow-up Program|Kepler Follow-up Program]], or KFOP, to conduct follow-up observations.

[[File:MilkywaykeplerfovbyCRoberts.jpg|thumb|left|The photometer's field of view in the constellations Cygnus, Lyra and Draco]]
Kepler's field of view covers 115 square degrees, around 0.25 percent of the sky, or "about two scoops of the Big Dipper". Thus, it would require around 400 Kepler-like telescopes to cover the whole sky.<ref>{{cite web |url=http://www.ballaerospace.com/page.jsp?page=186 |title=Kepler mission & program information |publisher=Ball Aerospace & Technologies |access-date=September 18, 2012}}</ref> The Kepler field contains portions of the constellations [[Astronomy:Cygnus (constellation)|Cygnus]], [[Astronomy:Lyra|Lyra]], and [[Astronomy:Draco (constellation)|Draco]].

The nearest star system in Kepler's field of view is the trinary star system Gliese 1245, 15 light years from the Sun. The brown dwarf WISE J2000+3629, 22.8 ± 1 light years from the Sun is also in the field of view, but is invisible to Kepler due to emitting light primarily in infrared wavelengths.

==Objectives and methods==

The scientific objective of the Kepler space telescope was to explore the structure and diversity of planetary systems.<ref>{{cite web |last1=Koch |first1=David |last2=Gould |first2=Alan |title=Overview of the Kepler Mission |url=http://www.cfa.harvard.edu/kepler/papers/2004/SPIE.Glasgow.Koch.pdf |publisher=SPIE |date=2004 |access-date=December 9, 2010}}</ref> This spacecraft observes a large sample of stars to achieve several key goals:

* To determine how many Earth-size and larger planets there are in or near the [[Astronomy:Habitable zone|habitable zone]] (often called "Goldilocks planets")<ref name="Muir">{{cite news |last1=Muir |first1=Hazel |url=https://www.newscientist.com/article/dn11710 |title='Goldilocks' planet may be just right for life |work=New Scientist |date=April 25, 2007 |access-date=April 2, 2009}}</ref> of a wide variety of spectral types of stars.
* To determine the range of size and shape of the orbits of these planets.
* To estimate how many planets there are in multiple-star systems.
* To determine the range of orbit size, brightness, size, mass and density of short-period giant planets.
* To identify additional members of each discovered planetary system using other techniques.
* Determine the properties of those stars that harbor planetary systems.

Most of the [[Astronomy:Exoplanet|exoplanet]]s previously detected by other projects were [[Astronomy:Giant planet|giant planet]]s, mostly the size of [[Astronomy:Jupiter|Jupiter]] and bigger. Kepler was designed to look for planets 30 to 600 times less massive, closer to the order of Earth's mass (Jupiter is 318 times more massive than Earth). The method used, the transit method, involves observing repeated [[Astronomy:Astronomical transit|transit]] of planets in front of their stars, which causes a slight reduction in the star's [[Astronomy:Apparent magnitude|apparent magnitude]], on the order of 0.01% for an Earth-size planet. The degree of this reduction in brightness can be used to deduce the diameter of the planet, and the interval between transits can be used to deduce the planet's orbital period, from which estimates of its orbital semi-major axis (using Kepler's laws) and its temperature (using models of stellar radiation) can be calculated.{{citation needed|date=March 2022}}

The probability of a random planetary orbit being along the line-of-sight to a star is the diameter of the star divided by the diameter of the orbit.<ref name="geomprob">{{cite web |author=David Koch and Alan Gould, curators |date=March 2009 |title=Kepler Mission: Characteristics of Transits (section 'Geometric Probability') |url=http://kepler.nasa.gov/sci/basis/character.html |publisher=NASA |access-date=September 21, 2009 |archive-url=https://web.archive.org/web/20090825002919/http://kepler.nasa.gov/sci/basis/character.html |archive-date=August 25, 2009 |url-status=dead}}</ref> For an Earth-size planet at 1&nbsp;[[Astronomy:Astronomical unit|AU]] transiting a Sun-like star the probability is 0.47%, or about 1 in 210.<ref name="geomprob"/> For a planet like Venus orbiting a Sun-like star the probability is slightly higher, at 0.65%;<ref name="geomprob"/>  If the host star has multiple planets, the probability of additional detections is higher than the probability of initial detection assuming planets in a given system tend to orbit in similar planes—an assumption consistent with current models of planetary system formation.<ref name="geomprob"/> For instance, if a ''Kepler''-like mission conducted by aliens observed Earth transiting the Sun, there is a 7% chance that it would also see [[Astronomy:Venus|Venus]] transiting.<ref name="geomprob"/>

Kepler's 115 deg<sup>2</sup> field of view gives it a much higher probability of detecting Earth-sized planets than the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]], which has a field of view of only [[Physics:Field of view#Astronomy|10 sq. arc-minutes]]. Moreover, Kepler is dedicated to detecting planetary transits, while the Hubble Space Telescope is used to address a wide range of scientific questions, and rarely looks continuously at just one starfield. Of the approximately half-million stars in Kepler's field of view, around 150,000 stars were selected for observation. More than 90,000 are G-type stars on, or near, the [[Astronomy:Main sequence|main sequence]]. Thus, Kepler was designed to be sensitive to wavelengths of 400–865&nbsp;nm where brightness of those stars peaks. Most of the stars observed by Kepler have apparent visual magnitude between 14 and 16 but the brightest observed stars have apparent visual magnitude of 8 or lower. Most of the planet candidates were initially not expected to be confirmed due to being too faint for follow-up observations.<ref name="arXiv-1001.0349v1">{{cite journal |title=Selection, Prioritization, and Characteristics of Kepler Target Stars |arxiv=1001.0349 |first1=N. M. |last1=Batalha |first2=W. J. |last2=Borucki |first3=D. G. |last3=Koch |first4=S. T. |last4=Bryson |first5=M. R. |last5=Haas |first6=T. M. |last6=Brown |first7=D. A. |last7=Caldwell |first8=R. L. |last8=Gilliland |first9=D. W. |last9=Latham |first10=S. |last10=Meibom |first11=D. G. |last11=Monet |display-authors=5 |date=January 3, 2010 |doi=10.1088/2041-8205/713/2/L109 |volume=713 |issue=2 |journal=The Astrophysical Journal |pages=L109–L114 |bibcode=2010ApJ...713L.109B|s2cid=39251116 }}</ref> All the selected stars are observed simultaneously, with the spacecraft measuring variations in their brightness every thirty minutes. This provides a better chance for seeing a transit. The mission was designed to maximize the probability of detecting planets orbiting other stars.<ref name=presskit /><ref name="mission_faq">{{cite web |title=Kepler Mission: Frequently Asked Questions |url=http://kepler.nasa.gov/about/faq.html |archive-url=https://web.archive.org/web/20070820000813/http://kepler.nasa.gov/about/faq.html |archive-date=August 20, 2007 |publisher=[[Organization:NASA|NASA]] |date=March 2009 |url-status=dead |access-date=March 14, 2009}}</ref>

Because Kepler must observe at least three transits to confirm that the dimming of a star was caused by a transiting planet, and because larger planets give a signal that is easier to check, scientists expected the first reported results to be larger Jupiter-size planets in tight orbits. The first of these were reported after only a few months of operation. Smaller planets, and planets farther from their sun would take longer, and discovering planets comparable to Earth were expected to take three years or longer.<ref name="keplerlaunch1">{{cite press release |title=Kepler Mission Rockets to Space in Search of Other Earth |url=https://science.nasa.gov/headlines/y2009/06mar_keplerlaunch.htm?friend |publisher=[[Organization:NASA|NASA]] |date=March 6, 2009 |access-date=March 14, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090315053534/http://science.nasa.gov/headlines/y2009/06mar_keplerlaunch.htm?friend |archive-date=March 15, 2009 }}</ref>

Data collected by Kepler is also being used for studying variable stars of various types and performing [[Astronomy:Asteroseismology|asteroseismology]],<ref>{{cite journal |last1=Grigahcène |first1=A. |date=2010 |title=Hybrid γ Doradus – δ Scuti pulsators: New insights into the physics of the oscillations from ''Kepler'' observations |arxiv=1001.0747 |display-authors=0 |doi=10.1088/2041-8205/713/2/L192 |volume=713 |issue=2 |journal=The Astrophysical Journal |pages=L192–L197 |bibcode=2010ApJ...713L.192G|s2cid=56144432 }}</ref> particularly on stars showing [[Astronomy:Solar-like oscillations|solar-like oscillations]].<ref>{{cite journal |last1=Chaplin |first1=W. J. |date=2010 |title=The asteroseismic potential of ''Kepler'': first results for solar-type stars |arxiv=1001.0506 |display-authors=0 |doi=10.1088/2041-8205/713/2/L169 |volume=713 |issue=2 |journal=The Astrophysical Journal |pages=L169–L175 |bibcode=2010ApJ...713L.169C|s2cid=67758571 }}</ref>

==Planet finding process==

===Finding planet candidates===
[[File:NASA-KeplerSpaceTelescope-ArtistConcept-20141027.jpg|thumb|Artist's impression of Kepler]]

Once Kepler has collected and sent back the data, raw light curves are constructed. Brightness values are then adjusted to take the brightness variations due to the rotation of the spacecraft into account. The next step is processing (folding) light curves into a more easily observable form and letting software select signals that seem potentially transit-like. At this point, any signal that shows potential transit-like features is called a threshold crossing event. These signals are individually inspected in two inspection rounds, with the first round taking only a few seconds per target. This inspection eliminates erroneously selected non-signals, signals caused by instrumental noise and obvious eclipsing binaries.<ref>{{cite web |url=http://exoplanetarchive.ipac.caltech.edu/docs/PurposeOfKOITable.html |title=Purpose of Kepler Objects of Interest (KOI) Activity Tables |website=NASA Exoplanet Archive |publisher=NASA Exoplanet Science Institute}}</ref>

Threshold crossing events that pass these tests are called [[Astronomy:Kepler object of interest|Kepler Objects of Interest]] (KOI), receive a KOI designation and are archived. KOIs are inspected more thoroughly in a process called dispositioning. Those which pass the dispositioning are called Kepler planet candidates. The KOI archive is not static, meaning that a Kepler candidate could end up in the false-positive list upon further inspection. In turn, KOIs that were mistakenly classified as false positives could end up back in the candidates list.<ref name="C4WDefault-7178839">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-new-data-q-and-a.html |title=New NASA Kepler Mission Data |date=May 31, 2013 |publisher=[[Organization:NASA|NASA]] |others=NASA Official: Brian Dunbar; Image credit: NASA Ames/W. Stenzel |access-date=April 20, 2014 |type=Interview |first=Michael |last=Haas |archive-url=https://web.archive.org/web/20140420090621/http://www.nasa.gov/mission_pages/kepler/news/kepler-new-data-q-and-a.html |archive-date=April 20, 2014 |url-status=live }}</ref>

Not all the planet candidates go through this process. [[Astronomy:Circumbinary planet|Circumbinary planet]]s do not show strictly periodic transits, and have to be inspected through other methods. In addition, third-party researchers use different data-processing methods, or even search planet candidates from the unprocessed light curve data. As a consequence, those planets may be missing KOI designation.

===Confirming planet candidates===
[[File:NewKeplerPlanetCandidates-20170619.jpg|thumb|right|Kepler mission – new exoplanet candidates – as of June 19, 2017.<ref name="NASA-20150723-MJ2">{{cite web |url=https://www.nasa.gov/image-feature/ames/kepler/new-kepler-planet-candidates |title=New Kepler Planet Candidates |publisher=NASA |editor-first=Rick |editor-last=Chen |date=June 19, 2017 |access-date=August 4, 2017}}</ref>]]
Once suitable candidates have been found from Kepler data, it is necessary to rule out false positives with follow-up tests.

Usually, Kepler candidates are imaged individually with more-advanced ground-based telescopes in order to resolve any background objects which could contaminate the brightness signature of the transit signal.<ref>{{cite journal |last1=Batalha |first1=Natalie M. |date=2010 |title=Pre-Spectroscopic False Positive Elimination of Kepler Planet Candidates |arxiv=1001.0392 |display-authors=0 |doi=10.1088/2041-8205/713/2/L103 |volume=713 |issue=2 |journal=The Astrophysical Journal |pages=L103–L108 |bibcode=2010ApJ...713L.103B|s2cid=119236240 }}</ref>  Another method to rule out planet candidates is [[Astronomy:Astrometry|astrometry]] for which Kepler can collect good data even though doing so was not a design goal. While Kepler cannot detect planetary-mass objects with this method, it can be used to determine if the transit was caused by a stellar-mass object.<ref>{{cite arXiv |last1=Monet |first1=David G. |date=2010 |title=Preliminary Astrometric Results from Kepler |class=astro-ph.IM |eprint=1001.0305|display-authors=0}}</ref>

====Through other detection methods====

There are a few different exoplanet detection methods which help to rule out false positives by giving further proof that a candidate is a real planet. One of the methods, called [[Astronomy:Doppler spectroscopy|doppler spectroscopy]], requires follow-up observations from ground-based telescopes. This method works well if the planet is massive or is located around a relatively bright star. While current spectrographs are insufficient for confirming planetary candidates with small masses around relatively dim stars, this method can be used to discover additional massive non-transiting planet candidates around targeted stars.{{citation needed|date=March 2022}}

[[File:329161main fullFFIHot300.png|thumb|A photo taken by Kepler with two points of interest outlined. Celestial north is towards the lower left corner.]]

In multiplanetary systems, planets can often be confirmed through transit timing variation by looking at the time between successive transits, which may vary if planets are gravitationally perturbed by each other. This helps to confirm relatively low-mass planets even when the star is relatively distant. Transit timing variations indicate that two or more planets belong to the same planetary system. There are even cases where a non-transiting planet is also discovered in this way.<ref>{{Cite news |url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=226 |url-status=dead |archiveurl=https://web.archive.org/web/20130128110114/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=226 |archivedate=2013-01-28 |title=The Transit Timing Variation (TTV) Planet-finding Technique Begins to Flower |date=2012-08-23 |work=NASA}}</ref>

[[Astronomy:Circumbinary planet|Circumbinary planet]]s show much larger transit timing variations between transits than planets gravitationally disturbed by other planets. Their transit duration times also vary significantly. Transit timing and duration variations for circumbinary planets are caused by the orbital motion of the host stars, rather than by other planets.<ref>{{cite arXiv |title=TASTE: The Asiago Survey for Timing transit variations of Exoplanets |eprint=1009.5905 |class=astro-ph.EP |first1=V. |last1=Nascimbeni |first2=G. |last2=Piotto |first3=L. R. |last3=Bedin |first4=M. |last4=Damasso |date=September 29, 2010}}</ref> In addition, if the planet is massive enough, it can cause slight variations of the host stars' orbital periods. Despite being harder to find circumbinary planets due to their non-periodic transits, it is much easier to confirm them, as timing patterns of transits cannot be mimicked by an eclipsing binary or a background star system.<ref>{{cite journal |title=Kepler-16: A Transiting Circumbinary Planet |journal=Science |first1=Laurance R. |last1=Doyle |first2=Joshua A. |last2=Carter |first3=Daniel C. |last3=Fabrycky |first4=Robert W. |last4=Slawson |first5=Steve B. |last5=Howell |display-authors=0 |volume=333 |issue=6049 |pages=1602–1606 |date=September 2011 |doi=10.1126/science.1210923 |bibcode=2011Sci...333.1602D |arxiv=1109.3432 |pmid=21921192|s2cid=206536332 }}</ref>

In addition to transits, planets orbiting around their stars undergo reflected-light variations—like the [[Astronomy:Moon|Moon]], they go through [[Astronomy:Planetary phase|phases]] from full to new and back again. Because Kepler cannot resolve the planet from the star, it sees only the combined light, and the brightness of the host star seems to change over each orbit in a periodic manner. Although the effect is small—the photometric precision required to see a close-in giant planet is about the same as to detect an Earth-sized planet in transit across a solar-type star—Jupiter-sized planets with an orbital period of a few days or less are detectable by sensitive space telescopes such as Kepler. In the long run, this method may help find more planets than the transit method, because the reflected light variation with orbital phase is largely independent of the planet's orbital inclination, and does not require the planet to pass in front of the disk of the star. In addition, the phase function of a giant planet is also a function of its thermal properties and atmosphere, if any. Therefore, the phase curve may constrain other planetary properties, such as the particle size distribution of the atmospheric particles.<ref>{{cite journal |last1=Jenkins |first1=J.M. |first2=Laurance R. |last2=Doyle |date=September 20, 2003 |title=Detecting reflected light from close-in giant planets using space-based photometers |journal=Astrophysical Journal |volume=1 |issue=595 |pages=429–445 |doi=10.1086/377165 |bibcode=2003ApJ...595..429J |arxiv=astro-ph/0305473|s2cid=17773111 }}</ref>

Kepler's photometric precision is often high enough to observe a star's brightness changes caused by [[Astronomy:Relativistic beaming|doppler beaming]] or a star's shape deformation by a companion. These can sometimes be used to rule out hot Jupiter candidates as false positives caused by a star or a brown dwarf when these effects are too noticeable.<ref name="arXiv-1402.6534v1">{{cite journal |title=Validation of Kepler's Multiple Planet Candidates. III: Light Curve Analysis & Announcement of Hundreds of New Multi-planet Systems |arxiv=1402.6534 |first1=Jason F. |last1=Rowe |first2=Stephen T. |last2=Bryson |first3=Geoffrey W. |last3=Marcy |first4=Jack J. |last4=Lissauer |first5=Daniel |last5=Jontof-Hutter |first6=Fergal |last6=Mullally |first7=Ronald L. |last7=Gilliland |first8=Howard |last8=Issacson |first9=Eric |last9=Ford |first10=Steve B. |last10=Howell |first11=William J. |last11=Borucki |first12=Michael |last12=Haas |first13=Daniel |last13=Huber |first14=Jason H. |last14=Steffen |first15=Susan E. |last15=Thompson |first16=Elisa |last16=Quintana |first17=Thomas |last17=Barclay |first18=Martin |last18=Still |first19=Jonathan |last19=Fortney |first20=T. N. |last20=Gautier III |first21=Roger |last21=Hunter |first22=Douglas A. |last22=Caldwell |first23=David R. Ciardi Edna |last23=Devore |first24=William |last24=Cochran |first25=Jon |last25=Jenkins |first26=Eric |last26=Agol |first27=Joshua A. |last27=Carter |first28=John |last28=Geary |display-authors=5 |date=February 26, 2014 |doi=10.1088/0004-637X/784/1/45 |volume=784 |issue=1 |journal=The Astrophysical Journal |page=45 |bibcode=2014ApJ...784...45R|s2cid=119118620 }}</ref> However, there are some cases where such effects are detected even by planetary-mass companions such as [[Astronomy:TrES-2b|TrES-2b]].<ref>{{cite journal |title=A comprehensive study of Kepler phase curves and secondary eclipses – temperatures and albedos of confirmed Kepler giant planets |arxiv=1404.4348 |first1=Daniel |last1=Angerhausen |first2=Emily |last2=DeLarme |first3=Jon A. |last3=Morse |date=April 16, 2014 |doi=10.1086/683797 |volume=127 |issue=957 |journal=Publications of the Astronomical Society of the Pacific |pages=1113–1130 |bibcode=2015PASP..127.1113A|s2cid=118462488 }}</ref>

====Through validation====

If a planet cannot be detected through at least one of the other detection methods, it can be confirmed by determining if the possibility of a Kepler candidate being a real planet is significantly larger than any false-positive scenarios combined. One of the first methods was to see if other telescopes can see the transit as well. The first planet confirmed through this method was [[Astronomy:Kepler-22b|Kepler-22b]] which was also observed with a Spitzer space telescope in addition to analyzing any other false-positive possibilities.<ref name="bbc20111205">{{cite news |url=https://www.bbc.co.uk/news/science-environment-16040655 |title=Kepler 22-b: Earth-like planet confirmed |date=December 5, 2011 |work=BBC Online |access-date=December 6, 2011}}</ref> Such confirmation is costly, as small planets can generally be detected only with space telescopes.

In 2014, a new confirmation method called "validation by multiplicity" was announced. From the planets previously confirmed through various methods, it was found that planets in most planetary systems orbit in a relatively flat plane, similar to the planets found in the Solar System. This means that if a star has multiple planet candidates, it is very likely a real planetary system.<ref name="NASA-20140226"/> Transit signals still need to meet several criteria which rule out false-positive scenarios. For instance, it has to have considerable signal-to-noise ratio, it has at least three observed transits, orbital stability of those systems have to be stable and transit curve has to have a shape that partly eclipsing binaries could not mimic the transit signal. In addition, its orbital period needs to be 1.6 days or longer to rule out common false positives caused by eclipsing binaries.<ref name="arXiv-1402.6352v1">{{cite journal |title=Validation of Kepler's Multiple Planet Candidates. II: Refined Statistical Framework and Descriptions of Systems of Special Interest |arxiv=1402.6352 |last1=Lissauer |first1=Jack J. |last2=Marcy |first2=Geoffrey W. |last3=Bryson |first3=Stephen T. |last4=Rowe |first4=Jason F. |last5=Jontof-Hutter |first5=Daniel |last6=Agol |first6=Eric |last7=Borucki |first7=William J. |last8=Carter |first8=Joshua A. |last9=Ford |first9=Eric B. |last10=Gilliland |first10=Ronald L. |last11=Kolbl |first11=Rea |last12=Star |first12=Kimberly M. |last13=Steffen |first13=Jason H. |last14=Torres |first14=Guillermo |display-authors=5 |date=February 25, 2014 |doi=10.1088/0004-637X/784/1/44 |volume=784 |issue=1 |journal=The Astrophysical Journal |page=44 |bibcode=2014ApJ...784...44L|s2cid=119108651 }}</ref> Validation by multiplicity method is very efficient and allows to confirm hundreds of Kepler candidates in a relatively short amount of time.

A new validation method using a tool called PASTIS has been developed. It makes it possible to confirm a planet even when only a single candidate transit event for the host star has been detected. A drawback of this tool is that it requires a relatively high signal-to-noise ratio from ''Kepler'' data, so it can mainly confirm only larger planets or planets around quiet and relatively bright stars. Currently, the analysis of Kepler candidates through this method is underway.<ref name="arXiv-1403.6725v1">{{cite journal |title=PASTIS: Bayesian Extrasolar Planet Validation. I. General Framework, Models, and Performance |arxiv=1403.6725 |last1=Díaz |first1=Rodrigo F. |last2=Almenara |first2=José M. |last3=Santerne |first3=Alexandre |last4=Moutou |first4=Claire |last5=Lethuillier |first5=Anthony |last6=Deleuil |first6=Magali |date=March 26, 2014 |doi=10.1093/mnras/stu601 |volume=441 |issue=2 |journal=Monthly Notices of the Royal Astronomical Society |pages=983–1004 |bibcode=2014MNRAS.441..983D|s2cid=118387716 }}</ref> PASTIS was first successful for validating the planet Kepler-420b.<ref name="arXiv-1406.6172v1">{{cite journal |title=SOPHIE Velocimetry of ''Kepler'' Transit Candidates: XII. KOI-1257 b: A Highly-Eccentric 3-Month Period Transiting Exoplanet |arxiv=1406.6172 |last1=Santerne |first1=A. |last2=Hébrard |first2=G. |last3=Deleuil |first3=M. |last4=Havel |first4=M. |last5=Correia |first5=A. C. M. |last6=Almenara |first6=J. -M. |last7=Alonso |first7=R. |last8=Arnold |first8=L. |last9=Barros |first9=S. C. C. |last10=Behrend |first10=R. |last11=Bernasconi |first11=L. |last12=Boisse |first12=I. |last13=Bonomo |first13=A. S. |last14=Bouchy |first14=F. |last15=Bruno |first15=G. |last16=Damiani |first16=C. |last17=Díaz |first17=R. F. |last18=Gravallon |first18=D. |last19=Guillot |first19=T. |last20=Labrevoir |first20=O. |last21=Montagnier |first21=G. |last22=Moutou |first22=C. |last23=Rinner |first23=C. |last24=Santos |first24=N. C. |last25=Abe |first25=L. |last26=Audejean |first26=M. |last27=Bendjoya |first27=P. |last28=Gillier |first28=C. |last29=Gregorio |first29=J. |last30=Martinez |first30=P. |last31=Michelet |first31=J. |last32=Montaigut |first32=R. |last33=Poncy |first33=R. |last34=Rivet |first34=J. -P. |last35=Rousseau |first35=G. |last36=Roy |first36=R. |last37=Suarez |first37=O. |last38=Vanhuysse |first38=M. |last39=Verilhac |first39=D. |display-authors=5 |date=June 24, 2014 |doi=10.1051/0004-6361/201424158 |volume=571 |journal=Astronomy & Astrophysics |page=A37 |bibcode=2014A&A...571A..37S|s2cid=118582477 }}</ref>

==Mission results==
[[File:NGC 6791 cluster.jpg|thumb|Detail of Kepler{{'s}} image of the investigated area showing [[Astronomy:Open cluster|open star cluster]] NGC 6791. Celestial north is towards the lower left corner.]]
[[File:Kepler First Light Detail TrES-2.jpg|thumb|Detail of Kepler{{'s}} image of the investigated area. The location of TrES-2b within this image is shown. Celestial north is towards the lower left corner.]]

The Kepler space telescope was in active operation from 2009 through 2013, with the first main results announced on January 4, 2010. As expected, the initial discoveries were all short-period planets. As the mission continued, additional longer-period candidates were found. {{As of|2018|11}}, Kepler has discovered 5,011 exoplanet candidates and 2,662 confirmed exoplanets.<ref>{{cite web |url=https://www.nasa.gov/kepler/discoveries |title=How many exoplanets has Kepler discovered? |publisher=NASA |date=October 27, 2017 |access-date=October 28, 2017}}</ref>
<ref>{{cite web|url=https://www.nasa.gov/kepler/missionstatistics|title=Kepler by the Numbers |publisher=NASA |date=November 2, 2018 |access-date=September 10, 2021}}</ref> As of August 2022, 2,056 exoplanet candidates remain to be confirmed and 2,711 are now confirmed exoplanets.<ref>{{Cite web |title=Exoplanet and Candidate Statistics  |url=https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html |access-date=2022-08-26 |website=exoplanetarchive.ipac.caltech.edu}}</ref>

===2009===
NASA held a press conference to discuss early science results of the Kepler mission on August 6, 2009.<ref name="KeplerBriefing20090806">{{cite web |url=http://www.nasa.gov/centers/ames/news/releases/2009/M09-94AR.html |title=NASA Announces Briefing About Kepler's Early Science Results |date=August 3, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=April 23, 2011}}</ref> At this press conference, it was revealed that Kepler had confirmed the existence of the previously known transiting exoplanet [[Astronomy:HAT-P-7b|HAT-P-7b]], and was functioning well enough to discover Earth-size planets.<ref name="KeplerPressConf20090806">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-discovery.html |title=NASA's Kepler Spies Changing Phases on a Distant World |date=August 6, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=August 6, 2009}}</ref><ref name="kepler_phases">{{cite journal |title=Kepler's Optical Phase Curve of the Exoplanet HAT-P-7b |journal=Science |date=August 7, 2009 |publisher=AAAS |location=Washington, DC |issn=1095-9203 |oclc=1644869 |volume=325 |issue=5941 |page=709 |doi=10.1126/science.1178312 |pmid=19661420 |bibcode=2009Sci...325..709B |first1=W. J. |last1=Borucki |first2=D. |last2=Koch |first3=J. |last3=Jenkins |first4=D. |last4=Sasselov |first5=R. |last5=Gilliland |first6=N. |last6=Batalha |first7=D. W. |last7=Latham |first8=D. |last8=Caldwell |first9=G. |last9=Basri |first10=T. |last10=Brown |first11=J. |last11=Christensen-Dalsgaard |first12=W. D. |last12=Cochran |first13=E. |last13=Devore |first14=E. |last14=Dunham |first15=A. K. |last15=Dupree |first16=T. |last16=Gautier |first17=J. |last17=Geary |first18=A. |last18=Gould |first19=S. |last19=Howell |first20=H. |last20=Kjeldsen |first21=J. |last21=Lissauer |first22=G. |last22=Marcy |first23=S. |last23=Meibom |first24=D. |last24=Morrison |first25=J. |last25=Tarter |s2cid=206522122 |display-authors=5 |url=https://zenodo.org/record/1230908 }}</ref>

Because Kepler's detection of planets depends on seeing very small changes in brightness, stars that vary in brightness by themselves ([[Astronomy:Variable star|variable star]]s) are not useful in this search.<ref name="Update20091105"/>  From the first few months of data, Kepler scientists determined that about 7,500 stars from the initial target list are such variable stars.  These were dropped from the target list, and replaced by new candidates.  On November 4, 2009, the Kepler project publicly released the light curves of the dropped stars.<ref>{{cite web |url=http://archive.stsci.edu/mast_news.php?out=html&desc=t&id=342 |title=Kepler dropped stars now public |date=November 4, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=April 23, 2011}}</ref> The first new planet candidate observed by Kepler was originally marked as a false positive because of uncertainties in the mass of its parent star. However, it was confirmed ten years later and is now designated [[Astronomy:Kepler-1658b|Kepler-1658b]].<ref name="Chontos et al">{{cite journal |last1=Chontos |first1=Ashley |last2=Huber |first2=Daniel |last3=Latham |first3=David W. |last4=Bieryla |first4=Allyson |last5=van Eylen |first5=Vincent |last6=Bedding |first6=Timothy R. |last7=Berger |first7=Travis |last8=Buchhave |first8=Lars A. |last9=Campante |first9=Tiago L. |last10=Chaplin |first10=William J |last11=Colman |first11=Isabel L. |last12=Coughlin |first12=Jeff L. |last13=Davies |first13=Guy |last14=Hirano |first14=Teruyuki |last15=Howard |first15=Andrew W. |last16=Isaacson |first16=Howard |title=The Curious Case of KOI 4: Confirming Kepler's First Exoplanet Detection|journal=The Astronomical Journal |volume=157 |issue=5 |pages=192 |arxiv=1903.01591 |date=March 2019 |doi=10.3847/1538-3881/ab0e8e |bibcode=2019AJ....157..192C |s2cid=119240124 |doi-access=free }}</ref><ref name="Chen">{{cite web |last1=Chen |first1=Rick |title=Kepler's First Planet Candidate Confirmed 10 Years Later |url=https://www.nasa.gov/feature/discovery-alert-keplers-first-planet-candidate-confirmed-10-years-later |website=NASA |access-date=March 6, 2019 |date=March 5, 2019}}</ref>

The first six weeks of data revealed five previously unknown planets, all very close to their stars.<ref name="sciencenews.org">{{cite web |url=http://www.sciencenews.org/view/generic/id/52465/title/Kepler_space_telescope_finds_its_first_extrasolar_planets |title=Kepler space telescope finds its first extrasolar planets |publisher=Sciencenews.org |date=January 30, 2010 |access-date=February 5, 2011 |archive-date=September 25, 2012 |archive-url=https://web.archive.org/web/20120925002416/http://www.sciencenews.org/view/generic/id/52465/title/Kepler_space_telescope_finds_its_first_extrasolar_planets |url-status=dead }}</ref><ref>{{cite web |last=MacRobert |first=Robert |title=Kepler's First Exoplanet Results – News Blog |url=http://www.skyandtelescope.com/community/skyblog/newsblog/80621797.html |publisher=Sky & Telescope |date=January 4, 2010 |access-date=April 21, 2011 |archive-date=September 14, 2011 |archive-url=https://web.archive.org/web/20110914041857/http://www.skyandtelescope.com/community/skyblog/newsblog/80621797.html |url-status=dead }}</ref> Among the notable results are one of the least dense planets yet found,<ref name="centauri-dreams.org">{{cite web |last=Gilster |first=Paul |url=http://www.centauri-dreams.org/?p=16640 |title=The Remarkable Kepler-11 |publisher=Tau Zero Foundation |date=February 2, 2011 |access-date=April 21, 2011}}</ref> two low-mass [[Astronomy:White dwarf|white dwarf]]s<ref name="vanKerkwijk2010">{{cite journal |first1=Marten H. |last1=van Kerkwijk |first2=Saul A. |last2=Rappaport |first3=René P. |last3=Breton |first4=Stephen |last4=Justham |first5=Philipp |last5=Podsiadlowski |first6=Zhanwen |last6=Han |title=Observations of Doppler Boosting in Kepler Light Curves |date=May 20, 2010 |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |issn=0004-637X |volume=715 |issue=1 |pages=51–58 |bibcode=2010ApJ...715...51V |doi=10.1088/0004-637X/715/1/51 |arxiv=1001.4539|s2cid=15893663 }}</ref> that were initially reported as being members of a new class of stellar objects,<ref name="hotter">{{cite web |last=Villard |first=Ray |title=Blazing Stellar Companion Defies Explanation |url=http://news.discovery.com/space/blazing-stellar-companion-defies-explanation.html |publisher=Discovery.com |access-date=April 20, 2011 |archive-date=March 2, 2012 |archive-url=https://web.archive.org/web/20120302113859/http://news.discovery.com/space/blazing-stellar-companion-defies-explanation.html |url-status=dead }}</ref> and  [[Astronomy:Kepler-16b|Kepler-16b]], a well-characterized planet orbiting a binary star.

===2010===
On June 15, 2010, the Kepler mission released data on all but 400 of the ~156,000 planetary target stars to the public. 706 targets from this first data set have viable exoplanet candidates, with sizes ranging from as small as Earth to larger than Jupiter. The identity and characteristics of 306 of the 706 targets were given. The released targets included five{{citation needed|date=May 2016}} candidate multi-planet systems, including six extra exoplanet candidates.<ref name=Borucki2010/>  Only 33.5 days of data were available for most of the candidates.<ref name=Borucki2010/>  NASA also announced data for another 400 candidates were being withheld to allow members of the ''Kepler'' team to perform follow-up observations.<ref>{{cite web |title=Kepler News: First 43 Days of Kepler Data Released |url=http://kepler.arc.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=42 |archive-url=https://web.archive.org/web/20100811220422/http://www.kepler.arc.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=42 |url-status=dead |archive-date=August 11, 2010 |publisher=[[Organization:NASA|NASA]] |date=May 15, 2010 |access-date=April 24, 2011}}</ref>  The data for these candidates was published February 2, 2011.<ref name=borucki/> (See the ''Kepler'' results for 2011 below.)

The Kepler results, based on the candidates in the list released in 2010, implied that most candidate planets have radii less than half that of Jupiter. The  results also imply that small candidate planets with periods less than thirty days are much more common than large candidate planets with periods less than thirty days and that the ground-based discoveries are sampling the large-size tail of the size distribution.<ref name="Borucki2010">{{cite report |last1=Borucki |first1=William J. |display-authors=0|title=Characteristics of Kepler Planetary Candidates Based on the First Data Set: The Majority are Found to be Neptune-Size and Smaller |arxiv=1006.2799 |date=2010 |doi=10.1088/0004-637X/728/2/117 |s2cid=93116 }}</ref>  This contradicted older theories which had suggested small and Earth-size planets would be relatively infrequent.<ref name="history">{{cite journal |last1=Woolfson |first1=M. M. |title=The Solar System: Its Origin and Evolution |journal=Journal of the Royal Astronomical Society |volume=34 |pages=1–20 |date=1993 |bibcode=1993QJRAS..34....1W}}  Page 18 in particular states that models that required a near collision of stars imply about 1% will have planets.</ref><ref>{{cite journal |title=Protoplanet Migration by Nebula Tides |last1=Ward |first1=W.R. |journal=Icarus |volume=126 |issue=2 |pages=261–281 |date=1997 |url=http://www.gps.caltech.edu/classes/ge133/reading/ward_migration.pdf |publisher=[[Company:Elsevier|Elsevier]] |access-date=April 23, 2011 |bibcode=1997Icar..126..261W |doi=10.1006/icar.1996.5647 |archive-date=June 14, 2011 |archive-url=https://web.archive.org/web/20110614091430/http://www.gps.caltech.edu/classes/ge133/reading/ward_migration.pdf |url-status=dead }}</ref>  Based on extrapolations from the ''Kepler'' data, an estimate of around 100 million habitable planets in the Milky Way may be realistic.<ref>{{cite web |url=http://www.ted.com/talks/dimitar_sasselov_how_we_found_hundreds_of_earth_like_planets.html |last1=Sasselov |first1=Dimitar |title=How we found hundreds of Earth-like planets |publisher=Ted.com |date=July 2010 |access-date=February 5, 2011 |archive-date=July 27, 2010 |archive-url=https://web.archive.org/web/20100727211015/http://www.ted.com/talks/dimitar_sasselov_how_we_found_hundreds_of_earth_like_planets.html |url-status=dead }}</ref>  Some media reports of the TED talk have led to the misunderstanding that ''Kepler'' had actually found these planets.  This was clarified in a letter to the Director of the NASA [[Organization:Ames Research Center|Ames Research Center]], for the Kepler Science Council dated August 2, 2010 states, "Analysis of the current Kepler data does not support the assertion that Kepler has found any Earth-like planets."<ref name='Earth-size'/><ref>{{cite journal |last1=Steffen |first1=Jason H. |display-authors=0 |title=Five Kepler target stars that show multiple transiting exoplanet candidates |doi=10.1088/0004-637X/725/1/1226 |pages=1226–1241 |volume=725 |issue=1 |journal=Astrophysical Journal |issn=0004-637X |arxiv=1006.2763 |date=November 9, 2010 |bibcode=2010ApJ...725.1226S|s2cid=14775394 }}</ref><ref name="arXiv-1006.2815v2">{{cite journal |title=Kepler Eclipsing Binary Stars. I. Catalog and Principal Characterization of 1879 Eclipsing Binaries in the First Data Release |arxiv=1006.2815 |first1=Andrej |last1=Prsa |first2=Natalie M. |last2=Batalha |first3=Robert W. |last3=Slawson |first4=Laurance R. |last4=Doyle |first5=William F. |last5=Welsh |first6=Jerome A. |last6=Orosz |first7=Sara |last7=Seager |first8=Michael |last8=Rucker |first9=Kimberly |last9=Mjaseth |first10=Scott G. |last10=Engle |first11=Kyle |last11=Conroy |first12=Jon M. |last12=Jenkins |first13=Douglas A. |last13=Caldwell |first14=David G. |last14=Koch |first15=William J. |last15=Borucki |display-authors=5 |date=January 21, 2011 |doi=10.1088/0004-6256/141/3/83 |volume=141 |issue=3 |journal=The Astronomical Journal |page=83 |bibcode=2011AJ....141...83P|s2cid=13440062 }}</ref>

In 2010, Kepler identified two systems containing objects which are smaller and hotter than their parent stars: [[Astronomy:KOI-74|KOI 74]] and [[Astronomy:KOI-81|KOI 81]].<ref>{{cite journal |first1=Jason F. |last1=Rowe |first2=William J. |last2=Borucki |first3=David |last3=Koch |first4=Steve B. |last4=Howell |first5=Gibor |last5=Basri |first6=Natalie |last6=Batalha |first7=Timothy M. |last7=Brown |first8=Douglas |last8=Caldwell |first9=William D. |last9=Cochran |first10=Edward |last10=Dunham |first11=Andrea K. |last11=Dupree |first12=Jonathan J. |last12=Fortney |first13=Thomas N. |last13=Gautier III |first14=Ronald L. |last14=Gilliland |first15=Jon |last15=Jenkins |first16=David W. |last16=Latham |first17=Jack J. |last17=Lissauer |first18=Geoff |last18=Marcy |first19=David G. |last19=Monet |first20=Dimitar |last20=Sasselov |first21=William F. |last21=Welsh |display-authors=5 |title=Kepler Observations of Transiting Hot Compact Objects |date=2010 |journal=The Astrophysical Journal Letters |volume=713 |issue=2 |pages=L150–L154 |bibcode=2010ApJ...713L.150R |doi=10.1088/2041-8205/713/2/L150 |arxiv=1001.3420|s2cid=118578253 }}</ref> These objects are probably low-mass [[Astronomy:White dwarf|white dwarf]]s produced by previous episodes of [[Physics:Mass transfer|mass transfer]] in their systems.<ref name="vanKerkwijk2010"/>

===2011===
[[File:Kepler 20 - planet lineup.jpg|thumb|left|A size comparison of the exoplanets Kepler-20e<ref name="Kepler20e-20111220">{{cite web |title=Kepler: A Search For Habitable Planets – Kepler-20e |url=http://kepler.nasa.gov/Mission/discoveries/kepler20e/ |archive-url=https://web.archive.org/web/20120310175648/http://kepler.nasa.gov/Mission/discoveries/kepler20e/ |url-status=dead |archive-date=March 10, 2012 |date=December 20, 2011 |publisher=NASA |access-date=December 23, 2011}}</ref> and Kepler-20f<ref name="Kepler20f-20111220">{{cite web |title=Kepler: A Search For Habitable Planets – Kepler-20f |url=http://kepler.nasa.gov/Mission/discoveries/kepler20f/ |archive-url=https://web.archive.org/web/20120310195733/http://kepler.nasa.gov/Mission/discoveries/kepler20f/ |url-status=dead |archive-date=March 10, 2012 |date=December 20, 2011 |publisher=NASA |access-date=December 23, 2011}}</ref> with Venus and Earth]]

On February 2, 2011, the Kepler team announced the results of analysis of the data taken between 2 May and September 16, 2009.<ref name="borucki">{{cite journal |last1=Borucki |first1=William J. |date=2011 |title=Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data |arxiv=1102.0541 |display-authors=0 |doi=10.1088/0004-637X/736/1/19 |volume=736 |issue=1 |journal=The Astrophysical Journal |page=19 |bibcode=2011ApJ...736...19B|s2cid=15233153 }}</ref> They found 1235 planetary candidates circling 997 host stars. (The numbers that follow assume the candidates are really planets, though the official papers called them only candidates. Independent analysis indicated that at least 90% of them are real planets and not false positives).<ref>{{cite journal |last1=Morton |first1=Timothy D. |last2=Johnson |first2=John Asher |date=2011 |title=On the Low False Positive Probabilities of Kepler Planet Candidates |journal=The Astrophysical Journal |volume=738 |issue=2 |pages=170 |arxiv=1101.5630|doi=10.1088/0004-637X/738/2/170 |bibcode=2011ApJ...738..170M |s2cid=35223956 }}</ref> 68 planets were approximately Earth-size, 288 [[Astronomy:Super-Earth|super-Earth]]-size, 662 Neptune-size, 165 Jupiter-size, and 19 up to twice the size of Jupiter. In contrast to previous work, roughly 74% of the planets are smaller than Neptune, most likely as a result of previous work finding large planets more easily than smaller ones.

That February 2, 2011 release of 1235 exoplanet candidates included 54 that may be in the "[[Astronomy:Habitable zone|habitable zone]]", including five less than twice the size of Earth.<ref name="5earths">{{cite web |title=NASA Finds Earth-size Planet Candidates in Habitable Zone, Six Planet System |url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=98 |archive-url=https://web.archive.org/web/20110429012133/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=98 |url-status=dead |archive-date=April 29, 2011 |date=February 2, 2011 |publisher=[[Organization:NASA|NASA]] |access-date=April 24, 2011}}</ref><ref name="Overbye">{{cite news |last1=Overbye |first1=Dennis |title=Kepler Planet Hunter Finds 1,200 Possibilities |work={{wipe|The New York Times}} |date=February 2, 2011 |url=https://www.nytimes.com/2011/02/03/science/03planet.html |access-date=April 24, 2011}}</ref> There were previously only two planets thought to be in the "habitable zone", so these new findings represent an enormous expansion of the potential number of "Goldilocks planets" (planets of the right temperature to support liquid water).<ref name="Borenstein">{{cite news |last1=Borenstein |first1=Seth |title=NASA spots scores of potentially livable worlds |publisher=MSNBC News |date=February 2, 2011 |url=http://www.nbcnews.com/id/41387915 |access-date=April 24, 2011}}</ref> All of the habitable zone candidates found thus far orbit stars significantly smaller and cooler than the Sun (habitable candidates around Sun-like stars will take several additional years to accumulate the three transits required for detection).<ref name="Alexander">{{cite web |last1=Alexander |first1=Amir |title=Kepler Discoveries Suggest a Galaxy Rich in Life |publisher=[[Organization:The Planetary Society|The Planetary Society]] |date=February 3, 2011 |url=http://www.planetary.org/news/2011/0203_Kepler_Discoveries_Suggest_a_Galaxy.html |access-date=February 4, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110205191952/http://www.planetary.org/news/2011/0203_Kepler_Discoveries_Suggest_a_Galaxy.html |archive-date=February 5, 2011 }}</ref> Of all the new planet candidates, 68 are 125% of [[Astronomy:Earth|Earth]]'s size or smaller, or smaller than all previously discovered exoplanets.<ref name="Overbye" /> "Earth-size" and "super-Earth-size" is defined as "less than or equal to 2 Earth radii (Re)" [(or, Rp ≤ 2.0 Re) – Table 5].<ref name=borucki/> Six such planet candidates [namely: KOI 326.01 (Rp=0.85), KOI 701.03 (Rp=1.73), KOI 268.01 (Rp=1.75), KOI 1026.01 (Rp=1.77), KOI 854.01 (Rp=1.91), KOI 70.03 (Rp=1.96) – Table 6]<ref name=borucki/> are in the "habitable zone."<ref name=5earths /> A more recent study found that one of these candidates (KOI&nbsp;326.01) is in fact much larger and hotter than first reported.<ref name="Grant">{{cite web |last1=Grant |first1=Andrew |title=Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion – It Isn't Habitable |url=http://blogs.discovermagazine.com/80beats/2011/03/08/exclusive-most-earth-like-exoplanet-gets-major-demotion%e2%80%94it-isnt-habitable/ |work=Discover Magazine |date=March 8, 2011 |access-date=April 24, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110309132609/http://blogs.discovermagazine.com/80beats/2011/03/08/exclusive-most-earth-like-exoplanet-gets-major-demotion%e2%80%94it-isnt-habitable/ |archive-date=March 9, 2011 }}</ref>

The frequency of planet observations was highest for exoplanets two to three times Earth-size, and then declined in inverse proportionality to the area of the planet. The best estimate (as of March 2011), after accounting for observational biases, was: 5.4% of stars host Earth-size candidates, 6.8% host super-Earth-size candidates, 19.3% host Neptune-size candidates, and 2.55% host Jupiter-size or larger candidates. Multi-planet systems are common; 17% of the host stars have multi-candidate systems, and 33.9% of all the planets are in multiple planet systems.<ref>{{cite journal |first=William J. |last=Borucki |display-authors=0 |date=2011 |title=Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data |arxiv=1102.0541 |bibcode=2011ApJ...736...19B |doi=10.1088/0004-637X/736/1/19 |journal=The Astrophysical Journal |volume=736 |issue=1 |page=19 |publisher=IOP Publishing |s2cid=15233153 |issn=0004-637X}}</ref>

By December 5, 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data.<ref name=KeplerDec2011Results/>

On December 20, 2011, the Kepler team announced the discovery of the first [[Astronomy:Terrestrial planet|Earth-size]] exoplanets, [[Astronomy:Kepler-20e|Kepler-20e]]<ref name="Kepler20e-20111220" /> and [[Astronomy:Kepler-20f|Kepler-20f]],<ref name="Kepler20f-20111220" /> orbiting a Sun-like star, [[Astronomy:Kepler-20|Kepler-20]].<ref name="NASA-20111220">{{cite web |last=Johnson |first=Michele |title=NASA Discovers First Earth-size Planets Beyond Our Solar System |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.html |publisher=[[Organization:NASA|NASA]] |date=December 20, 2011 |access-date=December 20, 2011}}</ref>

Based on Kepler's findings, astronomer [[Biography:Seth Shostak|Seth Shostak]] estimated in 2011 that "within a thousand light-years of Earth", there are "at least 30,000" habitable planets.<ref name="Shostak">{{cite news |last1=Shostak |first1=Seth |title=A Bucketful of Worlds |work=Huffington Post |date=February 3, 2011 |url=http://www.huffingtonpost.com/seth-shostak/a-bucketful-of-worlds_b_817921.html |access-date=April 24, 2011}}</ref> Also based on the findings, the Kepler team has estimated that there are "at least 50 billion planets in the Milky Way", of which "at least 500 million" are in the [[Astronomy:Habitable zone|habitable zone]].<ref name="BorensteinS">{{cite news |last1=Borenstein |first1=Seth |title=Cosmic census finds crowd of planets in our galaxy |agency=Associated Press |date=February 19, 2011 |url=http://apnews.excite.com/article/20110219/D9LG45NO0.html |access-date=April 24, 2011 |archive-url=https://web.archive.org/web/20110927053134/http://apnews.excite.com/article/20110219/D9LG45NO0.html |archive-date=September 27, 2011 |url-status=dead |df=mdy-all }}</ref> In March 2011, astronomers at NASA's [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] (JPL) reported that about "1.4&nbsp;to 2.7 percent" of all Sun-like stars are expected to have Earth-size planets "within the [[Astronomy:Habitable zone|habitable zone]]s of their stars". This means there are "two billion" of these "Earth analogs" in the Milky Way alone. The JPL astronomers also noted that there are "50 billion other galaxies", potentially yielding more than one sextillion "Earth analog" planets if all galaxies have similar numbers of planets to the Milky Way.<ref name="ChoiCQ">{{cite news |last1=Choi |first1=Charles Q. |url=http://www.space.com/11188-alien-earths-planets-sun-stars.html |title=New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone |date=March 21, 2011 |work=Space.com |access-date=April 24, 2011}}</ref>

===2012===
In January 2012, an international team of astronomers reported that each star in the [[Astronomy:Milky Way|Milky Way]] may host "[[Astronomy:Exoplanet|on average...at least 1.6 planets]]", suggesting that over 160 billion star-bound planets may exist in the Milky Way.<ref name="Space-20120111">{{cite news |last=Wall |first=Mike |title=160 Billion Alien Planets May Exist in Our Milky Way Galaxy |url=http://www.space.com/14200-160-billion-alien-planets-milky-galaxy.html |date=January 11, 2012 |work=Space.com |access-date=January 11, 2012}}</ref><ref name="Nature-20120111">{{cite journal |first1=A. |last1=Cassan |title=One or more bound planets per Milky Way star from microlensing observations |doi=10.1038/nature10684 |date=January 11, 2012 |journal=Nature |volume=481 |pages=167–169 |bibcode=2012Natur.481..167C |pmid=22237108 |issue=7380 |arxiv=1202.0903 |last2=Kubas |first2=D. |last3=Beaulieu |first3=J.-P. |last4=Dominik |first4=M. |last5=Horne |first5=K. |last6=Greenhill |first6=J. |last7=Wambsganss |first7=J. |last8=Menzies |first8=J. |last9=Williams |first9=A. <!--|first10=U. G. |first11=A. |first12=D. P. |first13=M. D. |first14=V. |first15=S. |first16=J. A. R. |first17=A. |first18=Ch. |first19=K. H. |first20=S. |first21=D. Dominis |first22=J. |first23=P. |first24=K. |first25=N. |first26=S. |first27=J.-B. |first28=R. |first29=K. R. |first30=K. C.--> |s2cid=2614136 |display-authors=5 }}</ref> Kepler also recorded distant [[Astronomy:Solar flare|stellar super-flares]], some of which are 10,000 times more powerful than the 1859 Carrington event.<ref name="bbc1">{{cite news |url=https://www.bbc.co.uk/news/science-environment-18089695 |title=Kepler telescope studies star superflares |work=BBC News |date=May 17, 2012 |access-date=May 31, 2012}}</ref> The superflares may be triggered by close-orbiting [[Astronomy:Jupiter|Jupiter]]-sized planets.<ref name=bbc1/> The Transit Timing Variation (TTV) technique, which was used to discover [[Astronomy:Kepler-9d|Kepler-9d]], gained popularity for confirming exoplanet discoveries.<ref>[https://web.archive.org/web/20130128110114/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=226 The Transit Timing Variation (TTV) Planet-finding Technique Begins to Flower]. NASA.gov.</ref> A planet in a system with four stars was also confirmed, the first time such a system had been discovered.<ref>[https://web.archive.org/web/20130128105815/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=233 Planet Hunters Find Circumbinary Planet in 4-Star System – 10.16.2012].</ref>

{{As of|2012}}, there were a total of 2,321 candidates.<ref name="KeplerDec2011Results">{{cite web |url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=165 |archive-url=https://web.archive.org/web/20120316051034/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?fuseaction=ShowNews&NewsID=165 |url-status=dead |archive-date=March 16, 2012 |title=Kepler-22b, Super-Earth in the habitable zone of a Sun-like Star |publisher=NASA |date=December 5, 2011}}</ref><ref>{{cite web |url=http://news.sciencemag.org/sciencenow/2011/09/super-earth-found-in-habitable-z.html |title='Super-Earth' Found in Habitable Zone |first=Govert |last=Schilling |date=September 12, 2011 |publisher=AAAS |url-status=dead |archive-url=https://web.archive.org/web/20110925020810/http://news.sciencemag.org/sciencenow/2011/09/super-earth-found-in-habitable-z.html |archive-date=September 25, 2011 }}</ref><ref>{{cite web |url=http://archive.stsci.edu/kepler/planet_candidates.html |title=Released Kepler Planetary Candidates |publisher=MAST |date=February 27, 2012 |access-date=November 26, 2012}}</ref> Of these, 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars.  The Kepler team estimated that 5.4% of all stars host Earth-size planet candidates, and that 17% of all stars have multiple planets.

===2013===
[[Image:Exoplanet Period-Mass Scatter Kepler.png|thumb|A chart showing Kepler discoveries, in context of all discovered exoplanets (through 2013), with some transit probabilities indicated for example scenarios.]]

According to a study by Caltech astronomers published in January 2013, the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets.<ref name="NASA-20130103">{{cite web |last=Claven |first=Whitney |title=Billions and Billions of Planets |url=http://www.nasa.gov/mission_pages/kepler/news/kepler20130103.html |date=January 3, 2013 |publisher=[[Organization:NASA|NASA]] |access-date=January 3, 2013}}</ref><ref name="Space-20130102">{{cite news |title=100 Billion Alien Planets Fill Our Milky Way Galaxy: Study |url=http://www.space.com/19103-milky-way-100-billion-planets.html |date=January 2, 2013 |work=Space.com |access-date=January 3, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130103060601/http://www.space.com/19103-milky-way-100-billion-planets.html |archive-date=January 3, 2013 }}</ref> The study, based on planets orbiting the star [[Astronomy:Kepler-32|Kepler-32]], suggests that [[Astronomy:Planetary system|planetary system]]s may be common around stars in the Milky Way. The discovery of 461 more candidates was announced on January 7, 2013.<ref name=kep>[http://www.nasa.gov/mission_pages/kepler/news/kepler-461-new-candidates.html NASA's Kepler Mission Discovers 461 New Planet Candidates]</ref> The longer Kepler watches, the more planets with long periods it can detect.<ref name=kep/>

{{Cquote|Since the last Kepler catalog was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20 percent and now totals 2,740 potential planets orbiting 2,036 stars|NASA<ref name=kep/>}}

A candidate, newly announced on January 7, 2013, was [[Astronomy:Kepler-69c|Kepler-69c]] (formerly, ''KOI-172.02''), an Earth-size exoplanet orbiting a star similar to the Sun in the [[Astronomy:Habitable zone|habitable zone]] and possibly habitable.<ref name="Space-20130109">{{cite news |last=Moskowitz |first=Clara |title=Most Earth-Like Alien Planet Possibly Found |url=http://www.space.com/19201-most-earth-like-alien-planet.html |date=January 9, 2013 |work=Space.com |access-date=January 9, 2013}}</ref>

In April 2013, a white dwarf was discovered bending the light of its companion red dwarf in the [[Astronomy:KOI-256|KOI-256]] star system.<ref name="nasa20130404">{{cite news |url=http://www.nasa.gov/mission_pages/kepler/news/kepler20130404.html |title=Gravity-Bending Find Leads to Kepler Meeting Einstein |publisher=NASA |date=April 4, 2013 |access-date=April 6, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150705131007/http://www.nasa.gov/mission_pages/kepler/news/kepler20130404.html |archive-date=July 5, 2015 }}</ref>

In April 2013, NASA announced the discovery of three new Earth-size exoplanets—[[Astronomy:Kepler-62e|Kepler-62e]], [[Astronomy:Kepler-62f|Kepler-62f]], and [[Astronomy:Kepler-69c|Kepler-69c]]—in the [[Astronomy:Habitable zone|habitable zone]]s of their respective host stars, [[Astronomy:Kepler-62|Kepler-62]] and [[Astronomy:Kepler-69|Kepler-69]]. The new exoplanets are considered prime candidates for possessing liquid water and thus a habitable environment.<ref name="NASA-20130418">{{cite web |last1=Johnson |first1=Michele |last2=Harrington |first2=J.D. |title=NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-62-kepler-69.html |date=April 18, 2013 |publisher=[[Organization:NASA|NASA]] |access-date=April 18, 2013}}</ref><ref name="NYT-20130418">{{cite news |last=Overbye |first=Dennis |title=2 Good Places to Live, 1,200 Light-Years Away |url=https://www.nytimes.com/2013/04/19/science/space/2-new-planets-are-most-earth-like-yet-scientists-say.html |date=April 18, 2013 |work={{wipe|The New York Times}} |access-date=April 18, 2013}}</ref><ref>{{cite web |url=https://www.youtube.com/watch?v=P8PJt-R5NxI | archive-url=https://ghostarchive.org/varchive/youtube/20211113/P8PJt-R5NxI| archive-date=2021-11-13 | url-status=live|title=NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date |publisher=YouTube |date=April 18, 2013 |access-date=April 19, 2013}}{{cbignore}}</ref> A more recent analysis has shown that Kepler-69c is likely more analogous to Venus, and thus unlikely to be habitable.<ref name="supervenus">{{cite journal |first1=Stephen R. |last1=Kane |first2=Thomas |last2=Barclay |first3=Dawn M. |last3=Gelino |title=A Potential Super-Venus in the Kepler-69 System |publisher=IOP Publishing |issn=2041-8205 |journal=The Astrophysical Journal Letters |arxiv=1305.2933 |date=2013 |bibcode=2013ApJ...770L..20K |volume=770 |page=L20 |doi=10.1088/2041-8205/770/2/L20 |issue=2|s2cid=9808447 }}</ref>

On May 15, 2013, NASA announced the space telescope had been crippled by failure of a [[Engineering:Reaction wheel|reaction wheel]] that keeps it pointed in the right direction. A second wheel had previously failed, and the telescope required three wheels (out of four total) to be operational for the instrument to function properly. Further testing in July and August determined that while Kepler was capable of using its damaged reaction wheels to prevent itself from entering safe mode and of downlinking previously collected science data it was not capable of collecting further science data as previously configured.<ref>{{cite web |url=http://www.nasa.gov/content/kepler-mission-manager-update-pointing-test-results |title=Kepler Mission Manager Update: Pointing Test Results |date=August 19, 2013 |publisher=[[Organization:NASA|NASA]] |access-date=September 9, 2013}}</ref> Scientists working on the Kepler project said there was a backlog of data still to be looked at, and that more discoveries would be made in the following couple of years, despite the setback.<ref>{{cite news |url=http://www.3news.co.nz/Planet-hunting-Kepler-broken-mission-may-be-over/tabid/1160/articleID/298431/Default.aspx |work=3 News NZ |title=Kepler broken – mission may be over |date=May 20, 2013 |access-date=May 20, 2013 |archive-url=https://web.archive.org/web/20140705045849/http://www.3news.co.nz/Planet-hunting-Kepler-broken-mission-may-be-over/tabid/1160/articleID/298431/Default.aspx |archive-date=July 5, 2014 |url-status=dead }}</ref>

Although no new science data from Kepler field had been collected since the problem, an additional sixty-three candidates were announced in July 2013 based on the previously collected observations.<ref name=recovery>[http://www.nasa.gov/content/kepler-mission-manager-update-preparing-for-recovery/ NASA – Kepler Mission Manager Update: Preparing for Recovery]</ref>

In November 2013, the second Kepler science conference was held. The discoveries included the median size of planet candidates getting smaller compared to early 2013, preliminary results of the discovery of a few circumbinary planets and planets in the habitable zone.<ref>{{cite conference |title=Agenda |conference=Second Kepler Science Conference – NASA Ames Research Center, Mountain View, CA. Nov. 4–8, 2013 |conference-url=http://nexsci.caltech.edu/conferences/KeplerII/agenda.shtml }}</ref>

===2014===
[[File:ExoplanetDiscoveries-Histogram-20140226.png|thumb|[[Histogram|Histogram]] of exoplanet discoveries. The yellow shaded bar shows newly announced planets including those verified by the multiplicity technique (February 26, 2014).]]

On February 13, over 530 additional planet candidates were announced residing around single planet systems. Several of them were nearly Earth-sized and located in the habitable zone. This number was further increased by about 400 in June 2014.<ref name="C4Wauto-2478260">{{cite web |url=http://exoplanetarchive.ipac.caltech.edu/ |title=Welcome to the NASA Exoplanet Archive |publisher=California Institute of Technology |date=February 27, 2014 |access-date=February 27, 2014 |archive-url=https://web.archive.org/web/20140227025004/http://exoplanetarchive.ipac.caltech.edu/ |archive-date=February 27, 2014 |url-status=live |quote=February 13, 2014: The Kepler project has updated dispositions for 534 KOIs in the Q1–Q16 KOI activity table. This brings the total number of Kepler candidates and confirmed planets to 3,841. For more information, see the Purpose of KOI Table document and the interactive tables.}}</ref>

On February 26, scientists announced that data from Kepler had confirmed the existence of 715 new exoplanets. A new statistical method of confirmation was used called "verification by multiplicity" which is based on how many planets around multiple stars were found to be real planets. This allowed much quicker confirmation of numerous candidates which are part of multiplanetary systems. 95% of the discovered exoplanets were smaller than [[Astronomy:Neptune|Neptune]] and four, including Kepler-296f, were less than 2 1/2 the size of [[Astronomy:Earth|Earth]] and were in [[Astronomy:Circumstellar habitable zone|habitable zones]] where surface temperatures are suitable for liquid [[Chemistry:Water|water]].<ref name="NASA-20140226">{{cite web |last1=Johnson |first1=Michele |last2=Harrington |first2=J.D. |title=NASA's Kepler Mission Announces a Planet Bonanza, 715 New Worlds |url=http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/ |date=February 26, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=February 26, 2014}}</ref><ref name="SP-20140226">{{cite web |last=Wall |first=Mike |title=Population of Known Alien Planets Nearly Doubles as NASA Discovers 715 New Worlds |url=http://www.space.com/24824-alien-planets-population-doubles-nasa-kepler.html |date=February 26, 2014 |work=Space.com |access-date=February 26, 2014}}</ref><ref name="BBC-20140227">{{cite news |last=Amos |first=Jonathan |title=Kepler telescope bags huge haul of planets |url=https://www.bbc.co.uk/news/science-environment-26362433 |date=February 26, 2014 |work=BBC News |access-date=February 27, 2014}}</ref><ref name="NYT-20140227">{{cite news |last=Overbye |first=Dennis |title=From Kepler Data, Astronomers Find Galaxy Filled With More but Smaller Worlds |url=https://www.nytimes.com/2014/02/27/science/space/diving-into-kepler-data-astronomers-discover-hundreds-of-planets.html |date=February 27, 2014 |work={{wipe|The New York Times}} |access-date=February 28, 2014}}</ref>

In March, a study found that small planets with orbital periods of less than one day are usually accompanied by at least one additional planet with orbital period of 1–50 days. This study also noted that ultra-short period planets are almost always smaller than 2 Earth radii unless it is a misaligned hot Jupiter.<ref name="arXiv-1403.2379v1">{{cite journal |title=A Study of the Shortest-Period Planets Found With Kepler |arxiv=1403.2379 |first1=Roberto |last1=Sanchis-Ojeda |first2=Saul |last2=Rappaport |first3=Joshua N. |last3=Winn |first4=Michael C. |last4=Kotson |first5=Alan M. |last5=Levine |first6=Ileyk |last6=El Mellah |date=March 10, 2014 |doi=10.1088/0004-637X/787/1/47 |volume=787 |issue=1 |journal=The Astrophysical Journal |page=47 |bibcode=2014ApJ...787...47S|s2cid=14380222 }}</ref>

On April 17, the Kepler team announced the discovery of [[Astronomy:Kepler-186f|Kepler-186f]], the first nearly Earth-sized planet located in the habitable zone. This planet orbits around a red dwarf.<ref name="NASA-9376058">{{cite web |url=http://www.nasa.gov/ames/kepler/nasas-kepler-discovers-first-earth-size-planet-in-the-habitable-zone-of-another-star/index.html |title=NASA's Kepler Discovers First Earth-Size Planet In The 'Habitable Zone' of Another Star |date=April 17, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=April 26, 2014 |editor=Jessica Culler |others=NASA Official: Brian Dunbar; Image credit(s): 2xNASA Ames/SETI Institute/JPL-Caltech; NASA Ames |first=Jessica |last=Culler |archive-url=https://web.archive.org/web/20140426070848/http://www.nasa.gov/ames/kepler/nasas-kepler-discovers-first-earth-size-planet-in-the-habitable-zone-of-another-star/index.html |archive-date=April 26, 2014 |url-status=live }}</ref>

In May 2014, K2 observations fields 0 to 13 were announced and described in detail.<ref name="NASA-20140529-all">{{cite web |last1=Barclay |first1=Thomas |last2=Dotson |first2=Jessie |title=K2 Campaign Fields – 0 to 13 |url=http://keplerscience.arc.nasa.gov/K2/Fields.shtml |archive-url=https://web.archive.org/web/20140426094927/http://keplerscience.arc.nasa.gov/K2/Fields.shtml |url-status=dead |archive-date=April 26, 2014 |date=May 29, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=April 4, 2015}}</ref> K2 observations began in June 2014.

In July 2014, the first discoveries from K2 field data were reported in the form of eclipsing binaries. Discoveries were derived from a Kepler engineering data set which was collected prior to campaign 0<ref name="NASA-20140529-00">{{cite web |last1=Barclay |first1=Thomas |last2=Dotson |first2=Jessie |title=K2 Campaign 0 (March 8, 2014 – May 30, 2014) |url=http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC00.shtml |archive-url=https://web.archive.org/web/20140801130152/http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC00.shtml |url-status=dead |archive-date=August 1, 2014 |date=May 29, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=April 4, 2015}}</ref> in preparation to the main ''K2'' mission.<ref>{{cite journal |title=Kepler Eclipsing Binary Stars. V. Identification of 31 Candidate Eclipsing Binaries in the K2 Engineering Dataset |journal=Publications of the Astronomical Society of the Pacific |first1=Kyle E. |last1=Conroy |first2=Andrej |last2=Prša |first3=Keivan G. |last3=Stassun |first4=Steven |last4=Bloemen |first5=Mahmoud |last5=Parvizi |first6=Billy |last6=Quarles |first7=Tabetha |last7=Boyajian |first8=Thomas |last8=Barclay |first9=Avi |last9=Shporer |first10=David W. |last10=Latham |first11=Michael |last11=Abdul-Masih |display-authors=5 |volume=126 |issue=944 |pages=914–922 |date=October 2014 |doi=10.1086/678953 |bibcode=2014PASP..126..914C |arxiv=1407.3780|s2cid=8232628 }}</ref>

On September 23, 2014, NASA reported that the ''K2'' mission had completed campaign 1,<ref name="NASA-20140529-01">{{cite web |last1=Barclay |first1=Thomas |last2=Dotson |first2=Jessie |title=K2 Campaign 1 (May 30, 2014 – August 21, 2014) |url=http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC01.shtml |archive-url=https://web.archive.org/web/20140801104443/http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC01.shtml |url-status=dead |archive-date=August 1, 2014 |date=May 29, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=April 4, 2015}}</ref> the first official set of science observations, and that campaign 2<ref name="NASA-20140529-02">{{cite web |last1=Barclay |first1=Thomas |last2=Dotson |first2=Jessie |title=K2 Campaign 2 (August 22, 2014 – November 11, 2014) |url=http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC02.shtml |archive-url=https://web.archive.org/web/20141105054749/http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC02.shtml |url-status=dead |archive-date=November 5, 2014 |date=May 29, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=April 4, 2015}}</ref> was underway.<ref name="NASA-20140923">{{cite web |last=Sobeck |first=Charlie |title=Mission Manager Update: C1 data on the ground; C2 underway |url=http://www.nasa.gov/ames/kepler/mission-manager-update-c1-data-on-the-ground-c2-underway |date=September 23, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=September 23, 2014}}</ref>

[[File:KeplerSpaceTelescope-SupernovaKSN2011b-20150520.png|thumb|Kepler observed KSN 2011b, a [[Astronomy:Type Ia supernova|Type Ia supernova]], in the process of exploding: before, during and after.<ref name="NASA-20150520" />]]

Campaign 3<ref name="NASA-20140529-03">{{cite web |last1=Barclay |first1=Thomas |last2=Dotson |first2=Jessie |title=K2 Campaign 3 (November 14, 2014 – February 6, 2014) |url=http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC03.shtml |archive-url=https://web.archive.org/web/20150102121432/http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC03.shtml |url-status=dead |archive-date=January 2, 2015 |date=May 29, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=April 4, 2015}}</ref> lasted from November 14, 2014, to February 6, 2015, and included "16,375 standard long cadence and 55 standard short cadence targets".<ref name="NASA-20140529-all" />

===2015===
{{prose|section|date=March 2023}}
* In January 2015, the number of confirmed Kepler planets exceeded 1000. At least two ([[Astronomy:Kepler-438b|Kepler-438b]] and [[Astronomy:Kepler-442b|Kepler-442b]]) of the discovered planets announced that month were likely [[Astronomy:Terrestrial planet|rocky]] and in the [[Astronomy:Habitable zone|habitable zone]].<ref name="NASA-20150106" /> Also in January 2015, NASA reported that five confirmed sub-earth-sized rocky [[Astronomy:Exoplanet|exoplanet]]s, all smaller than the planet [[Astronomy:Venus|Venus]], were found orbiting the 11.2 billion year old star [[Astronomy:Kepler-444|Kepler-444]], making this star system, at 80% of the age of the [[Astronomy:Universe|universe]], the oldest yet discovered.<ref name="ARX-20150126">{{cite journal |title=An Ancient Extrasolar System with Five Sub-Earth-size Planets |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |first1=T. L. |last1=Campante |first2=T. |last2=Barclay |first3=J. J. |last3=Swift |first4=D. |last4=Huber |first5=V. Zh. |last5=Adibekyan |first6=W. |last6=Cochran |first7=C. J. |last7=Burke |first8=H. |last8=Isaacson |first9=E. V. |last9=Quintana |first10=G. R. |last10=Davies |first11=V. |last11=Silva Aguirre |first12=D. |last12=Ragozzine |first13=R. |last13=Riddle |first14=C. |last14=Baranec |first15=S. |last15=Basu |first16=W. J. |last16=Chaplin |first17=J. |last17=Christensen-Dalsgaard |first18=T. S. |last18=Metcalfe |first19=T. R. |last19=Bedding |first20=R. |last20=Handberg |first21=D. |last21=Stello |first22=J. M. |last22=Brewer |first23=S. |last23=Hekker |first24=C. |last24=Karoff |first25=R. |last25=Kolbl |first26=N. M. |last26=Law |first27=M. |last27=Lundkvist |first28=A. |last28=Miglio |first29=J. F. |last29=Rowe |first30=N. C. |last30=Santos |first31=C. |last31=Van Laerhoven |first32=T. |last32=Arentoft |first33=Y. P. |last33=Elsworth |first34=D. A. |last34=Fischer |first35=S. D. |last35=Kawaler |first36=H. |last36=Kjeldsen |first37=M. N. |last37=Lund |first38=G. W. |last38=Marcy |first39=S. G. |last39=Sousa |first40=A. |last40=Sozzetti |first41=T. R. |last41=White |display-authors=5 |volume=799 |issue=2 |at=article 170 |date=February 2015 |doi=10.1088/0004-637X/799/2/170 |bibcode=2015ApJ...799..170C |arxiv=1501.06227|s2cid=5404044 }}</ref><ref name="AP-20150127">{{cite news |url=http://apnews.excite.com/article/20150127/us-sci--old_solar_system-56b7594709.html |title=Astronomers find solar system more than double ours in age |work=Excite.com |agency=Associated Press |first=Marcia |last=Dunn |date=January 27, 2015 |access-date=January 27, 2015}}</ref><ref name="UT-20150127">{{cite web |url=http://www.universetoday.com/118510/oldest-planetary-system-discovered-improving-the-chances-for-intelligent-life-everywhere/ |title=Oldest Planetary System Discovered, Improving the Chances for Intelligent Life Everywhere |work=Universe Today |first1=Nancy |last1=Atkinson |date=January 27, 2015 |access-date=January 27, 2015}}</ref>
* In April 2015, campaign 4<ref name="NASA-20140529-04">{{cite web |url=http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC04.shtml |archive-url=https://web.archive.org/web/20150224214845/http://keplerscience.arc.nasa.gov/K2/GuestInvestigationsC04.shtml |url-status=dead |archive-date=February 24, 2015 |title=K2 Campaign 4 (February 7, 2015 – April 24, 2015) |publisher=NASA |last1=Barclay |first1=Thomas |last2=Dotson |first2=Jessie |date=May 29, 2014 |access-date=April 4, 2015}}</ref> was reported to last between February 7, 2015, and April 24, 2015, and to include observations of nearly 16,000 target stars and two notable open star clusters, Pleiades and Hyades.<ref name="NASA-20150402">{{cite web |url=http://www.nasa.gov/ames/kepler/mission-manager-update-k2-in-campaign-4/ |title=Mission Manager Update: K2 in Campaign 4 |publisher=NASA |last1=Sobeck |first1=Charlie |last2=Johnson |first2=Michele |last3=Dunbar |first3=Brian |date=April 2, 2015 |access-date=April 4, 2015}}</ref>
* In May 2015, Kepler observed a newly discovered [[Astronomy:Supernova|supernova]], KSN 2011b ([[Astronomy:Type Ia supernova|Type 1a]]), before, during and after explosion. Details of the pre-nova moments may help scientists better understand [[Astronomy:Dark energy|dark energy]].<ref name="NASA-20150520">{{cite web |url=http://www.nasa.gov/ames/kepler/nasa-spacecraft-capture-rare-early-moments-of-baby-supernovae |title=NASA Spacecraft Capture Rare, Early Moments of Baby Supernovae |publisher=NASA |last1=Johnson |first1=Michele |last2=Chandler |first2=Lynn |date=May 20, 2015 |access-date=May 21, 2015}}</ref>
* On July 24, 2015, NASA announced the discovery of [[Astronomy:Kepler-452b|Kepler-452b]], a confirmed exoplanet that is near-Earth in size and found orbiting the habitable zone of a Sun-like star.<ref name="NASA-20150723">{{cite press release |url=http://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth |title=NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth |publisher=NASA |last1=Chou |first1=Felicia |last2=Johnson |first2=Michele |date=July 23, 2015 |access-date=July 23, 2015}}</ref><ref name="Jenkins2015">{{cite journal |title=Discovery and Validation of Kepler-452b: A 1.6 R⨁ Super Earth Exoplanet in the Habitable Zone of a G2 Star |journal=The Astronomical Journal |last1=Jenkins |first1=Jon M. |last2=Twicken |first2=Joseph D. |last3=Batalha |first3=Natalie M. |last4=Caldwell |first4=Douglas A. |last5=Cochran |first5=William D. |last6=Endl |first6=Michael |last7=Latham |first7=David W. |last8=Esquerdo |first8=Gilbert A. |last9=Seader |first9=Shawn|last10=Bieryla|first10=Allyson |last11=Petigura |first11=Erik |last12=Ciardi |first12=David R. |last13=Marcy |first13=Geoffrey W. |last14=Isaacson |first14=Howard |last15=Huber |first15=Daniel |last16=Rowe |first16=Jason F. |last17=Torres |first17=Guillermo |last18=Bryson |first18=Stephen T. |last19=Buchhave |first19=Lars |last20=Ramirez |first20=Ivan |last21=Wolfgang |first21=Angie |last22=Li |first22=Jie |last23=Campbell |first23=Jennifer R. |last24=Tenenbaum |first24=Peter |last25=Sanderfer |first25=Dwight |last26=Henze |first26=Christopher E. |last27=Catanzarite |first27=Joseph H. |last28=Gilliland |first28=Ronald L. |last29=Borucki |first29=William J. |display-authors=5 |volume=150 |issue=2 |page=56 |date=July 2015 |doi=10.1088/0004-6256/150/2/56 |bibcode=2015AJ....150...56J |arxiv=1507.06723|s2cid=26447864 }}</ref> The seventh Kepler planet candidate catalog was released, containing 4,696 candidates, and increase of 521 candidates since the previous catalog release in January 2015.<ref>{{cite news |url=https://www.nytimes.com/2015/07/24/science/space/kepler-data-reveals-what-might-be-best-goldilocks-planet-yet.html |title=NASA Says Data Reveals an Earth-Like Planet, Kepler 452b |work=The New York Times |first=Dennis |last=Overbye |date=July 23, 2015 |access-date=July 24, 2015}}</ref><ref>{{cite web |url=https://www.nasa.gov/ames/kepler/kepler-planet-candidates-july-2015 |title=Kepler Planet Candidates, July 2015 |publisher=NASA |first=Michele |last=Johnson |date=July 23, 2015 |access-date=July 24, 2015}}</ref>
* On September 14, 2015, astronomers reported unusual light fluctuations of [[Astronomy:KIC 8462852|KIC 8462852]], an [[Astronomy:F-type main-sequence star|F-type main-sequence star]] in the [[Astronomy:Constellation|constellation]] [[Astronomy:Cygnus (constellation)|Cygnus]], as detected by Kepler, while searching for [[Astronomy:Exoplanet|exoplanet]]s. Various hypotheses have been presented, including [[Astronomy:Comet|comet]]s, [[Astronomy:Asteroid|asteroid]]s, and an [[Philosophy:Extraterrestrial intelligence|alien civilization]].<ref name="WP-20151015">{{cite news |last=Kaplan |first=Sarah |title=The strange star that has serious scientists talking about an alien megastructure |url=https://www.washingtonpost.com/news/morning-mix/wp/2015/10/15/the-strange-star-that-has-serious-scientists-talking-about-an-alien-megastructure/ |newspaper={{wipe|The Washington Post}} |date=October 15, 2015 |access-date=October 15, 2015 |issn=0190-8286}}</ref><ref name="ATL-20151013">{{cite web |last1=Andersen |first1=Ross |title=The Most Mysterious Star in Our Galaxy |url=https://www.theatlantic.com/science/archive/2015/10/the-most-interesting-star-in-our-galaxy/410023/ |date=October 13, 2015 |work=The Atlantic |access-date=October 13, 2015}}</ref><ref name="MNRAS-20150911">{{cite journal |title=Planet Hunters IX. KIC 8462852 – Where's the flux? |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |last1=Boyajian |first1=T. S. |last2=LaCourse |first2=D. M. |last3=Rappaport |first3=S. A. |last4=Fabrycky |first4=D. |last5=Fischer |first5=D. A. |last6=Gandolfi |first6=D. |last7=Kennedy |first7=G. M. |last8=Liu |first8=M. C. |last9=Moor |first9=A. |last10=Olah |first10=K. |last11=Vida |first11=K. |last12=Wyatt |first12=M. C. |last13=Best |first13=W. M. J. |last14=Ciesla |first14=F. |last15=Csak |first15=B. |last16=Dupuy |first16=T. J. |last17=Handler |first17=G. |last18=Heng |first18=K. |last19=Korhonen |first19=H. |last20=Kovacs |first20=J. |last21=Kozakis |first21=T. |last22=Kriskovics |first22=L. |last23=Schmitt |first23=J. R. |last24=Szabo |first24=Gy. |last25=Szabo |first25=R. |last26=Wang |first26=J. |last27=Goodman |first27=S. |last28=Hoekstra |first28=A. |last29=Jek |first29=K. J. |display-authors=5 |volume=457 |issue=4 |pages=3988–4004 |date=April 2016 |doi=10.1093/mnras/stw218 |bibcode=2016MNRAS.457.3988B |arxiv=1509.03622|s2cid=54859232 }}</ref>

===2016===
By May 10, 2016, the Kepler mission had verified 1,284 new planets.<ref name="NASA-20160510a"/>  Based on their size, about 550 could be rocky planets. Nine of these orbit in their stars' [[Astronomy:Habitable zone|habitable zone]]: [[Astronomy:Kepler-560b|Kepler-560b]], [[Astronomy:Kepler-705b|Kepler-705b]], [[Astronomy:Kepler-1229b|Kepler-1229b]], Kepler-1410b, Kepler-1455b, [[Astronomy:Kepler-1544 b|Kepler-1544 b]], Kepler-1593b, [[Astronomy:Kepler-1606b|Kepler-1606b]], and [[Astronomy:Kepler-1638b|Kepler-1638b]].<ref name="NASA-20160510a"/>

==Mission status==
Kepler was launched in 2009. It was very successful at finding exoplanets, but failures in two of four reaction wheels crippled its extended mission in 2013. Without three functioning wheels, the telescope could not be pointed accurately.  On October 30, 2018, NASA announced that the spacecraft was out of fuel and its mission was officially ended.<ref>{{Cite web | url=https://www.theverge.com/2018/10/30/18044152/nasa-kepler-space-telescope-out-of-fuel-dead |title = The Kepler Space Telescope is dead|date = October 30, 2018 |work=The Verge |first=Mary Beth |last=Griggs}}</ref>

===Extension===
[[File:Neighborhood21.gif|thumb|Predicted structure of the Milky Way overlaid with the original Kepler search space.<ref name="QuickGuide" />]]

In April 2012, an independent panel of senior NASA scientists recommended that the Kepler mission be continued through 2016. According to the senior review, Kepler observations needed to continue until at least 2015 to achieve all the stated scientific goals.<ref>{{cite web |last1=Clark |first1=Stephen |url=http://www.spaceflightnow.com/news/n1204/04kepler/ |title=Kepler planet-hunting mission extended until 2016 |date=April 4, 2012 |work=Spaceflight Now |access-date=April 4, 2012}}</ref> On November 14, 2012, NASA announced the completion of Kepler's primary mission, and the beginning of its extended mission, which ended in 2018 when it ran out of fuel.<ref>{{cite web |title=Release : 12–394 – NASA's Kepler Completes Prime Mission, Begins Extended Mission |url=http://www.nasa.gov/home/hqnews/2012/nov/HQ_12-394_Kepler_Completes_Prime_Mission.html |publisher=NASA |access-date=November 17, 2012}}</ref>

===Reaction wheel issues===
In July 2012, one of Kepler's four [[Engineering:Reaction wheel|reaction wheel]]s (wheel 2) failed.<ref name="NASA-20130815" /> On May 11, 2013, a second wheel (wheel 4) failed, jeopardizing the continuation of the mission, as three wheels are necessary for its planet hunting.<ref name=prs/><ref name="nytimes.com"/> Kepler had not collected science data since May because it was not able to point with sufficient accuracy.<ref name=kep/> On July 18 and 22 reaction wheels 4 and 2 were tested respectively; wheel 4 only rotated counter-clockwise but wheel 2 ran in both directions, albeit with significantly elevated friction levels.<ref>{{cite web |title=Kepler Mission Manager Update: Initial Recovery Tests |url=http://www.nasa.gov/content/kepler-mission-manager-update-initial-recovery-tests/ |publisher=[[Organization:NASA|NASA]] |access-date=September 9, 2013 |date=July 24, 2013}}</ref>  A further test of wheel 4 on July 25 managed to achieve bi-directional rotation.<ref name="keprecoverytestNASA">{{cite web |url=http://www.nasa.gov/content/kepler-mission-manager-update-pointing-test |title=Kepler Mission Manager Update: Pointing Test |date=August 2, 2013 |publisher=[[Organization:NASA|NASA]] |access-date=August 3, 2013}}</ref> Both wheels, however, exhibited too much friction to be useful.<ref name="SP-20130815" /> On August 2, NASA put out a call for proposals to use the remaining capabilities of Kepler for other scientific missions.  Starting on August 8, a full systems evaluation was conducted.  It was determined that wheel 2 could not provide sufficient precision for scientific missions and the spacecraft was returned to a "rest" state to conserve fuel.<ref name="NASA-20130815" />   Wheel 4 was previously ruled out because it exhibited higher friction levels than wheel 2 in previous tests.<ref name=keprecoverytestNASA/> Sending astronauts to fix Kepler is not an option because it orbits the Sun and is millions of kilometers from Earth.<ref name="SP-20130815" />

On August 15, 2013, NASA announced that Kepler would not continue searching for planets using the transit method after attempts to resolve issues with two of the four reaction wheels failed.<ref name="NASA-20130815" /><ref name="NYT-20130815" /><ref name="SP-20130815" /> An engineering report was ordered to assess the spacecraft's capabilities, its two good reaction wheels and its thrusters.<ref name="NASA-20130815" /> Concurrently, a scientific study was conducted to determine whether enough knowledge can be obtained from Kepler's limited scope to justify its $18 million per year cost.

Possible ideas included searching for asteroids and comets, looking for evidence of supernovas, and finding huge exoplanets through [[Physics:Gravitational microlensing|gravitational microlensing]].<ref name="SP-20130815" />  Another proposal was to modify the software on Kepler to compensate for the disabled reaction wheels. Instead of the stars being fixed and stable in Kepler's field of view, they will drift. However, proposed software was to track this drift and more or less completely recover the mission goals despite being unable to hold the stars in a fixed view.<ref>{{cite arXiv |title=KeSeF - Kepler Self Follow-up Mission |eprint=1308.2252 |class=astro-ph.EP |first=Aviv |last=Ofir |date=August 9, 2013}}</ref>

Previously collected data continues to be analyzed.<ref name="kep2013">{{cite web |url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20130607.html |title=Kepler Mission Manager Update |date=June 7, 2013 |publisher=NASA |access-date=June 14, 2013}}</ref>

==={{anchor|K2}} Second Light (K2)===
In November 2013, a new mission plan named K2 "Second Light" was presented for consideration.<ref name="NASA-20131125">{{cite web |last1=Johnson |first1=Michele |editor1-last=Johnson |editor1-first=Michele |title=A Sunny Outlook for NASA Kepler's Second Light |url=http://www.nasa.gov/kepler/a-sunny-outlook-for-nasa-keplers-second-light |date=November 25, 2013 |publisher=[[Organization:NASA|NASA]] |access-date=December 12, 2013 |archive-url=https://web.archive.org/web/20140418000640/http://www.nasa.gov/kepler/a-sunny-outlook-for-nasa-keplers-second-light/ |archive-date=April 18, 2014 |url-status=live |others=NASA Official: Brian Dunbar; Image credits: NASA Ames; NASA Ames/W Stenzel }}</ref><ref name="NASA-20131211a">{{cite web |last1=Johnson |first1=Michele |editor1-last=Johnson |editor1-first=Michele |title=Kepler's Second Light: How K2 Will Work |publisher=[[Organization:NASA|NASA]] |date=December 11, 2013 |url=http://www.nasa.gov/kepler/keplers-second-light-how-k2-will-work/ |access-date=December 12, 2013 |others=NASA Official: Brian Dunbar; Image credit: NASA Ames/W Stenzel |archive-url=https://web.archive.org/web/20140418001138/http://www.nasa.gov/kepler/keplers-second-light-how-k2-will-work/ |archive-date=April 18, 2014 |url-status=live }}</ref><ref name="NASA-20131211b">{{cite web |first=Roger |last=Hunter |editor1-last=Johnson |editor1-first=Michele |title=Kepler Mission Manager Update: Invited to 2014 Senior Review |publisher=[[Organization:NASA|NASA]] |date=December 11, 2013 |url=http://www.nasa.gov/kepler/kepler-mission-manager-update-invited-to-2014-senior-review |access-date=December 12, 2013 |others=NASA Official: Brian Dunbar |archive-url=https://web.archive.org/web/20140418001405/http://www.nasa.gov/kepler/kepler-mission-manager-update-invited-to-2014-senior-review/ |archive-date=April 18, 2014 |url-status=live }}</ref><ref name=wall>{{cite web |last=Wall |first=Mike |title=NASA's Hobbled Planet-Hunting Spacecraft May Resume Search for Alien Worlds |work=Space.com |publisher=TechMediaNetwork |date=November 5, 2013 |url=http://www.space.com/23465-planet-hunting-kepler-spacecraft-future.html |access-date=April 17, 2014 |others=Image credit: NASA |archive-url=https://web.archive.org/web/20140418002147/http://www.space.com/23465-planet-hunting-kepler-spacecraft-future.html |archive-date=April 18, 2014 |url-status=live }}</ref> K2 would involve using Kepler's remaining capability, [[Astronomy:Photometry|photometric precision]] of about 300 parts per million, compared with about 20 parts per million earlier, to collect data for the study of "[[Astronomy:Supernova|supernova explosions]], [[Astronomy:Star formation|star formation]] and [[Astronomy:Solar System|Solar-System bodies]] such as [[Astronomy:Asteroid|asteroid]]s and [[Astronomy:Comet|comet]]s, ... " and for finding and studying more [[Astronomy:Exoplanet|exoplanet]]s.<ref name="NASA-20131125" /><ref name="NASA-20131211a" /><ref name=wall/> In this proposed mission plan, Kepler would search a much larger area in the plane of [[Astronomy:Earth's orbit|Earth's orbit]] around the [[Astronomy:Sun|Sun]].<ref name="NASA-20131125" /><ref name="NASA-20131211a" /><ref name=wall/> Celestial objects, including exoplanets, stars and others, detected by the K2 mission would be associated with the  [[Astronomy:Ecliptic Plane Input Catalog|EPIC]] [[Astronomy:List of astronomy acronyms|acronym]], standing for [[Astronomy:Ecliptic Plane Input Catalog|Ecliptic Plane Input Catalog]].
{{wide image|Kepler-K2-MissionTimeline-20140808.png|800px|align-cap=center|K2 mission timeline (August 8, 2014).<ref name="NASA-20140808">{{cite web |title=Kepler Mission Manager Update: K2 collecting data |url=http://www.nasa.gov/ames/kepler/kepler-mission-manager-update-k2-collecting-data/ |date=August 8, 2014 |publisher=[[Organization:NASA|NASA]] |access-date=August 9, 2014}}</ref>}}

In early 2014, the spacecraft underwent successful testing for the K2 mission.<ref name="C4WDefault-NASAkepler2014-02-14">{{cite web |url=http://www.nasa.gov/ames/kepler/kepler-mission-manager-update-k2-spacecraft-operation-tests-continue/ |title=Kepler Mission Manager Update: K2 spacecraft operation tests continue |publisher=[[Organization:NASA|NASA]] |date=February 14, 2014 |access-date=April 17, 2014 |first=Roger |last=Hunter |editor1-last=Johnson |editor1-first=Michele |others=NASA Official: Brian Dunbar; Image Credit: NASA Ames/T. Barclay |archive-url=https://web.archive.org/web/20140418002638/http://www.nasa.gov/ames/kepler/kepler-mission-manager-update-k2-spacecraft-operation-tests-continue/ |archive-date=April 18, 2014 |url-status=live }}</ref> From March to May 2014, data from a new field called Field 0 was collected as a testing run.<ref name="arXiv-1404.4417v1">{{cite journal |title=HAT-P-54b: A hot jupiter transiting a 0.64 Msun star in field 0 of the K2 mission |arxiv=1404.4417 |first1=G. Á. |last1=Bakos |first2=J. D. |last2=Hartman |first3=W. |last3=Bhatti |first4=A. |last4=Bieryla |first5=M. |last5=de Val-Borro |first6=D. W. |last6=Latham |first7=L. A. |last7=Buchhave |first8=Z. |last8=Csubry |first9=K. |last9=Penev |first10=G. |last10=Kovács |first11=B. |last11=Béky |first12=E. |last12=Falco |first13=T. |last13=Kovács |first14=A. W. |last14=Howard |first15=J. A. |last15=Johnson |first16=H. |last16=Isaacson |first17=G. W. |last17=Marcy |first18=G. |last18=Torres |first19=R. W. |last19=Noyes |first20=P. |last20=Berlind |first21=M. L. |last21=Calkins |first22=G. A. |last22=Esquerdo |first23=J. |last23=Lázár |first24=I. |last24=Papp |first25=P. |last25=Sári |display-authors=5 |date=April 17, 2014 |doi=10.1088/0004-6256/149/4/149 |volume=149 |issue=4 |journal=The Astronomical Journal |page=149 |bibcode=2015AJ....149..149B|s2cid=119239193 }}</ref> On May 16, 2014, NASA announced the approval of extending the Kepler mission to the K2 mission.<ref name="NASA-20140516" />  Kepler's photometric precision for the K2 mission was estimated to be 50&nbsp;ppm on a magnitude 12 star for a 6.5-hour integration.<ref name="C4WDefault-2446741">{{cite web |url=http://keplerscience.arc.nasa.gov/K2/ |title=Kepler Guest Observer Program |publisher=NASA |date=May 29, 2014 |access-date=June 12, 2014 |editor1-first=Martin |editor1-last=Still |others=NASA Official: Jessie Dotson |archive-url=https://web.archive.org/web/20140613045308/http://keplerscience.arc.nasa.gov/K2/ |archive-date=June 13, 2014 |url-status=dead }}</ref> In February 2014, photometric precision for the K2 mission using two-wheel, fine-point precision operations was measured as 44&nbsp;ppm on magnitude 12 stars for a 6.5-hour integration. The analysis of these measurements by NASA suggests the K2 photometric precision approaches that of the Kepler archive of three-wheel, fine-point precision data.<ref name="C4WDefault-9014554">{{cite web |url=http://keplerscience.arc.nasa.gov/K2/Performance.shtml |title=K2 Performance |publisher=NASA |date=May 29, 2014 |access-date=June 12, 2014 |editor1-first=Martin |editor1-last=Still |others=NASA Official: Jessie Dotson |archive-url=https://web.archive.org/web/20140613045540/http://keplerscience.arc.nasa.gov/K2/Performance.shtml |archive-date=June 13, 2014 |url-status=dead }}</ref>

On May 29, 2014, campaign fields 0 to 13 were reported and described in detail.<ref name="NASA-20140529-all" />
[[File:NASA-KeplerSecondLight-K2-Explained-20131211.jpg|thumb|right|350px|K2 proposal explained (December 11, 2013).<ref name="NASA-20131211a" />]]
Field 1 of the K2 mission is set towards the [[Astronomy:Leo (constellation)|Leo]]-[[Astronomy:Virgo (constellation)|Virgo]] region of the sky, while Field 2 is towards the "head" area of [[Astronomy:Scorpius|Scorpius]] and includes two globular clusters, [[Astronomy:Messier 4|Messier 4]] and [[Astronomy:Messier 80|Messier 80]],<ref name="arXiv-1405.7690v1">{{Cite journal |title=Cepheids and RR Lyrae Stars in the K2 Fields |journal=Information Bulletin on Variable Stars |first1=L. |last1=Molnár |first2=E. |last2=Plachy |first3=R. |last3=Szabó |volume=6108 |issue=1 |pages=1 |date=May 29, 2014 |bibcode=2014IBVS.6108....1M |arxiv=1405.7690}}</ref> and part of the [[Astronomy:Scorpius–Centaurus association|Scorpius–Centaurus association]], which is only about 11 million years old<ref name="Pecaut">{{Cite journal |first1=Mark J. |last1=Pecaut |first2=Eric E. |last2=Mamajek |first3=Eric J. |last3=Bubar |date=February 2012 |title=A Revised Age for Upper Scorpius and the Star Formation History among the F-type Members of the Scorpius-Centaurus OB Association |journal=Astrophysical Journal |volume=746 |issue=2 |pages=154 |arxiv=1112.1695 |bibcode=2012ApJ...746..154P |doi=10.1088/0004-637X/746/2/154|s2cid=118461108 }}</ref> and {{convert|380|-|470|ly|pc|order=flip|lk=on}} distant<ref>{{cite journal |title=A Hipparcos Census of Nearby OB Associations |first1=P. T. |last1=de Zeeuw |first2=R. |last2=Hoogerwerf |first3=J. H. J. |last3=de Bruijne |first4=A. G. A. |last4=Brown |first5=A. |last5=Blaauw |date=1999 |journal=Astronomical Journal |volume=117 |issue=1 |pages=354–399 |bibcode=1999AJ....117..354D |doi=10.1086/300682 |arxiv=astro-ph/9809227 |s2cid=16098861 }}</ref> with probably over 1,000 members.<ref>{{cite journal |bibcode=2002AJ....124.1670M |first1=E. E. |last1=Mamajek |first2=M. R. |last2=Meyer |first3=James |last3=Liebert |date=2002 |journal=Astronomical Journal |volume=124 |issue=3 |pages=1670–1694 |title=Post-T Tauri Stars in the Nearest OB Association |doi=10.1086/341952 |arxiv=astro-ph/0205417|s2cid=16855894 }}</ref>

On December 18, 2014, NASA announced that the K2 mission had detected its first confirmed exoplanet, a [[Astronomy:Super-Earth|super-Earth]] named [[Astronomy:HIP 116454 b|HIP 116454 b]]. Its signature was found in a set of engineering data meant to prepare the spacecraft for the full ''K2'' mission. Radial velocity follow-up observations were needed as only a single transit of the planet was detected.<ref name="NASA-20141218-FC">{{cite web |last1=Chou |first1=Felicia |last2=Johnson |first2=Michele |title=NASA's Kepler Reborn, Makes First Exoplanet Find of New Mission |url=http://www.nasa.gov/press/2014/december/nasa-s-kepler-reborn-makes-first-exoplanet-find-of-new-mission/ |publisher=[[Organization:NASA|NASA]] |date=December 18, 2014 |access-date=December 19, 2014 |id=Release 14-335}}</ref>

During a scheduled contact on April 7, 2016, Kepler was found to be operating in emergency mode, the lowest operational and most fuel intensive mode. Mission operations declared a spacecraft emergency, which afforded them priority access to NASA's Deep Space Network.<ref name="EmMode20160411">{{cite web |url=http://www.nasa.gov/feature/mission-manager-update-kepler-recovered-from-emergency-and-stable |title=Mission Manager Update: Kepler Recovered from Emergency and Stable |publisher=NASA |first=Charlie |last=Sobeck |date=April 11, 2016 |access-date=April 14, 2016}}</ref><ref name="nature20160410">{{cite news |url=http://www.nature.com/news/kepler-spacecraft-in-emergency-mode-1.19720 |title=Kepler spacecraft in emergency mode |work=Nature |first=Alexandra |last=Witze |date=April 10, 2016 |access-date=April 14, 2016}}</ref> By the evening of April 8 the spacecraft had been upgraded to safe mode, and on April 10 it was placed into point-rest state,<ref name="latimes20160411">{{cite news |url=http://www.latimes.com/science/sciencenow/la-sci-sn-nasa-kepler-emergency-safe-20160411-story.html |title=NASA Kepler spacecraft recovers from emergency mode, but what triggered it? |work={{wipe|Los Angeles Times}} |first=Amina |last=Khan |date=April 11, 2016 |access-date=April 14, 2016}}</ref> a stable mode which provides normal communication and the lowest fuel burn.<ref name="EmMode20160411" /> At that time, the cause of the emergency was unknown, but it was not believed that Kepler's reaction wheels or a planned maneuver to support ''K2''{{'s}} Campaign 9 were responsible. Operators downloaded and analyzed engineering data from the spacecraft, with the prioritization of returning to normal science operations.<ref name="EmMode20160411" /><ref name="sfnow20160411">{{cite news |url=http://spaceflightnow.com/2016/04/11/kepler-telescope-recovered-from-spacecraft-emergency/ |title=Kepler telescope recovered from spacecraft emergency |work=Spaceflight Now |first=Stephen |last=Clark |date=April 11, 2016 |access-date=April 14, 2016}}</ref> Kepler was returned to science mode on April 22.<ref name="nasa20160503">{{cite web |url=https://www.nasa.gov/feature/ames/kepler/mission-manager-qa-recovering-the-kepler-spacecraft-to-hunt-for-exoplanets-again |title=Mission Manager Q&A: Recovering The Kepler Spacecraft To Hunt For Exoplanets Again |publisher=NASA |first1=Michele |last1=Johnson |first2=Charlie |last2=Sobeck |date=May 3, 2016 |access-date=August 25, 2016}}</ref> The emergency caused the first half of Campaign 9 to be shortened by two weeks.<ref name="nasa20160609">{{cite web |url=https://www.nasa.gov/feature/ames/kepler/mission-manager-update-k2-marches-on |title=Mission Manager Update: K2 Marches On |publisher=NASA |first1=Michele |last1=Johnson |first2=Charlie |last2=Sobeck |date=June 9, 2016 |access-date=August 25, 2016}}</ref>

In June 2016, NASA announced a K2 mission extension of three additional years, beyond the expected exhaustion of on-board fuel in 2018.<ref>{{cite web |url=http://keplerscience.arc.nasa.gov/k2-mission-officially-extended-through-end-of-mission.html |archive-url=https://web.archive.org/web/20160814044546/http://keplerscience.arc.nasa.gov/k2-mission-officially-extended-through-end-of-mission.html |url-status=dead |archive-date=August 14, 2016 |title=K2 mission officially extended through end of mission |publisher=NASA |first=Knicole |last=Colon |date=June 9, 2016 |access-date=August 25, 2016}}</ref> In August 2018, NASA roused the spacecraft from sleep mode, applied a modified configuration to deal with thruster problems that degraded pointing performance, and began collecting scientific data for the 19th observation campaign, finding that the onboard fuel was not yet utterly exhausted.<ref>{{cite web|url=https://www.nasa.gov/ames/kepler/kepler-spacecraft-updates|title=Kepler and K2|website=Kepler Spacecraft Updates|date=September 5, 2018|access-date=September 7, 2018}}</ref>

==Data releases==
The Kepler team originally promised to release data within one year of observations.<ref>{{cite web |url=http://kepler.nasa.gov/Mission/faq/#a9 |archive-url=https://web.archive.org/web/20100527093519/http://kepler.nasa.gov/Mission/faq/#a9 |url-status=dead |archive-date=May 27, 2010 |title=Frequently Asked Questions from the Public |access-date=September 6, 2011 |quote=Data for each 3-month observation period will be made public within one year of the end the observation period.}}</ref> However, this plan was changed after launch, with data being scheduled for release up to three years after its collection.<ref name="release">{{cite web |title=NASA's Kepler Mission Data Release Schedule |url=http://keplergo.arc.nasa.gov/ArchiveSchedule.shtml |archive-url=https://web.archive.org/web/20091016015816/http://keplergo.arc.nasa.gov/ArchiveSchedule.shtml |url-status=dead |archive-date=October 16, 2009 |publisher=[[Organization:NASA|NASA]] |access-date=October 18, 2011}} On this schedule, the data from the quarter ending June 2010 was scheduled to be released in June 2013.</ref> This resulted in considerable criticism,<ref>{{cite news |title=In the Hunt for Planets, Who Owns the Data? |first=Dennis |last=Overbye |url=https://www.nytimes.com/2010/06/15/science/space/15kepler.html |work={{wipe|The New York Times}} |date=June 14, 2010}}</ref><ref>{{cite journal |url=http://www.nature.com/news/2010/100414/full/news.2010.182.html |title=Telescope team may be allowed to sit on exoplanet data |first=Eric |last=Hand |journal=Nature |date=April 14, 2010|doi=10.1038/news.2010.182 |doi-access=free }}</ref><ref>{{cite news |url=http://www.skyandtelescope.com/astronomy-news/keplers-exoplanets-aprogress-report/ |title=Kepler's Exoplanets: A Progress Report |first=Alan |last=MacRobert |date=August 2011 |newspaper=Sky and Telescope}}</ref><ref>{{cite web |title=Minutes of the Kepler Users Panel |url=http://keplergo.arc.nasa.gov/docs/KUP_minutes_Mar2011.pdf |archive-url=https://web.archive.org/web/20111015194243/http://keplergo.arc.nasa.gov/docs/KUP_minutes_Mar2011.pdf |url-status=dead |archive-date=October 15, 2011 |date=March 28–29, 2011 |first=Alex |last=Brown}}</ref><ref>{{cite web |url=http://news.discovery.com/space/kepler-exoplanet-controversy-erupts.html |title=Kepler Exoplanet Controversy Erupts |first=Nicole |last=Gugliucci |date=June 15, 2010 |publisher=Discovery News |access-date=October 4, 2011 |archive-date=November 12, 2012 |archive-url=https://web.archive.org/web/20121112182620/http://news.discovery.com/space/kepler-exoplanet-controversy-erupts.html |url-status=dead }}</ref> leading the Kepler science team to release the third quarter of their data one year and nine months after collection.<ref>{{cite web |url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=145 |archive-url=https://web.archive.org/web/20111019083528/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=145 |url-status=dead |archive-date=October 19, 2011 |title=NASA's Kepler Mission Announces Next Data Release to Public Archive|date=March 31, 2015}}</ref> The data through September 2010 (quarters 4, 5, and 6) was made public in January 2012.<ref>{{cite web |url=http://keplergo.arc.nasa.gov/ArchiveSchedule.shtml |archive-url=https://web.archive.org/web/20091016015816/http://keplergo.arc.nasa.gov/ArchiveSchedule.shtml |url-status=dead |archive-date=October 16, 2009 |title=Kepler Data Collection and Archive Timeline |access-date=January 1, 2012}}</ref>

===Follow-ups by others===
Periodically, the Kepler team releases a list of candidates (Kepler Objects of Interest, or KOIs) to the public. Using this information, a team of astronomers collected [[Astronomy:Radial velocity|radial velocity]] data using the [[Astronomy:SOPHIE échelle spectrograph|SOPHIE échelle spectrograph]] to confirm the existence of the candidate KOI-428b in 2010, later named [[Astronomy:Kepler-40b|Kepler-40b]].<ref name="exoplanet1">{{cite journal |title=SOPHIE velocimetry of ''Kepler'' transit candidates. II. KOI-428b: A hot Jupiter transiting a subgiant F-star |journal=Astronomy & Astrophysics |first1=A. |last1=Santerne |first2=R. F. |last2=Diaz |first3=F. |last3=Bouchy |first4=M. |last4=Deleuil |first5=C. |last5=Moutou |first6=G. |last6=Hebrard |first7=A. |last7=Eggenberger |first8=D. |last8=Ehrenreich |first9=C. |last9=Gry |first10=S. |last10=Udry |display-authors=5 |volume=528 |at=A63 |date=April 2011 |doi=10.1051/0004-6361/201015764 |bibcode=2011A&A...528A..63S |arxiv=1101.0196|s2cid=119275985 }}</ref> In 2011, the same team confirmed candidate KOI-423b, later named [[Astronomy:Kepler-39b|Kepler-39b]].<ref name="exoplanet2">{{Cite journal |title=SOPHIE velocimetry of Kepler transit candidates. III. KOI-423b: an 18 M<sub>Jup</sub> transiting companion around an F7IV star |journal=Astronomy & Astrophysics |first1=F. |last1=Bouchy |first2=A. S. |last2=Bonomo |first3=A. |last3=Santerne |first4=C. |last4=Moutou |first5=M. |last5=Deleuil |first6=R. F. |last6=Díaz |first7=A. |last7=Eggenberger |first8=D. |last8=Ehrenreich |first9=C. |last9=Gry |first10=T. |last10=Guillot |first11=M. |last11=Havel |first12=G. |last12=Hébrard |first13=S. |last13=Udry |display-authors=5 |volume=533 |at=A83 |date=September 2011 |doi=10.1051/0004-6361/201117095 |bibcode=2011A&A...533A..83B |arxiv=1106.3225|s2cid=62836749 }}</ref>

===Citizen scientist participation===
Since December 2010, Kepler mission data has been used for the [[Astronomy:Planet Hunters|Planet Hunters]] project, which allows volunteers to look for transit events in the light curves of Kepler images to identify planets that computer [[Algorithm|algorithm]]s might miss.<ref>{{cite magazine |last1=Andrews |first1=Bill |title=Become a Planet Hunter! |url=http://cs.astronomy.com/asycs/blogs/astronomy/archive/2010/12/20/become-a-planet-hunter.aspx |magazine=[[Astronomy:Astronomy (magazine)|Astronomy]] |date=December 20, 2010 |access-date=April 24, 2011}}</ref> By June 2011, users had found sixty-nine potential candidates that were previously unrecognized by the Kepler mission team.<ref>{{cite web |title=Planetometer |url=http://www.planethunters.org/planetometer |publisher=[[Astronomy:Zooniverse|Zooniverse]] |access-date=June 15, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110721124511/http://www.planethunters.org/planetometer |archive-date=July 21, 2011 }}</ref> The team has plans to publicly credit amateurs who spot such planets.

In January 2012, the BBC program ''Stargazing Live'' aired a public appeal for volunteers to analyse Planethunters.org data for potential new exoplanets. This led two amateur astronomers—one in Peterborough, England—to discover a new [[Astronomy:Neptune|Neptune]]-sized exoplanet, to be named Threapleton Holmes B.<ref>{{cite news |url=https://www.telegraph.co.uk/news/science/space/9026837/Amateur-stargazers-discover-new-planet.html |archive-url=https://ghostarchive.org/archive/20220112/https://www.telegraph.co.uk/news/science/space/9026837/Amateur-stargazers-discover-new-planet.html |archive-date=January 12, 2022 |url-access=subscription |url-status=live |work=The Daily Telegraph |title=Amateur stargazers discover new planet |date=January 20, 2012 |access-date=January 20, 2012}}{{cbignore}}</ref> One hundred thousand other volunteers were also engaged in the search by late January, analyzing over one million Kepler images by early 2012.<ref>{{cite news |url=https://www.bbc.co.uk/news/science-environment-16612181 |title=Stargazing viewer in planet coup |work=BBC News |date=January 18, 2012 |access-date=January 19, 2012}}</ref> One such exoplanet, [[Astronomy:PH1b|PH1b]] (or Kepler-64b from its Kepler designation), was discovered in 2012. A second exoplanet, [[Astronomy:PH2b|PH2b]] (Kepler-86b) was discovered in 2013.

In April 2017, ABC ''Stargazing Live'', a variation of BBC ''Stargazing Live'', launched the Zooniverse project "Exoplanet Explorers". While Planethunters.org worked with archived data, Exoplanet Explorers used recently downlinked data from the K2 mission. On the first day of the project, 184 transit candidates were identified that passed simple tests. On the second day, the research team identified a star system, later named [[Astronomy:K2-138b|K2-138]], with a Sun-like star and four [[Astronomy:Super-Earth|super-Earth]]s in a tight orbit. In the end, volunteers helped to identify 90 exoplanet candidates.<ref>{{cite web |url=https://www.zooniverse.org/projects/ianc2/exoplanet-explorers/about/results |title=We Got One!!! |series=Exoplanet Explorers |work=Zooniverse.org |access-date=April 18, 2017}}</ref><ref>{{cite web |url=https://blog.zooniverse.org/2017/04/07/stargazing-live-2017-thank-you-all/ |title=Stargazing Live 2017: Thank you all! |work=Zooniverse.org |date=April 7, 2017 |access-date=April 18, 2017}}</ref> The [[Citizen science|citizen scientists]] that helped discover the new star system will be added as co-authors in the research paper when published.<ref>{{cite news |url=http://www.abc.net.au/news/2017-04-06/stargazing-live-four-planets-discovered-in-new-solar-system/8423142 |title=Stargazing Live viewers find four-planet solar system via crowd-sourcing project |publisher=ABC News |first=Daniel |last=Miller |date=April 6, 2017 |access-date=April 18, 2017}}</ref>

==Confirmed exoplanets==
[[File:KeplerExoplanets-NearEarthSize-HabitableZone-20150106.png|thumb|right|upright=1.5|Confirmed small exoplanets in habitable zones (Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b).<ref name="NASA-20150106" />]]

Exoplanets discovered using ''Kepler''{{'s}} data, but confirmed by outside researchers, include Kepler-39b,<ref name="exoplanet2"/> Kepler-40b,<ref name="exoplanet1"/> [[Astronomy:Kepler-41b|Kepler-41b]],<ref name="KOI-196b">{{cite web |first=Cyril |last=Dedieu |url=http://exoplanet.eu/star.php?st=KOI-196 |title=Star: KOI-196 |access-date=December 21, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120111063437/http://exoplanet.eu/star.php?st=KOI-196 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |archive-date=January 11, 2012 }}</ref> [[Astronomy:Kepler-43b|Kepler-43b]],<ref name="KOI-135b">{{cite web |url=http://exoplanet.eu/star.php?st=KOI-135 |title=Star: KOI-135 |access-date=December 21, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120101111049/http://exoplanet.eu/star.php?st=KOI-135 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |archive-date=January 1, 2012 }}</ref> [[Astronomy:Kepler-44b|Kepler-44b]],<ref name="KOI-204b">{{cite web |url=http://exoplanet.eu/star.php?st=KOI-204 |title=Star: KOI-204 |access-date=December 21, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120101104353/http://exoplanet.eu/star.php?st=KOI-204 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |archive-date=January 1, 2012 }}</ref> [[Astronomy:Kepler-45b|Kepler-45b]],<ref name="KOI-254b">{{cite web |url=http://exoplanet.eu/star.php?st=KOI-254 |title=Star: KOI-254 |access-date=December 21, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120119195842/http://exoplanet.eu/star.php?st=KOI-254 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |archive-date=January 19, 2012 }}</ref> as well as the planets orbiting [[Astronomy:Kepler-223|Kepler-223]]<ref name="KOI-730">{{cite web |url=http://exoplanet.eu/star.php?st=KOI-730 |title=Star: KOI-730 |access-date=December 21, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120616124736/http://exoplanet.eu/star.php?st=KOI-730 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |archive-date=June 16, 2012 }}</ref> and [[Astronomy:Kepler-42|Kepler-42]].<ref name="KOI-961">{{cite web |url=http://exoplanet.eu/star.php?st=KOI-961 |archive-url=https://web.archive.org/web/20120208054043/http://exoplanet.eu/star.php?st=KOI-961 |url-status=dead |archive-date=February 8, 2012 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |title=Star: KOI-961 |access-date=January 1, 2012}}</ref> The "KOI" acronym indicates that the star is a [[Astronomy:Kepler object of interest|''K''epler ''O''bject of ''I''nterest]].

==Kepler Input Catalog==
{{Main|Astronomy:Kepler Input Catalog}}

The [[Astronomy:Kepler Input Catalog|Kepler Input Catalog]] is a publicly searchable database of roughly 13.2 million targets used for the Kepler Spectral Classification Program and the Kepler mission.<ref name="kic">{{cite web |url=http://archive.stsci.edu/kepler/kic10/help/search_help.html |title=MAST KIC Search Help |publisher=[[Organization:Space Telescope Science Institute|Space Telescope Science Institute]] |access-date=April 23, 2011}}</ref><ref>{{cite web |url=http://archive.stsci.edu/kepler/kic10/search.php |title=KIC10 Search |access-date=April 23, 2011}}</ref> The catalog alone is not used for finding Kepler targets, because only a portion of the listed stars (about one-third of the catalog) can be observed by the spacecraft.<ref name=kic/>

==Solar System observations==
Kepler has been assigned an observatory code {{Obscode|C55}} in order to report its astrometric observations of [[Astronomy:Small Solar System bodies|small Solar System bodies]] to the [[Organization:Minor Planet Center|Minor Planet Center]]. In 2013 the alternative ''NEOKepler'' mission was proposed, a search for [[Astronomy:Near-Earth object|near-Earth object]]s, in particular [[Astronomy:Potentially hazardous asteroid|potentially hazardous asteroid]]s (PHAs). Its unique orbit and larger field of view than existing survey telescopes allow it to look for objects inside Earth's orbit. It was predicted a 12-month survey could make a significant contribution to the hunt for PHAs as well as potentially locating targets for NASA's [[Astronomy:Asteroid Redirect Mission|Asteroid Redirect Mission]].<ref name="Stevenson2013">{{cite arXiv |title=NEOKepler: Discovering Near-Earth Objects Using the Kepler Spacecraft |eprint=1309.1096 |class=astro-ph.EP |first1=Kevin B. |last1=Stevenson |first2=Daniel |last2=Fabrycky |first3=Robert |last3=Jedicke |first4=William |last4=Bottke |first5=Larry |last5=Denneau |date=September 2013}}</ref> Kepler's first discovery in the Solar System, however, was {{LoMP|506121|{{mp|506121|2016 BP|81}}}}, a 200-kilometer cold [[Astronomy:Classical Kuiper belt object|classical Kuiper belt object]] located [[Astronomy:Trans-Neptunian object|beyond the orbit of Neptune]].<ref name="MPC-object">{{cite web |url=http://www.minorplanetcenter.net/db_search/show_object?object_id=506121 |title=506121 (2016 BP81) |work=Minor Planet Center |access-date=March 28, 2018}}</ref>

==Retirement==
[[File:KeplerSpaceTelescope -Retirement-ArtistConcept-20181030.jpg|thumb|Artwork commissioned by NASA to commemorate the retirement of Kepler in October–November 2018.<ref name="NASA-20181030" /><ref name="NYT-20181030" />]]
On October 30, 2018, [[Organization:NASA|NASA]] announced that the Kepler space telescope, having run out of fuel, and after nine years of service and the discovery of over 2,600 [[Astronomy:Exoplanet|exoplanet]]s, has been officially retired, and will maintain its current, safe orbit, away from Earth.<ref name="NASA-20181030">{{cite web |last1=Chou |first1=Felicia |last2=Hawkes |first2=Alison |last3=Cofield |first3=Calia |title=NASA Retires Kepler Space Telescope |url=https://www.jpl.nasa.gov/news/news.php?feature=7272 |date=October 30, 2018 |publisher=[[Organization:NASA|NASA]] |access-date=October 30, 2018 }}</ref><ref name="NYT-20181030">{{cite news |last=Overbye |first=Dennis |title=Kepler, the Little NASA Spacecraft That Could, No Longer Can |url=https://www.nytimes.com/2018/10/30/science/nasa-kepler-exoplanet.html |date=October 30, 2018 |work={{wipe|The New York Times}} |access-date=October 30, 2018 }}</ref> The spacecraft was deactivated with a "goodnight" command sent from the mission's control center at the Laboratory for Atmospheric and Space Physics on November 15, 2018.<ref name="space.com-1">{{Cite web|last=Wall|first=Mike|title=Farewell, Kepler: NASA Shuts Down Prolific Planet-Hunting Space Telescope|url=https://www.space.com/42461-kepler-exoplanet-hunting-telescope-shuts-down.html|website=Space.com|access-date=November 16, 2018 |archive-url=https://web.archive.org/web/20181116223512/https://www.space.com/42461-kepler-exoplanet-hunting-telescope-shuts-down.html|archive-date=November 16, 2018|date=November 16, 2018|quote=NASA decommissioned the Kepler space telescope last night (Nov. 15), beaming "goodnight" commands to the sun-orbiting observatory. [...] The final commands were sent from Kepler's operations center at the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics...|url-status=live}}</ref> Kepler's retirement coincides with the 388th anniversary of [[Biography:Johannes Kepler|Johannes Kepler]]'s death in 1630.<ref name="jpl-1">{{Cite web|last1=Chou|first1=Felicia|last2=Hawkes|first2=Alison|last3=Cofield|first3=Calla|title=Kepler Telescope Bids 'Goodnight' with Final Commands|url=https://www.jpl.nasa.gov/news/news.php?feature=7284|website=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]]|access-date=November 16, 2018|archive-url=https://web.archive.org/web/20181116223850/https://www.jpl.nasa.gov/news/news.php?feature=7284|archive-date=November 16, 2018|date=November 16, 2018|quote=Coincidentally, Kepler's "goodnight" falls on the same date as the 388-year anniversary of the death of its namesake, German astronomer Johannes Kepler...|url-status=live}}</ref>

== See also ==
{{div col|colwidth=36em}}
* [[Astronomy:Kepler-22b|Kepler-22b]], the first exoplanet confirmed by Kepler to have an average orbital distance within its star's [[Astronomy:Habitable zone|habitable zone]]
* List of exoplanets discovered using the Kepler spacecraft
* List of exoplanets
* [[Astronomy:List of multiplanetary systems|List of multiplanetary systems]]
* [[Astronomy:List of stars that dim oddly|List of stars that dim oddly]]
* [[Organization:Hunt for Exomoons with Kepler|Hunt for Exomoons with Kepler]]
* [[Biography:William J. Borucki|William J. Borucki]], the chief investigator for ''Kepler''
* [[Astronomy:NASA Exoplanet Archive|NASA Exoplanet Archive]], online exoplanet catalog
{{div col end}}

'''Other space-based exoplanet search projects'''
{{div col|colwidth=18em}}
* [[Astronomy:CHEOPS|CHEOPS]] {{small|(2019)}}
* [[Astronomy:CoRoT|CoRoT]] {{small|(2006–2012)}}
* ''[[Engineering:Gaia (spacecraft)|Gaia]]'' {{small|(since 2013)}}
* PlanetQuest {{small|(since 2002)}}
* [[Engineering:PLATO (spacecraft)|PLATO]] {{small|(2026)}}
* [[Astronomy:Transiting Exoplanet Survey Satellite|TESS]] {{small|(since 2018)}}
{{div col end}}

'''Other ground-based exoplanet search projects'''
{{div col|colwidth=18em}}
* [[Astronomy:Automated Planet Finder|APF]] {{small|(since 2013)}}
* [[Astronomy:HATNet Project|HATNet]]  {{small|(since 2001)}}
* [[Astronomy:High Accuracy Radial Velocity Planet Searcher|HARPS]] {{small|(since 2003)}}
* [[Astronomy:Next-Generation Transit Survey|NGTS]] {{small|(since 2015)}}
* PlanetQuest {{small|(since 2002)}}
* SuperWASP {{small|(since 2002)}}
{{div col end}}
<!--
'''Others (cancelled or decommissioned)'''
{{div col|colwidth=18em}}
* [[Engineering:Darwin (spacecraft)|Darwin]] {{small|(abandoned)}}
* [[Engineering:Eddington (spacecraft)|Eddington]] {{small|(cancelled)}}
* [[Astronomy:Space Interferometry Mission|SIM]] {{small|(cancelled)}}
* [[Astronomy:Terrestrial Planet Finder|TPF]] {{small|(cancelled)}}
* [[Astronomy:Trans-Atlantic Exoplanet Survey|TrES]] {{small|(decommissioned)}}
{{div col end}}
-->

==Notes==
{{notelist-ua}}

== References ==
{{Reflist}}

==External links==
* by NASA's Ames Research Center
* [http://www.nasa.gov/kepler Kepler website] by [[Organization:NASA|NASA]]
* [https://web.archive.org/web/20121110210233/http://keplerscience.arc.nasa.gov/ Kepler Science Center] by NASA's Ames Research Center
* [http://archive.stsci.edu/kepler/ Kepler public data archive] by the [[Organization:Space Telescope Science Institute|Space Telescope Science Institute]]
* [https://archive.nytimes.com/www.nytimes.com/interactive/science/space/keplers-tally-of-planets.html Kepler – Tally of Exoplanets] (''NYT''; October 30, 2018)
* [http://www.mso.anu.edu.au/saga/ Strömgren survey for Asteroseismology and Galactic Archaeology]

'''[[Astronomy:Exoplanet|Exoplanet]] catalogs and databases'''
* [https://exoplanetarchive.ipac.caltech.edu/ NASA Exoplanet Archive]
* [https://exoplanet.eu/home/ Extrasolar Planets Encyclopaedia] by the Paris Observatory
* [http://phl.upr.edu/projects/habitable-exoplanets-catalog The Habitable Exoplanets Catalog] by UPR Arecibo
* [http://planetquest.jpl.nasa.gov/newworldsatlas New Worlds Atlas] by the NASA/JPL PlanetQuest

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