Astronomy:Planet-hosting stars

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Planet-hosting stars are stars which host planets, therefore forming planetary systems. This article describes the correlations between stars' characteristics and the characteristics of the planets that orbit them, and other connections between stars and their planets.

Proportion of stars with planets

Most stars have planets [citation needed] but exactly what proportion of stars have planets is uncertain because not all planets can yet be detected. That said it has been calculated that there is at least one planet on average per star.[1] One in five Sun-like stars[lower-alpha 1] are expected to have an "Earth-sized"[lower-alpha 2] planet in the habitable zone. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets in small orbits. Thus many known exoplanets are "hot Jupiters": planets of Jovian mass or larger in very small orbits with periods of only a few days. A 2005 survey of radial-velocity-detected planets found that about 1.2% of Sun-like stars have a hot jupiter, where "Sun-like star" refers to any main-sequence star of spectral classes late-F, G, or early-K without a close stellar companion.[2] This 1.2% is more than double the frequency of hot jupiters detected by the Kepler spacecraft, which may be because the Kepler field of view covers a different region of the Milky Way where the metallicity of stars is different.[3] It is further estimated that 3% to 4.5% of Sun-like stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses.[4]

It is known that small planets (of roughly Earth-like mass or somewhat larger) are more common than giant planets.[5] It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that perhaps 20% of Sun-like stars have at least one giant planet whereas at least 40% may have planets of lower mass.[4][6][7] A 2012 study of gravitational microlensing data collected between 2002 and 2007 concludes the proportion of stars with planets is much higher and estimates an average of 1.6 planets orbiting between 0.5 and 10 AU per star in the Milky Way, the authors of this study conclude that "stars are orbited by planets as a rule, rather than the exception".[1] In November 2013 it was announced that 22±8% of Sun-like[lower-alpha 1] stars have an Earth-sized[lower-alpha 2] planet in the habitable[lower-alpha 3] zone.[8][9]

Whatever the proportion of stars with planets, the total number of exoplanets must be very large. Because the Milky Way has at least 200 billion stars, it must also contain tens or hundreds of billions of planets.

Type of star, spectral classification

See also planets orbiting Herbig Ae/Be stars, T Tauri stars, Subgiants, Yellow giants, Red giants, B-type stars, A-type stars, Subdwarf B stars, F-type stars, G-type stars, K-type stars, Red dwarfs, Brown dwarfs, White dwarfs, or Pulsars.
The Morgan-Keenan spectral classification

Most known exoplanets orbit stars roughly similar to the Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is that planet-search programs have tended to concentrate on such stars. In addition, statistical analyses indicate that lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[4][10] Nevertheless, many planets around red dwarfs have been discovered by the Kepler spacecraft by the transit method, which can detect smaller planets.

Stars of spectral category A typically rotate very quickly, which makes it very difficult to measure the small Doppler shifts induced by orbiting planets because the spectral lines are very broad.[11] However, this type of massive star eventually evolves into a cooler red giant that rotates more slowly and thus can be measured using the radial-velocity method.[11] A few tens of planets have been found around red giants.

Observations using the Spitzer Space Telescope indicate that extremely massive stars of spectral category O, which are much hotter than the Sun, produce a photo-evaporation effect that inhibits planetary formation.[12] When the O-type star goes supernova any planets that had formed would become free-floating due to the loss of stellar mass unless the natal kick of the resulting remnant pushes it in the same direction as an escaping planet.[13] Fallback disks of matter that failed to escape orbit during a supernova may form planets around neutron stars and black holes.[14]

Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, vs. 1 in 16 for Sun-like stars and only 1 in 50 for red dwarfs. On the other hand, microlensing surveys indicate that long-period Neptune-mass planets are found around 1 in 3 red dwarfs. [15] Kepler Space Telescope observations of planets with up to one year periods show that occurrence rates of Earth- to Neptune-sized planets (1 to 4 Earth radii) around M, K, G, and F stars are successively higher towards cooler, less massive stars.[16]

At the low-mass end of star-formation are sub-stellar objects that don't fuse hydrogen: the brown dwarfs and sub-brown dwarfs, of spectral classification L,T and Y. Planets and protoplanetary disks have been discovered around brown dwarfs, and disks have been found around sub-brown dwarfs (e.g. OTS 44).

Rogue planets ejected from their system could retain a system of satellites.[17]

Metallicity

Stars with a higher metallicity than the Sun are more likely to have planets, especially giant planets, than stars with lower metallicity.

Ordinary stars are composed mainly of the light elements hydrogen and helium. They also contain a small proportion of heavier elements, and this fraction is referred to as a star's metallicity (even if the elements are not metals in the traditional sense),[2] denoted [m/H] and expressed on a logarithmic scale where zero is the Sun's metallicity.

A 2012 study of the Kepler spacecraft data found that smaller planets, with radii smaller than Neptune's were found around stars with metallicities in the range −0.6 < [m/H] < +0.5 (about four times less than that of the Sun to three times more),[lower-alpha 4] whereas larger planets were found mostly around stars with metallicities at the higher end of this range (at solar metallicity and above). In this study small planets occurred about three times as frequently as large planets around stars of metallicity greater than that of the Sun, but they occurred around six times as frequently for stars of metallicity less than that of the Sun. The lack of gas giants around low-metallicity stars could be because the metallicity of protoplanetary disks affects how quickly planetary cores can form and whether they accrete a gaseous envelope before the gas dissipates. However, Kepler can only observe planets very close to their star and the detected gas giants probably migrated from further out, so a decreased efficiency of migration in low-metallicity disks could also partly explain these findings.[18]

A 2014 study found that not only giant planets, but planets of all sizes have an increased occurrence rate around metal-rich stars compared to metal-poor stars, although the larger the planet, the greater this increase as the metallicity increases. The study divided planets into three groups based on radius: gas giants, gas dwarfs, and terrestrial planets with the dividing lines at 1.7 and 3.9 Earth radii. For these three groups the planet occurrence rates are 9.30, 2.03, and 1.72 times higher for metal-rich stars than for metal-poor stars, respectively. There is a bias against detecting smaller planets because metal-rich stars tend to be larger, making it more difficult to detect smaller planets, which means that these increases in occurrence rates are lower limits.[19]

It has also been shown that Sun-like stars with planets are much more likely to be deficient in lithium, although this correlation is not seen at all in other types of stars.[20] However, this claimed relationship has become a point of contention in the planetary astrophysics community, being frequently denied[21][22] but also supported.[23][24]

Multiple stars

Stellar multiplicity increases with stellar mass: the likelihood of stars being in multiple systems is about 25% for red dwarfs, about 45% for Sun-like stars, and rises to about 80% for the most massive stars. Of the multiple stars about 75% are binaries and the rest are higher-order multiplicities.[25]

More than one hundred planets have been discovered orbiting one member of a binary star system (e.g. 55 Cancri, possibly Alpha Centauri Bb),[26] and several circumbinary planets have been discovered which orbit around both members of a binary star (e.g. PSR B1620-26 b, Kepler-16b). A few dozen planets in triple star systems are known (e.g. 16 Cygni Bb)[27] and two in quadruple systems Kepler 64 and 30 Arietis.[28]

The Kepler results indicate circumbinary planetary systems are relatively common (as of October 2013 the spacecraft had found seven circumbinary planets out of roughly 1000 eclipsing binaries searched). One puzzling finding is that although half of the binaries have an orbital period of 2.7 days or less, none of the binaries with circumbinary planets have a period less than 7.4 days. Another surprising Kepler finding is circumbinary planets tend to orbit their stars close to the critical instability radius (theoretical calculations indicate the minimum stable separation is roughly two to three times the size of the stars' separation).[29]

In 2014, from statistical studies of searches for companion stars, it was inferred that around half of exoplanet host stars have a companion star, usually within 100AU.[30][31] This means that many exoplanet host stars that were thought to be single are binaries, so in many cases it is not known which of the stars a planet actually orbits, and the published parameters of transiting planets could be significantly incorrect because the planet radius and distance from star are derived from the stellar parameters. Follow-up studies with imaging (such as speckle imaging) are needed to find or rule out companions (and radial velocity techniques would be required to detect binaries really close together) and this has not yet been done for most exoplanet host stars. Examples of known binary stars where it is not known which of the stars a planet orbits are Kepler-132 and Kepler-296,[32] although a 2015 study found that the Kepler-296 planets were likely orbiting the brighter star.[33]

Open clusters

Most stars form in open clusters, but very few planets have been found in open clusters and this led to the hypothesis that the open-cluster environment hinders planet formation. However, a 2011 study concluded that there have been an insufficient number of surveys of clusters to make such a hypothesis.[34] The lack of surveys was because there are relatively few suitable open clusters in the Milky Way. Recent discoveries of both giant planets[35] and low-mass planets[36] in open clusters are consistent with there being similar planet occurrence rates in open clusters as around field stars.

The open cluster NGC 6811 contains two known planetary systems Kepler-66 and Kepler-67.

Age

Asteroseismology

Stellar activity

Further reading

References

  1. ↑ 1.0 1.1 For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars
  2. ↑ 2.0 2.1 For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
  3. ↑ For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
  4. ↑ Converting log scale [m/H] to multiple of solar metallicity: [(10−0.6 ≈ 1/4), (100.5 ≈ 3)]
  1. ↑ 1.0 1.1 Cassan, A.; Kubas, D.; Beaulieu, J. P.; Dominik, M et al. (2012). "One or more bound planets per Milky Way star from microlensing observations". Nature 481 (7380): 167–169. doi:10.1038/nature10684. PMID 22237108. Bibcode: 2012Natur.481..167C. 
  2. ↑ 2.0 2.1 Marcy, G. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement 158: 24–42. doi:10.1143/PTPS.158.24. Bibcode: 2005PThPS.158...24M. http://ptp.ipap.jp/link?PTPS/158/24. Retrieved 2020-05-07. 
  3. ↑ The Frequency of Hot Jupiters Orbiting Nearby Solar-Type Stars, J. T. Wright, G. W. Marcy, A. W. Howard, John Asher Johnson, T. Morton, D. A. Fischer, (Submitted on 10 May 2012)
  4. ↑ 4.0 4.1 4.2 Andrew Cumming; R. Paul Butler; Geoffrey W. Marcy et al. (2008). "The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets". Publications of the Astronomical Society of the Pacific 120 (867): 531–554. doi:10.1086/588487. Bibcode: 2008PASP..120..531C. 
  5. ↑ Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler, Andrew W. Howard et al. (Submitted on 13 Mar 2011)
  6. ↑ Amos, Jonathan (19 October 2009). "Scientists announce planet bounty". BBC News. http://news.bbc.co.uk/2/hi/science/nature/8314581.stm. 
  7. ↑ David P. Bennett; Jay Anderson; Ian A. Bond; Andrzej Udalski et al. (2006). "Identification of the OGLE-2003-BLG-235/MOA-2003-BLG-53 Planetary Host Star". Astrophysical Journal Letters 647 (2): L171–L174. doi:10.1086/507585. Bibcode: 2006ApJ...647L.171B. 
  8. ↑ Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu. http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/. 
  9. ↑ Petigura, E. A.; Howard, A. W.; Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences 110 (48): 19273. doi:10.1073/pnas.1319909110. Bibcode: 2013PNAS..11019273P. 
  10. ↑ Bonfils, Xavier; Forveille, Thierry; Delfosse, Xavier; Udry, Stéphane; Mayor, Michel; Perrier, Christian; Bouchy, François; Pepe, Francesco et al. (2005). "The HARPS search for southern extra-solar planets VI: A Neptune-mass planet around the nearby M dwarf Gl 581". Astronomy and Astrophysics 443 (3): L15–L18. doi:10.1051/0004-6361:200500193. Bibcode: 2005A&A...443L..15B. 
  11. ↑ 11.0 11.1 Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants, John A. Johnson, Debra A. Fischer, Geoffrey W. Marcy, Jason T. Wright, Peter Driscoll, R. P. Butler, Saskia Hekker, Sabine Reffert, Steven S. Vogt, 19 Apr 2007
  12. ↑ L. Vu (3 October 2006). "Planets Prefer Safe Neighborhoods". Spitzer Science Center. http://www.spitzer.caltech.edu/Media/happenings/20061003/. 
  13. ↑ Limits on Planets Orbiting Massive Stars from Radio Pulsar Timing , Thorsett, S.E. Dewey, R.J. 16-Sep-1993
  14. ↑ The fate of fallback matter around newly born compact objects, Rosalba Perna, Paul Duffell, Matteo Cantiello, Andrew MacFadyen, (Submitted on 17 Dec 2013)
  15. ↑ J. A. Johnson (2011). "The Stars that Host Planets". Sky & Telescope (April): 22–27. 
  16. ↑ A stellar-mass-dependent drop in planet occurrence rates, Gijs D. Mulders, Ilaria Pascucci, Daniel Apai, (Submitted on 28 Jun 2014)
  17. ↑ The Survival Rate of Ejected Terrestrial Planets with Moons by J. H. Debes, S. Sigurdsson
  18. ↑ Buchhave, L. A. (2012). "An abundance of small exoplanets around stars with a wide range of metallicities". Nature. doi:10.1038/nature11121. Bibcode: 2012Natur.486..375B. 
  19. ↑ Revealing A Universal Planet-Metallicity Correlation For Planets of Different Sizes Around Solar-Type Stars, Ji Wang, Debra A. Fischer, (Submitted on 29 Oct 2013 (v1), last revised 16 Oct 2014 (this version, v3))
  20. ↑ Israelian, G. (2009). "Enhanced lithium depletion in Sun-like stars with orbiting planets". Nature 462 (7270): 189–191. doi:10.1038/nature08483. PMID 19907489. Bibcode: 2009Natur.462..189I. "... confirm the peculiar behaviour of Li in the effective temperature range 5600–5900 K ... We found that the immense majority of planet host stars have severely depleted lithium ... At higher and lower temperatures planet-host stars do not appear to show any peculiar behaviour in their Li abundance.". 
  21. ↑ Baumann, P.; Ramírez, I. et al. (2010). "Lithium depletion in solar-like stars: no planet connection". Astronomy and Astrophysics 519: A87. doi:10.1051/0004-6361/201015137. ISSN 0004-6361. 
  22. ↑ Ramírez, I.; Fish, J. R. et al. (2012). "Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, galactic chemical evolution, and exoplanets". The Astrophysical Journal 756 (1): 46. doi:10.1088/0004-637X/756/1/46. ISSN 0004-637X. 
  23. ↑ Figueira, P.; Faria, J. P. et al. (2014). "Exoplanet hosts reveal lithium depletion". Astronomy & Astrophysics 570: A21. doi:10.1051/0004-6361/201424218. ISSN 0004-6361. 
  24. ↑ Delgado Mena, E.; Israelian, G. et al. (2014). "Li depletion in solar analogues with exoplanets". Astronomy & Astrophysics 562: A92. doi:10.1051/0004-6361/201321493. ISSN 0004-6361. 
  25. ↑ Stellar Multiplicity, Gaspard Duchêne (1,2), Adam Kraus (3) ((1) UC Berkeley, (2) Institut de Planétologie et d'Astrophysique de Grenoble, (3) Harvard-Smithsonian CfA), (Submitted on 12 Mar 2013)
  26. ↑ BINARY CATALOGUE OF EXOPLANETS , Maintained by Richard Schwarz], retrieved 28 Sept 2013
  27. ↑ "Archived copy". http://www.univie.ac.at/adg/schwarz/multi.html. 
  28. ↑ Schwarz, Richard; Bazsó, Ákos (2019). "Catalogue of exoplanets in binary star systems". doi:10.1093/mnras/stw1218. https://www.univie.ac.at/adg/schwarz/multiple.html. 
  29. ↑ Welsh, William F.; Doyle, Laurance R. (2013). "Worlds with Two Suns". Scientific American 309 (5): 40. doi:10.1038/scientificamerican1113-40. 
  30. ↑ One Planet, Two Stars: A System More Common Than Previously Thought , www.universetoday.com, by Shannon Hall on September 4, 2014
  31. ↑ Most Sub-Arcsecond Companions of Kepler Exoplanet Candidate Host Stars are Gravitationally Bound, Elliott P. Horch, Steve B. Howell, Mark E. Everett, David R. Ciardi, 3 Sep 2014
  32. ↑ Validation of Kepler's Multiple Planet Candidates. II: Refined Statistical Framework and Descriptions of Systems of Special Interest, Jack J. Lissauer, Geoffrey W. Marcy, Stephen T. Bryson, Jason F. Rowe, Daniel Jontof-Hutter, Eric Agol, William J. Borucki, Joshua A. Carter, Eric B. Ford, Ronald L. Gilliland, Rea Kolbl, Kimberly M. Star, Jason H. Steffen, Guillermo Torres, (Submitted on 25 Feb 2014)
  33. ↑ The Five Planets in the Kepler-296 Binary System All Orbit the Primary: A Statistical and Analytical Analysis, Thomas Barclay, Elisa V. Quintana, Fred C. Adams, David R. Ciardi, Daniel Huber, Daniel Foreman-Mackey, Benjamin T. Montet, Douglas Caldwell, 7 May 2015
  34. ↑ Ensemble analysis of open cluster transit surveys: upper limits on the frequency of short-period planets consistent with the field, Jennifer L. van Saders, B. Scott Gaudi, (Submitted on 15 Sep 2010)
  35. ↑ Three planetary companions around M67 stars, A. Brucalassi (1,2), L. Pasquini (3), R. Saglia (1,2), M. T. Ruiz (4), P. Bonifacio (5), L. R. Bedin (6), K. Biazzo (7), C. Melo (8), C. Lovis (9), S. Randich (10) ((1) MPI Munich, (2) UOM-LMU Munchen, (3) ESO Garching, (4) Astron. Dpt. Univ. de Chile, (5) GEPI Paris, (6) INAF-OAPD, (7) INAF-OACT, (8) ESO Santiago, (9) Obs. de Geneve, (10) INAF-OAFI) (Submitted on 20 Jan 2014)
  36. ↑ The same frequency of planets inside and outside open clusters of stars, Søren Meibom, Guillermo Torres, Francois Fressin, David W. Latham, Jason F. Rowe, David R. Ciardi, Steven T. Bryson, Leslie A. Rogers, Christopher E. Henze, Kenneth Janes, Sydney A. Barnes, Geoffrey W. Marcy, Howard Isaacson, Debra A. Fischer, Steve B. Howell, Elliott P. Horch, Jon M. Jenkins, Simon C. Schuler & Justin Crepp Nature 499, 55–58 (04 July 2013) doi:10.1038/nature12279 Received 06 November 2012 Accepted 02 May 2013 Published online 26 June 2013




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