View source for Astronomy:Gliese 581g

{{Short description|Former candidate super-Earth orbiting Gliese 581}}
{{Infobox planet
| name                     = Gliese 581g
| image                    = Exoplanet Comparison Gliese 581 g.png
| caption                  = Size comparison of Gliese 581g with Earth and Neptune<br />(Based on selected hypothetical [[:File:Exoplanet Comparison Gliese 581 g.png#Summary|modeled compositions]])
<!-- DISCOVERY           -->
| discoverer               = Steven S. Vogt et al.<ref name="Vogt 2010"/>
| discovery_site           = Keck Observatory, Hawaii<ref name="Smith 2010"/><ref name="Overbye 2012"/><ref name="Alleyne 2010"/>
| discovered               = September 29, 2010<ref name="Vogt 2010"/><ref name="Sanders 2010"/>
| discovery_method         = [[Astronomy:Doppler spectroscopy|Radial velocity]]<ref name="Vogt 2010"/>
<!-- DESIGNATIONS        -->
<!-- ORBITAL             -->
| epoch                    = [[Astronomy:Julian day|JD]] 2451409.762<ref name="Vogt 2010"/>
| apsis                    = astron
| semimajor                = {{convert|0.13|AU|km|abbr=on}}<ref name="Vogt 2012"/>
| eccentricity             = 0<ref name="Vogt 2010"/>
| period                   = 32<ref name = "Vogt 2012"/> [[Day|d]]
| mean_anomaly             = 271 ± 48<ref name="Vogt 2010"/>
| semi-amplitude           = 1.29 ± 0.19<ref name="Vogt 2010"/>
| star                     = [[Astronomy:Gliese 581|Gliese 581]]<ref name="Vogt 2010"/><ref name="Quenqua 2014"/>
<!-- PHYS CHARS          -->
| single_temperature       = {{convert|242 |K|C F}} to {{convert|261|K|C F}}<ref name="Stephens 2010"/>
<!-- ATMOSPHERE          -->
<!-- NOTES               -->
}}
'''Gliese 581g''' {{IPAc-en|ˈ|ɡ|l|iː|z|ə}} was a candidate [[Astronomy:Exoplanet|exoplanet]] postulated to orbit within the Gliese 581 system, twenty light-years from Earth.<ref name="Robertson 2014"/> It was discovered by the [[Astronomy:Lick–Carnegie Exoplanet Survey|Lick–Carnegie Exoplanet Survey]], and was the sixth [[Astronomy:Planet|planet]] claimed to orbit the [[Astronomy:Star|star]];<ref name="Hsu 2010"/> however, its existence could not be confirmed by the [[Organization:European Southern Observatory|European Southern Observatory]] (ESO) / [[Astronomy:High Accuracy Radial Velocity Planet Searcher|High Accuracy Radial Velocity Planet Searcher]] (HARPS) survey team, and was ultimately refuted.<ref name="Robertson 2014"/><ref name="Trifonov2018"/><ref name="Dodson-Robinson2022"/> It was thought to be near the middle of the [[Astronomy:Habitable zone|habitable zone]] of its [[Astronomy:Star|star]],<ref name="Howell 2016"/> meaning it could sustain liquid water—a necessity for all known life—on its surface, if there are favorable atmospheric conditions on the planet.

Gliese 581g was claimed to be detected by astronomers of the [[Astronomy:Lick–Carnegie Exoplanet Survey|Lick–Carnegie Exoplanet Survey]]. The authors stated that data sets from both the High Resolution Echelle Spectrometer (HIRES) and HARPS were needed to sense the planet; however, the ESO/HARPS survey team could not confirm its existence. The planet remained unconfirmed as consensus for its existence could not be reached. Additional reanalysis only found evidence for four planets, but the discoverer, Steven S. Vogt, did not agree with those conclusions. In 2012, a reanalysis by Vogt supported its existence.<ref name="Wall; July 23, 2012"/> A new study in 2014 concluded that it was a false positive,<ref name="Grant 2014"/><ref name="Robertson 2014"/> a conclusion which has been further confirmed by subsequent studies.<ref name="Dodson-Robinson2022"/> The planet was thought to be tidally locked to its star. If the planet has a dense [[Astronomy:Atmosphere|atmosphere]], it may be able to circulate heat. The actual [[Social:Habitability|habitability]] of the planet depends on the composition of its surface and the atmosphere. It was thought to have temperatures around −37 to −11&nbsp;°C (−35 to 10&nbsp;°F). By comparison, [[Astronomy:Earth|Earth]] has an average [[Astronomy:Planetary equilibrium temperature|surface temperature]] of 15&nbsp;°C (59&nbsp;°F)—while [[Astronomy:Mars|Mars]] has an average surface temperature of about −63&nbsp;°C (−81&nbsp;°F). The planet was said by Vogt to have a "100%" chance of supporting life.<ref name="Press Release 10-172 - Video"/> The supposed detection of Gliese 581g was said to foreshadow what Vogt called "a second Age of Discovery".<ref name="Vogt 2010"/>

== History ==
{{anchor|History of Gliese 581 g}}

=== Discovery ===
[[File:KeckTwilight-hi.png|thumb|right|The W. M. Keck Observatory at twilight, where Gliese 581g was discovered]]
[[File:GJ581orbits Vogt2010.svg|thumb|left|The six-planet model of the Gliese 581 system with circular orbits<ref name="Vogt 2010"/>]]

The planet's discovery was claimed in September 2010,<ref name="Howell 2016"/> to have been detected by astronomers in the [[Astronomy:Lick–Carnegie Exoplanet Survey|Lick–Carnegie Exoplanet Survey]], led by principal investigator Steven Vogt,<ref name="Howell 2016"/> professor of astronomy and astrophysics at the [[Organization:University of California, Santa Cruz|University of California, Santa Cruz]],<ref name="Howell 2016"/> and co-investigator R. Paul Butler of the Carnegie Institution of Washington. The discovery was made using [[Astronomy:Doppler spectroscopy|radial velocity measurements]],<ref name="Howell 2016"/><ref name="Vogt 2010"/> combining 122 observations obtained over 11<ref name="Howell 2016"/> years from the HIRES instrument of the [[Astronomy:W. M. Keck Observatory|W. M. Keck Observatory]]<ref name="Howell 2016"/> with 119 measurements obtained over 4.3 years from the HARPS<ref name="Howell 2016"/> instrument of the [[Astronomy:ESO 3.6 m Telescope|ESO 3.6 m Telescope]]<ref name="Howell 2016"/> at La Silla Observatory.<ref name="Alleyne 2010"/> In addition, brightness measurements of the star were confirmed with a robotic telescope<ref name="Howell 2016"/> from [[Organization:Tennessee State University|Tennessee State University]].<ref name="Vogt 2010"/>

After subtracting the signals of the previously known Gliese 581 planets, ''[[Astronomy:Gliese 581b|b]]'', ''[[Astronomy:Gliese 581c|c]]'', ''[[Astronomy:Gliese 581d|d]]'' and ''[[Astronomy:Gliese 581e|e]]'', the signals of two<ref name="Howell 2016"/> additional planets were apparent: a 445-day signal from a newly recognized outermost planet designated ''f'', and the 37-day signal from Gliese 581g.<ref name="Vogt 2010"/><ref name="Alexander 2010"/> The probability that the detection of the latter was spurious was estimated at only 2.7 in a million.<ref name="Vogt 2010"/> The authors stated that while the 37-day signal is "clearly visible in the HIRES data set alone", "the HARPS data set alone is not able to reliably sense this planet" and concluded, "It is really necessary to combine both data sets to sense all these planets reliably".<ref name="Vogt 2010"/> The Lick–Carnegie team explained the results of their research in a paper published in the ''Astrophysical Journal'', which were also made available in [[Preprint|preprint]]<ref name="Vogt, preprint 2010"/> version on [[ArXiv|arXiv]].<ref name="Howell 2016"/> Although not sanctioned by the [[Organization:International Astronomical Union|IAU]]'s naming conventions, Vogt's team informally referred to the planet as "Zarmina's World" after his wife,<ref name="Meichsner 2010"/> and in some cases simply as Zarmina.

During a press release announcing the discovery, Vogt et al. acknowledged that the "Gliese 581 system has a somewhat checkered history of habitable planet claims," as two previously discovered planets in the same system, Gliese 581c and d, were also regarded as potentially habitable, but later evaluated as being outside the conservatively defined habitable zone.<ref name="Howell 2016"/>

=== Nondetection in new HARPS data analysis ===
Two weeks after the announcement of the discovery of Gliese 581g, another team—led by Michael Mayor of the Geneva Observatory<ref name="Howell 2016"/>—reported that in a new analysis of 179 measurements taken by the HARPS<ref name="Howell 2016"/> spectrograph over 6.5 years, neither planet ''g''<ref name="Howell 2016"/> nor planet ''f'' was detectable.<ref name="Kerr 2010"/><ref name="Forveille 2011"/><ref name="Bonfils 2011"/> An astronomer who works on HARPS data at the Geneva Observatory, Francesco Pepe, said in an email for an Astrobiology Magazine article republished on Space.com, "The reason for that is that, despite the extreme accuracy of the instrument and the many data points, the signal amplitude of this potential fifth planet is very low and basically at the level of the measurement noise".<ref name="Howell 2016"/><ref name="Mullen 2010"/> The Geneva team had also published their paper on [[ArXiv|arXiv]],<ref name="Howell 2016"/> but it appeared to not<ref name="Howell 2016"/> have been accepted for publication{{why?|date=January 2022}}.

Vogt responded to the latest concerns by saying, "I am not overly surprised by this as these are very weak signals, and adding 60 points onto 119 does not necessarily translate to big gains in sensitivity."<ref name="Grossman October 2010"/> More recently, Vogt added, "I feel confident that we have accurately and honestly reported our uncertainties and done a thorough and responsible job extracting what information this data set has to offer. I feel confident that anyone independently analyzing this data set will come to the same conclusions."<ref name="Wall 2010"/>

Differences in the two groups' results may involve the planetary orbital characteristics assumed in calculations. According to [[Organization:Massachusetts Institute of Technology|Massachusetts Institute of Technology]] astronomer [[Biography:Sara Seager|Sara Seager]], Vogt postulated the planets around Gliese 581 had perfectly circular orbits whereas the Swiss group thought the orbits were more [[Astronomy:Orbital eccentricity|eccentric]].<ref name="Cowen 2010"/> This difference in approach may be the reason for the disagreement, according to [[Biography:Alan Boss|Alan Boss]].<ref name="Cowen 2010"/> Butler remarked that with additional observations, "I would expect that on the time scale of a year or two this should be settled."<ref name="Kerr 2010"/> Other astronomers also supported a deliberate evaluation: Seager stated, "We will have consensus at some point; I don't think we need to vote right now." Ray Jayawardhana noted, "Given the extremely interesting implications of such a discovery, it's important to have independent confirmation."<ref name="Cowen 2010"/> Gliese 581g is listed as "retracted" in the ''[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]]''.<ref name="Extrasolar Planets Encyclopaedia"/>

=== Further analyses of HIRES/HARPS data ===
In December 2010, a claimed methodological error was reported—by a group led by Rene Andrae of the [[Organization:Max Planck Institute for Astronomy|Max Planck Institute for Astronomy]]—in the data analysis that led to the discovery of Gliese 581f and g.<ref name="Andrae 2010"/><ref name="Howell 2016"/>

In 2011, another reanalysis—performed by a group led by Philip Gregory of the University of British Columbia—found no clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set.<ref name="Howell 2016"/><ref name="Gregory 2011"/> The claim was made that the HARPS data provided only some evidence for 5 planet signals, while incorporation of both data sets actually degraded the evidence for more than four planets (i.e., none for 581f or 581g).<ref name="Gregory 2011"/> [[Biography:Mikko Tuomi|Mikko Tuomi]] of the [[Organization:University of Hertfordshire|University of Hertfordshire]] performed a Bayesian reanalysis of the HARPS and HIRES data with the result that they "do not imply the conclusion that there are two additional companions orbiting GJ 581".<ref name="Tuomi 2011"/>

"I have studied [the paper] in detail and do not agree with his conclusions,"<ref name="Canadian Broadcasting Corporation 2011"/> Steven Vogt said in reply, concerned that Gregory has considered the HIRES data as more uncertain.<ref name="Wall 2011"/> "The question of Gliese 581g's existence won't be settled definitively until researchers gather more high-precision radial velocity data", Vogt said. However Vogt expects further analysis to strengthen the case for the planet.<ref name="Bhattacharjee 2011"/>

By performing a number of statistical tests, [[Biography:Guillem Anglada-Escudé|Guillem Anglada-Escudé]] of the Carnegie Institute of Washington concluded that the existence of Gl 581g was well supported by the available data, despite the presence of a statistical degeneracy that derives from an alias of the first eccentric harmonic of another planet in the system.<ref name="Anglada-Escudé 2010"/> In a preprint posted to arXiv, Anglada-Escudé and Rebekah Dawson claimed that, "with the data we have, the most likely explanation is that this planet is still there."<ref name="Anglada-Escudé 2010"/><ref name="Grossman 2011"/>

=== 2012 reanalysis of HARPS data ===
In July 2012, Vogt reanalyzed the 2011 data proposed by Forveille{{who?|date=January 2022}} et al., noting that there were five objects (Gliese 581b, e, c, g, d, with no evidence for f). Planet g was orbiting around 0.13 AU with an orbital period of thirty-two days, placing it inside the habitable zone. Vogt concluded that the object had a [[Astronomy:Minimum mass|minimum mass]] of 2.2 M and had a false positive probability of less than 4%. Vogt also said that they couldn't come to same conclusion as the Geneva team, without removing data points, "I don't know whether this omission was intentional or a mistake," he said, "I can only say that, if it was a mistake, they've been making that same mistake more than once now, not only in this paper, but in other papers as well."<ref name="Howell 2016"/> Vogt then said that the planet was there as long as all of the planets had circular orbits, and that the circular orbits work because “of dynamic stability, goodness-of-fit, and principle of parsimony (Occam's Razor)."<ref name="Howell 2016"/><ref name="Overbye 2012"/><ref name="Vogt 2012"/>

=== Further studies and refutation ===
Two studies in 2013 did not find evidence of Gliese 581g, only finding evidence for four—or three—planets in the system.<ref name="Baluev2013"/><ref name="Hatzes2013"/>

A study in 2014—published in ''Science''—<ref name="Chung 2014"/> led by postdoctoral<ref name="Chung 2014"/> researcher Paul Robertson concluded that [[Astronomy:Gliese 581d|Gliese 581d]] is "an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g."<ref name="Robertson 2014"/><ref name="Quenqua 2014"/><ref name="Chung 2014"/> "They were very high value targets if they were real," Robertson said, "But unfortunately we found out that they weren't."<ref name="Chung 2014"/> It was pointed out—during a press release by Penn State University—that sunspots could sometimes masquerade as planetary signals.<ref name="Howell 2016"/> An additional study concluded that Gliese 581g's existence depends on Gliese 581d's eccentricity.<ref name="Robertson 2014"/> The planet was later delisted from the Habitable Exoplanets Catalog, which is run by the University of Puerto Rico at Arecibo.<ref name="Howell 2016"/> Later, in October that year, Abel Mendez wrote—in a blog post characterizing "false starts" in exoplanet habitability—<ref name="Howell 2016"/><ref name="Mendez 2014"/> that the planet does not exist.

In 2015, a pair of researchers led by Guillem Anglada-Escudé of the University of London questioned the methodology of the 2014 study and suggested planet [[Astronomy:Gliese 581d|Gliese 581d]] really could exist, despite stellar variability, and the 2014 refutation of the existence of Gliese 581d and g was triggered by poor and inadequate analysis of the data, saying that the statistical method used by Robertson's team was "simply inadequate for identifying small planets like Gliese 581d", urging that the data be reanalyzed using a "more accurate model."<ref name="SCI-20150306"/><ref name="Howell 2016"/><ref name="Astronomy Now 2015"/> However, this response did not make any claim for the existence of Gliese 581g, and was published along with a rebuttal by the team that published the 2014 refutation.<ref name="Robertson2015"/> Most further studies have confirmed the stellar, rather than planetary, origin of the signal corresponding to Gliese 581d, and consequently Gliese 581g,<ref name="SuárezMascareño2015"/><ref name="Hatzes2016"/><ref name="Trifonov2018"/> with one such study explicitly refuting g.<ref name="Dodson-Robinson2022"/>

== Physical characteristics ==
{{anchor|Physical characteristics of Gliese 581 g}}

=== Tidal locking ===
Because of Gliese 581g's proximity to its parent star, it is predicted to be [[Astronomy:Tidal locking|tidally locked]] to Gliese 581. Just as Earth's [[Astronomy:Moon|Moon]] always presents the same face to the Earth, the length of Gliese 581g's sidereal day would then precisely match the length of its year, meaning it would be permanently light<ref name="Howell 2016"/> on one half and permanently dark<ref name="Howell 2016"/> on the other half of its surface.<ref name="Vogt 2010"/><ref name="Berardelli 2010"/>

=== Atmosphere ===
[[File:Gliese 581 system compared to solar system.jpg|thumb|right|Planetary orbits in the Gliese 581 system compared to those of the Solar System ("g" designates Gliese 581g)]]
An atmosphere that is dense will circulate heat, potentially allowing a wide area on the surface to be habitable.<ref name="Alpert 2005"/> For example, [[Astronomy:Venus|Venus]] has a [[Astronomy:Solar time|solar]] rotation rate approximately 117 times slower than Earth's, producing prolonged days and nights. Despite the uneven distribution of sunlight over time intervals shorter than several months, unilluminated areas of Venus are kept almost as hot as the day side by globally circulating winds.<ref name="Lorenz 2001"/> Simulations have shown that an atmosphere containing appropriate levels of CO<sub>2</sub> and H<sub>2</sub>O need only be a tenth the pressure of Earth's atmosphere (100 [[Bar (unit)|mbar]]) to effectively distribute heat to the night side.<ref name="Joshi 1997"/> Current technology cannot determine the atmospheric or surface composition of the planet due to the overpowering light of its parent star.<ref name="Shiga 2010"/>

Whether or not a tidally locked planet with the orbital characteristics of Gliese 581g is actually habitable depends on the composition of the atmosphere and the nature of the planetary surface. A comprehensive modeling study<ref name="Pierrehumbert 2010"/> including atmospheric dynamics, realistic radiative transfer and the physics of formation of sea ice (if the planet has an ocean) indicates that the planet can become as hot as Venus if it is dry and allows carbon dioxide to accumulate in its atmosphere. The same study identified two habitable states for a water-rich planet. If the planet has a very thin atmosphere, a thick ice crust forms over most of the surface, but the substellar point remains hot enough to yield a region of thin ice or even episodically open water. If the planet has an atmosphere with Earthlike pressures, containing approximately 20% (molar) carbon dioxide, then the greenhouse effect is sufficiently strong to maintain a pool of open water under the substellar point with temperatures comparable to the Earth's tropics. This state has been dubbed "Eyeball Earth" by the author.<ref name="Pierrehumbert 2010"/> Modeling of the effect of tidal locking on Gliese 581g's possible atmosphere, using a [[Earth:General circulation model|general circulation model]] employing an atmosphere with Earthlike surface pressure but a highly idealized representation of radiative processes, indicates that for a solid-surface planet the locations of maximum warmth would be distributed in a sideways [[Engineering:Chevron (insignia)|chevron]]-shaped pattern centered near the substellar point.<ref name="Heng 2010"/><ref name="Grossman November 2010"/>

=== Climate ===
[[File:Gliese 581 - 2010.jpg|thumb|250px|right|The habitable zone of Gliese 581 compared with the Solar System's habitable zone, showing Gliese 581g near the center]]
It is estimated that the average global [[Physics:Thermal equilibrium|equilibrium temperature]] (the temperature in the absence of atmospheric effects) of Gliese 581g would range from 209 to 228&nbsp;[[Kelvin|K]] (−64 to −45&nbsp;°C, or −84 to −49&nbsp;°F) for [[Physics:Bond albedo|Bond albedo]]s (reflectivities) from 0.5 to 0.3 (with the latter being more characteristic of the inner [[Astronomy:Solar System|Solar System]]). Adding an Earthlike [[Earth:Greenhouse effect|greenhouse effect]] would yield an average surface temperature in the range of 236 to 261&nbsp;K (−37 to −12&nbsp;°C, or −35 to 10&nbsp;°F).<ref name="Vogt 2010"/><ref name="Stephens 2010"/> Gliese 581g would be in an orbit where a silicate weathering thermostat could operate, and this could lead to accumulation of sufficient carbon dioxide in the atmosphere to permit liquid water to exist at the surface, provided the planet's composition and tectonic behavior could support sustained outgassing.<ref name="Pierrehumbert 2010"/>

{|   class="wikitable" style="margin-right: 0.5em;" align="left"
! Temperature<br />comparisons
! [[Astronomy:Mercury (planet)|Mercury]]
! [[Astronomy:Venus|Venus]]
! Earth
! align="center" style="background:#add8e6;"| Gliese 581g
! [[Astronomy:Mars|Mars]]
|-
| align="center" |Global<br />equilibrium <br />temperature|| 431 K <br />158&nbsp;°C <br />316&nbsp;°F ||307 K <br />34&nbsp;°C <br />93&nbsp;°F ||255 K <br />−18&nbsp;°C<br />−0.4&nbsp;°F ||align="center" style="background:#e7f6fb;"| 209 K to 228 K<br />−64&nbsp;°C to −45&nbsp;°C<br />−83&nbsp;°F to −49&nbsp;°F||206 K <br />−67&nbsp;°C<br />−88.6&nbsp;°F
|-
| align="center" | Adjusted for 
[[Earth:Greenhouse effect|greenhouse]] 
[[Earth:Greenhouse effect|effect]]
|{{N/a}}||737 K <br />464&nbsp;°C<br />867&nbsp;°F || 288 K <br />15&nbsp;°C<br />59&nbsp;°F|| style="background:#e7f6fb;" align="center" | 236 K to 261 K <br /> −37&nbsp;°C to −12&nbsp;°C <br /> −35&nbsp;°F to 10&nbsp;°F
{{Small|Assumes Earth atmosphere}}
|210 K <br />−63&nbsp;°C<br />−81&nbsp;°F
|-
| align="center" | Tidally <br />locked   || align="center" | 3:2 || align="center" | Almost || align="center" | No || style="background:#e7f6fb;" align="center" | Likely || align="center" | No
|-
| align="center" | Global <br /> [[Physics:Bond albedo|Bond albedo]]   ||  align="center" |  0.142 ||  align="center" |  0.9 ||align="center" | 0.29 ||align="center" style="background:#e7f6fb;"| 0.5 to 0.3 ||align="center" | 0.25
|-
| colspan="6" align="left" | Refs.<ref name="Vogt 2010"/><ref name="Stephens 2010"/><ref name="NASA"/><ref name="Mallama 2006"/><ref name="Mallama 2007"/><ref>[Overview – Mercury – Solar System Exploration: NASA Science https://solarsystem.nasa.gov/planets/mercury/overview/]</ref>
|}

By comparison, Earth's present global equilibrium temperature is 255 K (−18&nbsp;°C), which is raised to 288 K (15&nbsp;°C) by greenhouse effects. However, when life evolved [[Earth:Archean|early in Earth's history]], the Sun's energy output is thought to have been [[Faint young Sun paradox|only about 75%]] of its current value,<ref name="Sagan 1972"/> which would have correspondingly lowered Earth's equilibrium temperature under the same [[Earth:Albedo|albedo]] conditions. Yet Earth maintained equable temperatures in that era, perhaps with a more intense greenhouse effect,<ref name="Pavlov 2000"/> or a lower albedo,<ref name="Rosing 2010"/> than at present.

Current [[Astronomy:Mars#Climate|Martian surface temperatures]] vary from lows of about −87&nbsp;°C (−125&nbsp;°F) during polar winter to highs of up to −5&nbsp;°C (23&nbsp;°F) in summer.<ref name="NASA"/> The wide range is due to the rarefied atmosphere, which cannot store much solar heat, and the low [[Physics:Volumetric heat capacity|thermal inertia]] of the soil.<ref name="MGCM Press release"/> Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars.<ref name="Boyce 2005"/>

== Habitability ==
<!-- linked from redirect Habitability of Gliese 581 g -->
{{Further|Astronomy:Habitability of red dwarf systems|Astronomy:Planetary habitability}}
The planet is thought to be located within the habitable zone of its parent star, a red dwarf, which is cooler than the Sun. That means planets need to orbit closer to the star than in the [[Astronomy:Solar System|Solar System]] to maintain liquid water on their surface. While habitability is generally defined by the planets ability to support liquid water, there are many factors that can influence it. This includes the atmosphere of the planet and the variability of its parent star in terms of emitting energy.<ref name="Howell 2016"/>

In an interview with Lisa-Joy Zgorski of the [[Organization:National Science Foundation|National Science Foundation]], Steven Vogt was asked what he thought about the chances of life existing on Gliese 581g. Vogt was optimistic: {{cquote|I'm not a biologist, nor do I want to play one on TV. Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that... the chances of life on this planet are 100%. I have almost no doubt about it."<ref name="Press Release 10-172 - Video"/> In the same article Dr. Seager is quoted as saying "Everyone is so primed to say here's the next place we're going to find life, but this isn't a good planet for follow-up.<ref name="Press Release 10-172 - Video"/>}}
According to Vogt, the long lifetime of red dwarfs improves the chances of life being present. "It's pretty hard to stop life once you give it the right conditions", he said.<ref name="Borenstein 2010"/> According to the Associated Press interview with Steven Vogt: {{cquote|Life on other planets doesn't mean [[Astronomy:Extraterrestrial life|E.T.]] Even a simple single-cell bacteria or the equivalent of shower mold would shake perceptions about the uniqueness of life on Earth.<ref name="Borenstein 2010"/>}}

== Implications ==
Scientists have monitored only a relatively small number of stars in the search for exoplanets. The discovery of a potentially habitable planet like Gliese 581g so early in the search might mean that habitable planets are more widely distributed than had been previously believed.<ref name="Carnegie Institution for Science 2010"/> According to Vogt, the discovery "implies an interesting lower limit on η{{Sub|⊕}}{{what?|date=January 2022}} as there are only ~116 known solar-type or later stars ... out to the 6.3 parsec distance of GJ 581".<ref name="Vogt 2010"/> This finding foreshadows what Vogt calls a new, second [[Age of Discovery]] in [[Astronomy:Exoplanetology|exoplanetology]]:

{{Quote|Confirmation by other teams through additional high-precision RVs would be most welcome. But if GJ 581g is confirmed by further RV scrutiny, the mere fact that a habitable planet has been detected this soon, around such a nearby star, suggests that η{{Sub|⊕}} could well be on the order of a few tens of percent, and thus that either we have just been incredibly lucky in this early detection, or we are truly on the threshold of a second Age of Discovery.<ref name="Vogt 2010"/>}}

If the fraction of stars with potentially habitable planets (η{{Sub|⊕}}, "[[Astronomy:Drake equation#eta-earth|eta-Earth]]") is on the order of a few tens of percent as Vogt proposes, and the Sun's stellar neighborhood is a typical sample of the galaxy, then the discovery of Gliese 581g in the habitable zone of its star points to the [[Astronomy:Drake equation|potential of billions of Earthlike planets]] in our [[Astronomy:Milky Way|Milky Way]] galaxy alone.<ref name="Berardelli 2010"/>

==See also==
* [[Astronomy:Lists of exoplanets|Lists of exoplanets]]
* [[Astronomy:List of nearest exoplanets|List of nearest exoplanets]]

==References==
{{Reflist|refs=

<ref name="Anglada-Escudé 2010">{{cite arXiv |eprint= 1011.0186 |author1=Guillem Anglada-Escudé |title=Aliases of the first eccentric harmonic : Is GJ 581g a genuine planet candidate? |year=2010|class=astro-ph.EP }}</ref>

<ref name="Andrae 2010">{{cite arXiv |eprint=1012.3754|title=Dos and don'ts of reduced chi-squared|author1=Rene Andrae|author2=Tim Schulze-Hartung|author3=Peter Melchior|year=2010|class=astro-ph.IM}}</ref>

<ref name="Alexander 2010">{{cite web|last1=Alexander|first1=Amir|title=Billions and Billions? Discovery of Habitable Planet Suggests Many More are Out There|url=http://planetary.org/news/2010/1006_Billions_and_Billions_Discovery_of.html|publisher=[[Organization:The Planetary Society|The Planetary Society]]|access-date=October 8, 2010|archive-url=https://web.archive.org/web/20101009211355/http://planetary.org/news/2010/1006_Billions_and_Billions_Discovery_of.html|archive-date=2010-10-09 |url-status=dead}}</ref>

<ref name="Alleyne 2010">{{cite news|url=https://www.telegraph.co.uk/science/space/8033124/Gliese-581g-the-most-Earth-like-planet-yet-discovered.html|title=Gliese 581g the most Earth like planet yet discovered|last=Alleyne|first=Richard|date=September 29, 2010|work=The Daily Telegraph|location=London|archive-url=https://web.archive.org/web/20101002104629/http://www.telegraph.co.uk/science/space/8033124/Gliese-581g-the-most-Earth-like-planet-yet-discovered.html|archive-date=October 2, 2010|url-status=dead|access-date=September 30, 2010}}</ref>

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<ref name="Hatzes2016">{{cite journal |last1=Hatzes |first1=Artie P. |title=Periodic Hα variations in GL 581: Further evidence for an activity origin to GL 581d |journal=Astronomy & Astrophysics |date=January 2016 |volume=585 |pages=A144 |doi=10.1051/0004-6361/201527135 |arxiv=1512.00878 |bibcode=2016A&A...585A.144H |s2cid=55623630 }}</ref>

<ref name="Trifonov2018">{{cite journal |last1=Trifonov |first1=T. |last2=Kürster |first2=M. |display-authors=0 |date=February 2018 |title=The CARMENES search for exoplanets around M dwarfs. First visual-channel radial-velocity measurements and orbital parameter updates of seven M-dwarf planetary systems |journal=Astronomy & Astrophysics |volume=609 |issue= |pages=A117 |doi=10.1051/0004-6361/201731442 |arxiv=1710.01595 |bibcode=2018A&A...609A.117T}}</ref>

<ref name="Dodson-Robinson2022">{{Cite journal |last1=Dodson-Robinson |first1=Sarah E. |last2=Delgado |first2=Victor Ramirez |last3=Harrell |first3=Justin |last4=Haley |first4=Charlotte L. |date=2022-04-01 |title=Magnitude-squared Coherence: A Powerful Tool for Disentangling Doppler Planet Discoveries from Stellar Activity |journal=The Astronomical Journal |volume=163 |issue=4 |pages=169 |doi=10.3847/1538-3881/ac52ed |arxiv=2201.13342 |bibcode=2022AJ....163..169D |s2cid=246430514 |issn=0004-6256 |doi-access=free }}</ref>

}}

==External links==
* {{cite web |date=2010-09-29 |author = National Science Foundation|url = https://www.nsf.gov/news/news_videos.jsp?cntn_id=117765&media_id=68454&org=NSF|title = Steven Vogt and Paul Butler lead a team that discovered the first potentially habitable exoplanet|quote = Video: Steven Vogt of UC Santa Cruz and UC Observatories and Paul Butler of the Carnegie Institution of Washington join NSF's Lisa-Joy Zgorski to announce the discovery of the first exoplanet that has the potential to support life.|}}
* {{cite web |date=2010-09-29 |url=http://www.nasa.gov/home/hqnews/2010/sep/HQ_10-237_Exoplanet_Findings.html |title=NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet |work=Release 10-237 |publisher=[[Organization:NASA|NASA]]}}

{{Extraterrestrial life}}
{{Sky|15|19|27|-|07|43|19|20.2}}
{{DEFAULTSORT:Gliese 581G}}
[[Category:Exoplanet candidates]]

{{Sourceattribution|Gliese 581g}}