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Gliese 581g , unofficially known as Zarmina (or Zarmina's World), was a candidate
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
postulated to orbit within the Gliese 581 system, twenty light-years from Earth. It was discovered by the
Lick–Carnegie Exoplanet Survey The Lick–Carnegie Exoplanet Survey (LCES) is a search for exoplanets using the Keck I optical telescope of the W. M. Keck Observatory in Hawaii. The survey is sponsored by NASA and the National Science Foundation. The survey comprises a decade o ...
, and was the sixth
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
claimed to orbit the
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
; however, its existence could not be confirmed by the
European Southern Observatory The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental research organisation made up of 16 member states for ground-based ast ...
(ESO) /
High Accuracy Radial Velocity Planet Searcher The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
(HARPS) survey team, and was ultimately refuted. It was thought to be near the middle of the
habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kast ...
of its
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
, meaning it could sustain liquid water—a necessity for all known
life Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy ...
—on its surface, if there are favorable
atmospheric An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
conditions on the planet. Gliese 581g was claimed to be detected by astronomers of the
Lick–Carnegie Exoplanet Survey The Lick–Carnegie Exoplanet Survey (LCES) is a search for exoplanets using the Keck I optical telescope of the W. M. Keck Observatory in Hawaii. The survey is sponsored by NASA and the National Science Foundation. The survey comprises a decade o ...
. The authors stated that data sets from both the
High Resolution Echelle Spectrometer The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when comp ...
(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 A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a youn ...
, but the discoverer, Steven S. Vogt, did not agree with those conclusions. In 2012, a reanalysis by Vogt supported its existence. A new study in 2014 concluded that it was a false positive, a conclusion which has been further confirmed by subsequent studies. The planet was thought to be tidally locked to its star. If the planet has a dense
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
, it may be able to circulate heat. The actual habitability of the planet depends on the composition of its surface and the atmosphere. It was thought to have
temperatures Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...
around −37 to −11 °C (−35 to 10 °F). By comparison,
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
has an average surface temperature of 15 °C (59 °F)—while
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
has an average surface temperature of about −63 °C (−81 °F). The planet was said by Vogt to have a "100%" chance of supporting life. The supposed detection of Gliese 581g was said to foreshadow what Vogt called "a second Age of Discovery".


History


Discovery

The planet's discovery was claimed in September 2010, to have been detected by astronomers in the
Lick–Carnegie Exoplanet Survey The Lick–Carnegie Exoplanet Survey (LCES) is a search for exoplanets using the Keck I optical telescope of the W. M. Keck Observatory in Hawaii. The survey is sponsored by NASA and the National Science Foundation. The survey comprises a decade o ...
, led by principal investigator
Steven Vogt Steven Scott Vogt (born December 20, 1949) is an American astronomer of German descent whose main interest is the search for extrasolar planets. He is credited, along with R. Paul Butler, for discovering Gliese 581 g, the first potentially ...
, professor of astronomy and astrophysics at the
University of California, Santa Cruz The University of California, Santa Cruz (UC Santa Cruz or UCSC) is a public land-grant research university in Santa Cruz, California. It is one of the ten campuses in the University of California system. Located on Monterey Bay, on the ed ...
, and co-investigator R. Paul Butler of the
Carnegie Institution of Washington The Carnegie Institution of Washington (the organization's legal name), known also for public purposes as the Carnegie Institution for Science (CIS), is an organization in the United States established to fund and perform scientific research. Th ...
. The discovery was made using radial velocity measurements, combining 122 observations obtained over 11 years from the
HIRES Hires may refer to: * High Resolution Fly's Eye, ultra-high-energy cosmic ray observatory * High Resolution Echelle Spectrometer or High Resolution Echelle Spectrograph, W. M. Keck Observatory's spectrometer * Hires Big H, restaurant chain headquar ...
instrument of the W. M. Keck Observatory with 119 measurements obtained over 4.3 years from the
HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
instrument of the ESO 3.6 m Telescope at
La Silla Observatory La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory (ESO). Several other telescopes are located at the site and are partly maintained by ESO. The observatory is ...
. In addition, brightness measurements of the star were confirmed with a robotic telescope from
Tennessee State University Tennessee State University (Tennessee State, Tenn State, or TSU) is a public historically black land-grant university in Nashville, Tennessee, United States. Founded in 1912, it is the only state-funded historically black university in Tenness ...
. After subtracting the signals of the previously known Gliese 581 planets, '' b'', '' c'', '' d'' and '' e'', the signals of two additional planets were apparent: a 445-day signal from a newly recognized outermost planet designated '' f'', and the 37-day signal from Gliese 581g. The probability that the detection of the latter was
spurious Spurious may refer to: * Spurious relationship in statistics * Spurious emission or spurious tone in radio engineering * Spurious key in cryptography * Spurious interrupt in computing * Spurious wakeup in computing * ''Spurious'', a 2011 novel b ...
was estimated at only 2.7 in a million. 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". The Lick–Carnegie team explained the results of their research in a paper published in the ''
Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and Jam ...
'', which were also made available in
preprint In academic publishing, a preprint is a version of a scholarly or scientific paper that precedes formal peer review and publication in a peer-reviewed scholarly or scientific journal. The preprint may be available, often as a non-typeset versi ...
version on
arXiv arXiv (pronounced "archive"—the X represents the Greek letter chi ⟨χ⟩) is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation, but not peer review. It consists of ...
. Although not sanctioned by the IAU's naming conventions, Vogt's team informally refers to the planet as "Zarmina's World" after his wife, 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.


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 The Geneva Observatory (french: Observatoire de Genève, german: Observatorium von Genf) is an astronomical observatory at Sauverny (CH) in the municipality of Versoix, Canton of Geneva, in Switzerland. It shares its buildings with the astronomy d ...
—reported that in a new analysis of 179 measurements taken by the
HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
spectrograph over 6.5 years, neither planet ''g'' nor planet ''f'' was detectable. An astronomer who works on HARPS data at the Geneva Observatory,
Francesco Pepe Francesco, the Italian (and original) version of the personal name " Francis", is the most common given name among males in Italy. Notable persons with that name include: People with the given name Francesco * Francesco I (disambiguation), sev ...
, 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". The Geneva team had also published their paper on
arXiv arXiv (pronounced "archive"—the X represents the Greek letter chi ⟨χ⟩) is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation, but not peer review. It consists of ...
, but it appeared to not have been accepted for publication. 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." 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." Differences in the two groups' results may involve the planetary orbital characteristics assumed in calculations. According to
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of th ...
astronomer
Sara Seager Sara Seager (born 21 July 1971) is a Canadian-American astronomer and planetary scientist. She is a professor at the Massachusetts Institute of Technology and is known for her work on extrasolar planets and their atmospheres. She is the au ...
, Vogt postulated the planets around Gliese 581 had perfectly circular orbits whereas the Swiss group thought the orbits were more
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
. This difference in approach may be the reason for the disagreement, according to
Alan Boss Alan P. Boss (born in Lakewood, Ohio) is a United States astrophysicist and planetary scientist. Life and career Educated at the University of South Florida and the University of California, Santa Barbara, Boss is a prominent scientist in stellar ...
. Butler remarked that with additional observations, "I would expect that on the time scale of a year or two this should be settled." 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 Ray Jayawardhana is the Harold Tanner Dean of the Cornell University College of Arts and Sciences and a Professor of Astronomy at Cornell University, effective September 1, 2018. He was formerly Dean of Science and a Professor of physics & astron ...
noted, "Given the extremely interesting implications of such a discovery, it's important to have independent confirmation." Gliese 581g is listed as "retracted" in the ''
Extrasolar Planets Encyclopaedia The Extrasolar Planets Encyclopaedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with in ...
''.


Further analyses of HIRES/HARPS data

In December 2010, a claimed methodological error was reported—by a group led by Rene Andrae of the Max Planck Institute for Astronomy—in the data analysis that led to the discovery of Gliese 581f and g. In 2011, another reanalysis—performed by a group led by Philip Gregory of the
University of British Columbia The University of British Columbia (UBC) is a public research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks among the top thr ...
—found no clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set. 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). Mikko Tuomi of the
University of Hertfordshire The University of Hertfordshire (UH) is a public university in Hertfordshire, United Kingdom. The university is based largely in Hatfield, Hertfordshire. Its antecedent institution, Hatfield Technical College, was founded in 1948 and was ident ...
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". "I have studied
he paper He or HE may refer to: Language * He (pronoun), an English pronoun * He (kana), the romanization of the Japanese kana へ * He (letter), the fifth letter of many Semitic alphabets * He (Cyrillic), a letter of the Cyrillic script called ''He'' ...
in detail and do not agree with his conclusions," Steven Vogt said in reply, concerned that Gregory has considered the HIRES data as more uncertain. "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. By performing a number of statistical tests, 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. 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."


2012 reanalysis of HARPS data

In July 2012, Vogt reanalyzed the 2011 data proposed by Forveille 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 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." 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)."


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. A study in 2014—published in ''Science''— led by postdoctoral researcher Paul Robertson concluded that Gliese 581d is "an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g." "They were very high value targets if they were real," Robertson said, "But unfortunately we found out that they weren't." It was pointed out—during a press release by
Penn State University The Pennsylvania State University (Penn State or PSU) is a Public university, public Commonwealth System of Higher Education, state-related Land-grant university, land-grant research university with campuses and facilities throughout Pennsylvan ...
—that sunspots could sometimes masquerade as planetary signals. An additional study concluded that Gliese 581g's existence depends on Gliese 581d's eccentricity. The planet was later delisted from the Habitable Exoplanets Catalog, which is run by the
University of Puerto Rico at Arecibo The University of Puerto Rico at Arecibo (UPRA or UPR Arecibo) is a public college in Arecibo, Puerto Rico. It is part of the University of Puerto Rico. UPR-Arecibo was previously the (CRA, 'Arecibo Regional College') and (CUTA, 'Arecibo Techno ...
. Later, in October that year, Abel Mendez wrote—in a blog post characterizing "false starts" in exoplanet habitability— that the planet does not exist. In 2015, a pair of researchers led by Guillem Anglada-Escudé of the
University of London The University of London (UoL; abbreviated as Lond or more rarely Londin in post-nominals) is a federal public research university located in London, England, United Kingdom. The university was established by royal charter in 1836 as a degr ...
questioned the methodology of the 2014 study and suggested planet 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." 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. Most further studies have confirmed the stellar, rather than planetary, origin of the signal corresponding to Gliese 581d, and consequently Gliese 581g, with one such study explicitly refuting g.


Physical characteristics


Tidal locking

Because of Gliese 581g's proximity to its parent star, it is predicted to be tidally locked to Gliese 581. Just as Earth's
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
always presents the same face to the Earth, the length of Gliese 581g's
sidereal day Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coor ...
would then precisely match the length of its year, meaning it would be permanently light on one half and permanently dark on the other half of its surface.


Atmosphere

An atmosphere that is dense will circulate heat, potentially allowing a wide area on the surface to be habitable. For example,
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
has a 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. Simulations have shown that an atmosphere containing appropriate levels of CO2 and H2O need only be a tenth the pressure of Earth's atmosphere (100
mbar The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000  Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea l ...
) to effectively distribute heat to the night side. Current technology cannot determine the atmospheric or surface composition of the planet due to the overpowering light of its parent star. 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 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. Modeling of the effect of tidal locking on Gliese 581g's possible atmosphere, using a
general circulation model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic ter ...
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
chevron Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...
-shaped pattern centered near the substellar point.


Climate

It is estimated that the average global
equilibrium temperature The planetary equilibrium temperature is a theoretical temperature that a planet would be if it were a black body being heated only by its parent star. In this model, the presence or absence of an atmosphere (and therefore any greenhouse effect) is ...
(the temperature in the absence of atmospheric effects) of Gliese 581g would range from 209 to 228  K (−64 to −45 °C, or −84 to −49 °F) for
Bond albedo The Bond albedo (or ''spheric albedo'' or ''planetary albedo'' or ''bolometric albedo''), named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic ra ...
s (reflectivities) from 0.5 to 0.3 (with the latter being more characteristic of the inner
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
). Adding an Earthlike
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
would yield an average surface temperature in the range of 236 to 261 K (−37 to −12 °C, or −35 to 10 °F). 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. By comparison, Earth's present global equilibrium temperature is 255 K (−18 °C), which is raised to 288 K (15 °C) by greenhouse effects. However, when life evolved early in Earth's history, the Sun's energy output is thought to have been only about 75% of its current value, which would have correspondingly lowered Earth's equilibrium temperature under the same
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
conditions. Yet Earth maintained equable temperatures in that era, perhaps with a more intense greenhouse effect, or a lower albedo, than at present. Current Martian surface temperatures vary from lows of about −87 °C (−125 °F) during polar winter to highs of up to −5 °C (23 °F) in summer. The wide range is due to the rarefied atmosphere, which cannot store much solar heat, and the low
thermal inertia In thermodynamics, a material's thermal effusivity, thermal inertia or thermal responsivity is a measure of its ability to exchange thermal energy with its surroundings. It is defined as the square root of the product of the material's thermal ...
of the soil. Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars.


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
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
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. In an interview with Lisa-Joy Zgorski of the
National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581g. Vogt was optimistic: 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. According to the Associated Press interview with Steven Vogt:


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. According to Vogt, the discovery "implies an interesting lower limit on η as there are only ~116 known solar-type or later stars ... out to the 6.3 parsec distance of GJ 581". This finding foreshadows what Vogt calls a new, second
Age of Discovery The Age of Discovery (or the Age of Exploration), also known as the early modern period, was a period largely overlapping with the Age of Sail, approximately from the 15th century to the 17th century in European history, during which seafa ...
in
exoplanetology This page describes exoplanet orbital and physical parameters. Orbital parameters Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determi ...
: If the fraction of stars with potentially habitable planets (η, " 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 potential of billions of Earthlike planets in our
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
galaxy alone.


See also

*
Lists of exoplanets These are lists of exoplanets. Most of these were discovered by the Kepler space telescope. There are an additional 2,054 potential exoplanets from Kepler's first mission yet to be confirmed, as well as 978 from its " Second Light" mission and ...
*
List of nearest exoplanets There are known exoplanets, or planets outside the Solar System that orbit a star, as of ; only a small fraction of these are located in the vicinity of the Solar System. Within , there are 97 exoplanets listed as confirmed by the NASA Exoplan ...


References


External links

* * {{DEFAULTSORT:Gliese 581G Exoplanet candidates Gliese 581