Gliese 581 g
Encyclopedia
Gliese 581 g also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet
proven nonexistent by the Geneva Team, orbiting the red dwarf
star Gliese 581
, 20.5 light-years from Earth in the constellation
of Libra
. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth in order of increasing distance from the star. The discovery was announced by the Lick-Carnegie Exoplanet Survey
in late September 2010, after a decade of observation.
Studies indicate the planet is situated near the middle of the habitable zone
of its parent star, where temperatures are neither too hot nor too cold to sustain Earth-like life (it is expected to be around 20 Farenheit degrees cooler than Earth, however). If it is a rocky planet
, favorable atmospheric conditions could permit the presence of liquid water, a necessity for all known life, on its surface. With a mass 3.1 to 4.3 times Earth's, Gliese 581 g is considered a super-Earth
and is the planet closest in size to Earth known in a habitable zone. This makes it the most Earth-like Goldilocks planet found outside the Solar System
and the exoplanet with the greatest recognized potential for harboring life.
The detection of Gliese 581 g after such a short period of searching
and at such close proximity has led some astronomers to hypothesize that the proportion of stars with habitable planets may be greater than ten percent.
, led by principal investigator Steven S. Vogt
, professor of astronomy and astrophysics at the University of California, Santa Cruz
and co-investigator R. Paul Butler
of the Carnegie Institution of Washington. The discovery was made using radial velocity measurements
, combining 122 observations obtained over 11 years from the High Resolution Echelle Spectrometer (HIRES) instrument of the Keck 1 telescope with 119 measurements obtained over 4.3 years from the High Accuracy Radial Velocity Planet Searcher
(HARPS) instrument of the European Southern Observatory
's 3.6 m telescope at La Silla Observatory
.
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 581 g. The probability that the detection of the latter was spurious 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
. 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 some cases simply as Zarmina.
Differences in the two groups' results may involve the planetary orbital characteristics assumed in calculations. According to MIT astronomer Sara Seager
, Vogt postulated the planets around Gliese 581 had perfectly circular orbits whereas the Swiss group thought the orbits were more eccentric
. This difference in approach may be the reason for the disagreement, according to Alan Boss
. 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." and Ray Jayawardhana
noted, "Given the extremely interesting implications of such a discovery, it's important to have independent confirmation." Gliese 581 g is listed as "unconfirmed" in the Extrasolar Planets Encyclopaedia
.
"I have studied [the paper] 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.
By performing a number of statistical tests, Guillen Anglada-Escude of the Carnegie Institute of Washington concluded that the existence of Gl 581 g was well supported by the available data. This 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 forthcoming paper, Anglada-Escude and Rebekah Dawson claim “With the data we have, the most likely explanation is that this planet is still there.”
Mikko Tuomi of the University of Turku performed a Bayesian re-analysis 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".
from its parent star. It is believed to have a mass of 3.1 to 4.3 times Earth's and a radius of 1.3 to 2.0 times Earth's (1.3 to 1.5 times larger if predominantly rocky, 1.7 to 2.0 times larger if predominantly water ice). Its mass indicates it is probably a rocky planet with a solid surface. The planet's surface gravity
is expected to be in the range of 1.1 to 1.7 times Earth's, enough to hold on to an atmosphere likely to be denser than Earth's.
to Gliese 581. Just as Earth's Moon
always presents the same face to the Earth, the length of Gliese 581 g's sidereal day 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. Tidal locking also means the planet would have no axial tilt
and therefore no seasonality
in a conventional sense.
With one side of the planet always facing the star, temperatures could range from blazing hot in the bright side to freezing cold in the dark side if atmospheric heat transport is limited. The atmosphere's inventory of volatile compound
s such as water and carbon dioxide
could then permanently freeze out on the dark side. However, an atmosphere of the expected density would be likely to moderate these extremes.
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 greenhouse and need only be a tenth the pressure of Earth's atmosphere (100 mb
) 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 tide-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 581 g's possible atmosphere, using a 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 chevron-shaped pattern centered near the substellar point.
(−64 to −45 °C, or −84 to −49 °F) for Bond albedo
s (reflectivities) from 0.5 to 0.3 (with the latter being more characteristic of the inner Solar System
). Adding an Earth-like greenhouse effect
yields an average surface temperature in the range of 236 to 261 K (−37 to −12 °C, or −35 to 10 °F). Gliese 581g is in an orbit where a silicate weathering thermostat can operate, and this can 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 can 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
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
of the soil. Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars
.
Two previously discovered planets in the same system, Gliese 581 c
and d
(inward and outward from planet g, respectively), were also regarded as potentially habitable following their discovery. Both were later evaluated as being outside the conservatively defined habitable zone, leading Vogt et al. to remark that "The GJ 581 system has a somewhat checkered history of habitable planet claims". However, a subsequent downward revision of the period of planet d from 83 to 67 days has bolstered its habitability prospects, although a large greenhouse effect would be needed.
, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581 g. Vogt was optimistic: "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, my own personal feeling is that the chances of life on this planet are 100%. I have almost no doubt about it." 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."
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. "Life on other planets doesn't mean E.T. Even a simple single-cell bacteria or the equivalent of shower mold
would shake perceptions about the uniqueness of life on Earth."
in exoplanetology
:
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 581 g in the habitable zone of its star points to the potential of billions of Earth-like planets
in our Milky Way
galaxy alone.
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
proven nonexistent by the Geneva Team, orbiting the red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
star Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...
, 20.5 light-years from Earth in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Libra
Libra (constellation)
Libra is a constellation of the zodiac. Its name is Latin for weighing scales, and its symbol is . It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east.-Notable features:]...
. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth in order of increasing distance from the star. The discovery was announced by the Lick-Carnegie Exoplanet Survey
Lick-Carnegie Exoplanet Survey
The Lick–Carnegie Exoplanet Survey 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 of observations. The survey is led by Steven Vogt, professor...
in late September 2010, after a decade of observation.
Studies indicate the planet is situated near the middle of the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
of its parent star, where temperatures are neither too hot nor too cold to sustain Earth-like life (it is expected to be around 20 Farenheit degrees cooler than Earth, however). If it is a rocky planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
, favorable atmospheric conditions could permit the presence of liquid water, a necessity for all known life, on its surface. With a mass 3.1 to 4.3 times Earth's, Gliese 581 g is considered a super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...
and is the planet closest in size to Earth known in a habitable zone. This makes it the most Earth-like Goldilocks planet found outside the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
and the exoplanet with the greatest recognized potential for harboring life.
The detection of Gliese 581 g after such a short period of searching
Discoveries of extrasolar planets
An extrasolar planet is a planet located outside the Solar System. As of , astronomers have announced confirmed detections of such planets. This is a list of the most notable discoveries.-1988–1992:...
and at such close proximity has led some astronomers to hypothesize that the proportion of stars with habitable planets may be greater than ten percent.
Discovery
The planet was detected by astronomers in the Lick-Carnegie Exoplanet SurveyLick-Carnegie Exoplanet Survey
The Lick–Carnegie Exoplanet Survey 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 of observations. The survey is led by Steven Vogt, professor...
, led by principal investigator Steven S. Vogt
Steven S. Vogt
Steven Scott Vogt is an American astronomer 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 habitable planet outside of our own solar system....
, professor of astronomy and astrophysics at the University of California, Santa Cruz
University of California, Santa Cruz
The University of California, Santa Cruz, also known as UC Santa Cruz or UCSC, is a public, collegiate university; one of ten campuses in the University of California...
and co-investigator R. Paul Butler
R. Paul Butler
R. Paul Butler is an astronomer who searches for extrasolar planets.He received a BA and an MS from San Francisco State University, completing a Master's thesis with Geoffrey Marcy, and then completed his doctoral studies at the University of Maryland, College Park in 1993...
of the Carnegie Institution of Washington. The discovery was made using radial velocity measurements
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
, combining 122 observations obtained over 11 years from the High Resolution Echelle Spectrometer (HIRES) instrument of the Keck 1 telescope with 119 measurements obtained over 4.3 years from the High Accuracy Radial Velocity Planet Searcher
High Accuracy Radial Velocity Planet Searcher
The High Accuracy Radial velocity Planet Searcher is a high-precision echelle spectrograph installed in 2002 on ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003...
(HARPS) instrument of the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
's 3.6 m telescope at La Silla Observatory
La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory organisation, and several others are partly maintained by ESO...
.
After subtracting the signals of the previously known Gliese 581 planets, b
Gliese 581 b
Gliese 581 b or Gl 581 b is an extrasolar planet orbiting the star Gliese 581. It is the first planet of six discovered in the system so far, and the second in order from the star.- Discovery :...
, c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...
, d
Gliese 581 d
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....
and e
Gliese 581 e
Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra...
, the signals of two additional planets were apparent: a 445-day signal from a newly recognized outermost planet designated f
Gliese 581 f
Gliese 581 f or Gl 581 f is an unconfirmed planet in the Gliese 581 system, located 20 light years from Earth. It is the fifth planet discovered in the system and the sixth known in order from the star. Its discovery was announced September 29, 2010...
, and the 37-day signal from Gliese 581 g. The probability that the detection of the latter was spurious 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
Astrophysical Journal
The Astrophysical Journal is a peer-reviewed scientific journal covering astronomy and astrophysics. It was founded in 1895 by the American astronomers George Ellery Hale and James Edward Keeler. It publishes three 500-page issues per month....
. Although not sanctioned by the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
's naming conventions, Vogt's team informally refers to the planet as "Zarmina's World" after his wife, and some cases simply as Zarmina.
Nondetection in new HARPS data analysis
Two weeks after the announcement of the discovery of Gliese 581 g, astronomer Francesco Pepe of the Geneva Observatory reported that in a new analysis of 179 measurements taken by the HARPS spectrograph over 6.5 years, neither planet g nor planet f was detectable. 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." He cautioned that not finding the planet in this study does not make a strong case for it not existing, because both data sets may be needed to detect it. 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 MIT astronomer Sara Seager
Sara Seager
Sara Seager is a Canadian-American astronomer who is currently a professor at the Massachusetts Institute of Technology and known for her work on extrasolar planets. She was born in Toronto, Canada. In 1994, she earned the degree of Bachelor of Science in Mathematics and Physics from the...
, Vogt postulated the planets around Gliese 581 had perfectly circular orbits whereas the Swiss group thought the orbits were more eccentric
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
. This difference in approach may be the reason for the disagreement, according to Alan Boss
Alan Boss
Alan P. Boss is a United States astrophysicist and NASA 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 and planetary system formation and the study of extrasolar planets who has made...
. 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." and Ray Jayawardhana
Ray Jayawardhana
Ray Jayawardhana is an astronomer at the University of Toronto, where he holds the Canada Research Chair in Observational Astrophysics, and an award-winning science writer...
noted, "Given the extremely interesting implications of such a discovery, it's important to have independent confirmation." Gliese 581 g is listed as "unconfirmed" in the Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia 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 individual "note" pages for each planet and a full list...
.
Erroneous data analysis
In December 2010, a methodological error has been revealed in the data analysis that led to the "discovery" of Gliese 581 f and g. The team around Steven Vogt inferred the number of exoplanets by using a reduced chi-square, although the orbital models are nonlinear in the model parameters. Therefore, reduced chi-square is not a trustworthy diagnostic. In fact, an investigation of the fit residuals showed that the data used by Vogt's team actually prefers a model with four planets, not six, in agreement with the results of Francesco Pepe's team.Further analyses of HIRES/HARPS data
Another re-analysis found no clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set. The claim was made that the HARPS data only provided 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 581 f or 581 g)."I have studied [the paper] 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.
By performing a number of statistical tests, Guillen Anglada-Escude of the Carnegie Institute of Washington concluded that the existence of Gl 581 g was well supported by the available data. This 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 forthcoming paper, Anglada-Escude and Rebekah Dawson claim “With the data we have, the most likely explanation is that this planet is still there.”
Mikko Tuomi of the University of Turku performed a Bayesian re-analysis 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".
Physical characteristics
Gliese 581 g has an orbital period of 37 days, orbiting at a distance of 0.146 AUAstronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
from its parent star. It is believed to have a mass of 3.1 to 4.3 times Earth's and a radius of 1.3 to 2.0 times Earth's (1.3 to 1.5 times larger if predominantly rocky, 1.7 to 2.0 times larger if predominantly water ice). Its mass indicates it is probably a rocky planet with a solid surface. The planet's surface gravity
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
is expected to be in the range of 1.1 to 1.7 times Earth's, enough to hold on to an atmosphere likely to be denser than Earth's.
Tidal locking and habitability
Because of Gliese 581 g's proximity to its parent star, it is predicted to be tidally lockedTidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
to Gliese 581. Just as Earth's Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
always presents the same face to the Earth, the length of Gliese 581 g's sidereal day 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. Tidal locking also means the planet would have no axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
and therefore no seasonality
Seasonality
In statistics, many time series exhibit cyclic variation known as seasonality, periodic variation, or periodic fluctuations. This variation can be either regular or semi regular....
in a conventional sense.
With one side of the planet always facing the star, temperatures could range from blazing hot in the bright side to freezing cold in the dark side if atmospheric heat transport is limited. The atmosphere's inventory of volatile compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s such as water and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
could then permanently freeze out on the dark side. However, an atmosphere of the expected density would be likely to moderate these extremes.
Atmospheric effects
An atmosphere that is dense will circulate heat, potentially allowing a wide area on the surface to be habitable. For example, VenusVenus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
has a solar
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...
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 greenhouse and need only be a tenth the pressure of Earth's atmosphere (100 mb
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
) 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 tide-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 581 g's possible atmosphere, using a 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 chevron-shaped pattern centered near the substellar point.
Temperatures
It is estimated that the average global equilibrium temperature (the temperature in the absence of atmospheric effects) of Gliese 581 g ranges from 209 to 228 KKelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(−64 to −45 °C, or −84 to −49 °F) for Bond albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...
s (reflectivities) from 0.5 to 0.3 (with the latter being more characteristic of the inner Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
). Adding an Earth-like greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
yields an average surface temperature in the range of 236 to 261 K (−37 to −12 °C, or −35 to 10 °F). Gliese 581g is in an orbit where a silicate weathering thermostat can operate, and this can 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 can support sustained outgassing.
| Temperature comparisons |
Venus Venus Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows... |
Earth | Gliese 581 g | Mars Mars Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance... |
| Global equilibrium temperature |
307 K 34 °C 93 °F |
255 K −18 °C −0.4 °F |
209 K to 228 K −64 °C to −45 °C −83 °F to −49 °F |
206 K −67 °C −88.6 °F |
| + Venus' GHG Greenhouse gas A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone... effect |
737 K 464 °C 867 °F |
|||
| + Earth's GHG effect |
288 K 15 °C 59 °F |
236 K to 261 K −37 °C to −12 °C −35 °F to 10 °F |
||
| + Mars' GHG effect |
210 K −63 °C −81 °F |
|||
| Tidally locked |
Almost | No | Probably | No |
| Global Bond Albedo Bond albedo The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space... |
0.9 | 0.29 | 0.5 to 0.3 | 0.25 |
| Refs. | ||||
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
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
, the Sun's energy output is thought to have been only about 75%
Faint young Sun paradox
The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised...
of its current value, which would have correspondingly lowered Earth's equilibrium temperature under the same albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
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
Volumetric heat capacity
Volumetric heat capacity , also termed volume-specific heat capacity, describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase change...
of the soil. Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars
Mars Ocean Hypothesis
The Mars Ocean Hypothesis states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet’s geologic history....
.
Two previously discovered planets in the same system, Gliese 581 c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...
and d
Gliese 581 d
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....
(inward and outward from planet g, respectively), were also regarded as potentially habitable following their discovery. Both were later evaluated as being outside the conservatively defined habitable zone, leading Vogt et al. to remark that "The GJ 581 system has a somewhat checkered history of habitable planet claims". However, a subsequent downward revision of the period of planet d from 83 to 67 days has bolstered its habitability prospects, although a large greenhouse effect would be needed.
Potential for life
In an interview with Lisa-Joy Zgorski of the National Science FoundationNational Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581 g. Vogt was optimistic: "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, my own personal feeling is that the chances of life on this planet are 100%. I have almost no doubt about it." 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."
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. "Life on other planets doesn't mean E.T. Even a simple single-cell bacteria or the equivalent of shower mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...
would shake perceptions about the uniqueness of life on Earth."
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 581 g 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 the fraction of stars that have at least one potentially habitable planet as there are only ~116 known solar-type or later stars out to the 6.3 parsec distance of Gliese 581." This finding foreshadows what Vogt calls a new, second Age of DiscoveryAge of Discovery
The Age of Discovery, also known as the Age of Exploration and the Great Navigations , was a period in history starting in the early 15th century and continuing into the early 17th century during which Europeans engaged in intensive exploration of the world, establishing direct contacts with...
in exoplanetology
Exoplanetology
Exoplanetology, or exoplanetary science, is an integrated science related to the study of extrasolar planets or exoplanets. It employs an interdisciplinary approach which includes astrobiology, astrophysics, astronomy, planetary science , geochemistry, astrochemistry and astrogeology.Exoplanetology...
:
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 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.
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 581 g in the habitable zone of its star points to the potential of billions of Earth-like planets
Drake equation
The Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...
in our Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy alone.
See also
- Habitability and climate of Gliese 581 c
- Habitability of red dwarf systemsHabitability of red dwarf systemsDetermining the habitability of red dwarf systems could help reveal the likelihood of extraterrestrial life, as red dwarfs make up most stars in the Milky Way Galaxy...
- List of extrasolar planets
- Polar nightPolar nightThe polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.-Description:...
/ TwilightTwilightTwilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...
/ Midnight sunMidnight sunThe midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...