| abstract
| - The Gliese 581 planetary system with circular orbits, including unconfirmed planet g but not unconfirmed outermost planet f. Parent star Star Gliese 581 Constellation Libra Right ascension (α) 15h 19m 26s Declination (δ) −07° 43′ 20″ Apparent magnitude (mV) 10.55 Distance 20.3 ± 0.3 ly (6.2 ± 0.1 pc) Spectral type M3V Mass (m) 0.31 M☉ Radius (r) 0.29 R☉ Temperature (T) 3480 ± 48 K Metallicity [Fe/H] −0.33 ± 0.12 Age 7 – 11 Gyr Orbital elements Epoch JD 2451409.762[1] Semimajor axis (a) 0.14601 ± 0.00014[1] AU Eccentricity (e) 0[1] Orbital period (P) 36.562 ± 0.052[1] d (0.100 y) (877 h) Mean anomaly (M) 271 ± 48[1]° Semi-amplitude (K) 1.29 ± 0.19[1] m/s Physical characteristics Minimum mass (m sin i) 3.1 ± 0.4[1] M⊕ Discovery information Discovery date September 29, 2010 Discoverer(s) Steven S. Vogt et al. Detection method Radial Velocity Discovery site Keck Observatory, Hawaii Discovery status Unconfirmed[2] Database references Extrasolar Planets Encyclopaedia data SIMBAD data Gliese 581 g (pronounced /ˈɡliːzə/), also Gl 581 g or GJ 581 g, is an unconfirmed extrasolar planet, orbiting the red dwarf star Gliese 581, 20.5 light-years (1.94×1014 km) 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 after a decade of observation in late September 2010. Studies indicate that the planet is situated near the middle of the habitable zone of its parent star, where temperatures are neither too hot nor too cold. 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.[3] The detection of Gliese 581 g after such a short period of searching and at such close proximity leads astronomers to believe that the proportion of stars with habitable planets may be greater than ten percent.[1]Contents 1 Discovery 1.1 Nondetection in new HARPS data analysis 2 Physical characteristics 3 Habitability 3.1 Atmospheric effects 3.2 Temperatures 3.3 Potential for life 4 Implications 5 See also 6 Notes and references 7 External links Discovery The planet was detected by astronomers in the Lick-Carnegie Exoplanet Survey, led by principal investigator Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz and co-investigator 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.[1][4] 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.[1][5] The probability that the detection of the latter was spurious was estimated at only 2.7 in a million.[1] The authors stated that the 37-day signal is "clearly visible in the HIRES data set alone" but that "the HARPS data set alone is not able to reliably sense this planet" and concluded that "It is really necessary to combine both data sets to sense all these planets reliably."[1] The Lick–Carnegie team explained the results of their research in a paper published in the Astrophysical Journal. Vogt informally named the planet "Zarmina's World" after his wife.[6] 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.[7][8] 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 because both data sets may be needed to detect the planet, non-detection in this study does not make a strong case for the planet not existing.[9] 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."[10] Differences in the two groups' results may involve the planetary orbital characteristics assumed in calculations. According to MIT astronomer Sara Seager, Vogt postulated that the planets around Gliese 581 had perfectly circuluar orbits whereas the Swiss group thought the orbits were more elongated.[11] This difference in approach may be the reason for the disagreement, according to Alan Boss.[11] Butler remarked that with additional observations, "I would expect that on the time scale of a year or two this should be settled."[7] 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.”[11] Gliese 581 g is listed as "unconfirmed" in the Extrasolar Planets Encyclopaedia.[12] Physical characteristics Gliese 581 g has an orbital period of 37 days, orbiting at a distance of 0.146 AU from its parent star.[1] It is believed to have a mass of 3.1 to 4.3 times that of the Earth and a radius of 1.3 to 2.0 times that of Earth (1.3 to 1.5 times Earth's if predominantly rocky, 1.7 to 2.0 times Earth's if predominantly water ice). Its mass indicates that 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 that is likely to be denser than Earth's.[1] Habitability Main article: Habitability of red dwarf systems Because of Gliese 581 g's proximity to its parent star, it is predicted to be tidally locked to Gliese 581. Just as the 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 that it would be permanently light on one half and permanently dark on the other half of its surface.[1][13] 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 light side to freezing cold in the dark side if atmospheric heat transport is limited. The atmosphere's inventory of volatile compounds such as water and carbon dioxide could then permanently freeze out on the dark side. However, the atmosphere is expected to be thick enough to avoid these extremes. Atmospheric effects Planetary orbits in the Gliese 581 system compared to those of our own Solar System ("g" designates Gliese 581g) An atmosphere as thick as that believed to exist on Gliese 581 g will circulate heat, potentially allowing a wide area on the surface to be habitable.[14] For example, Venus 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.[15] Simulations have shown that an atmosphere containing appropriate levels of greenhouse CO2 and H2O need only be a tenth the pressure of Earth's atmosphere (100 mb) to effectively distribute heat to the night side.[16] Current technology cannot determine the atmospheric or surface composition of the planet due to the overpowering light of its parent star.[17] The greater mass of Gliese 581 g would tend to compress its atmosphere (i.e., reduce its scale height) relative to Earth's. 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 K (−64 to −45 °C, or −84 to −49 °F) for Bond albedos (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).[1][18] A factor that could potentially give Gliese 581 g a greenhouse effect greater than Earth's is the possibility that the more massive planet also has a more massive atmosphere.[1]Temperature Comparisons Venus Earth Gliese 581 g Mars Global Equilibrium Temperature 307 K (34 °C) 255 K (−18 °C) 209 K (−64 °C) to 228 K (−45 °C) 206 K (−67 °C) + Venus' GHG effect 737 K (464 °C) + Earth's GHG effect 288 K (15 °C) 236 K (−37 °C) to 261 K (−12 °C) + Mars' GHG effect 210 K (−63 °C) Tidally locked Almost No Probably No Refs. [1][18][19] 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,[20] 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,[21] or a lower albedo,[22] 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.[23] The wide range is due to the rarefied atmosphere, which cannot store much solar heat, and the low thermal inertia of the soil.[24] Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars.[25] 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.[26] 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".[1] 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.[1] Potential for life Further information: Planetary habitability In an interview with Lisa-Joy Zgorski of the National Science Foundation, 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."[27] According to the Associated Press interview with Steven Vogt, "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."[28] Implications Keck telescope at twilight 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."[29] This finding foreshadows what Vogt calls a new, second Age of Discovery in exoplanetology:[30] 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.[1] 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.[31] See also Astronomy portal Space portal
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