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Gliese 581g

Coordinates: Sky map 15h 19m 27s, −07° 43′ 19″
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Gliese 581 g (Template:Pron-en) or Gl 581 g, is an extrasolar planet, orbiting the red dwarf star Gliese 581, 20.5 light-years[1] 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 planet was discovered by the Lick-Carnegie Exoplanet Survey after a decade of observation. Despite the Gliese 581 system having a "somewhat checkered history of habitable planet claims,"[2] results from the study imply that the planet is located in the habitable zone of its parent star, where liquid water is considered a strong possibility.

The discovery of Gliese 581 g was announced in late September 2010, and it is believed to be the most Earth-like planet and the exoplanet candidate with the greatest potential for harboring life found. The detection of Gliese 581 g in such a short period of time and at such a close proximity leads astronomers to believe that the proportion of stars with habitable planets may be greater than ten percent.[2]

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 11 years of data from the HIRES instrument of the Keck 1 telescope and the HARPS instrument of ESO's 3.6m telescope at La Silla Observatory.[2][3]

Gliese 581 g has an orbital period of 37 days, orbiting at a distance of 0.146 AU from its parent star.[2] 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.[2]

The Lick–Carnegie team explained the results of their research in a paper published in the Astrophysical Journal.

Habitability

Main article: Habitability of red dwarf systems

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."[4] 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."[5]

Tidal locking

Because of Gliese 581 g's proximity to its parent star, it is predicted to be tidally locked to Gliese 581. Just as our 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.[2][6] Tidal locking means the planet would have no axial tilt and therefore no seasonality in a conventional sense.

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).[2][7] A factor that could potentially give Gliese 581 g a greenhouse effect greater than Earth's is the possibility that the more massive planet has a correspondingly more massive atmosphere.[2]

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, with continuous Earth-like temperatures imaginable along the terminator (the area between the bright and the dark side), informally known as the twilight zone because of the permanent state of sunrise/sunset-like twilight . By comparison, Martian surface temperatures vary from lows of about −87 °C (−125 °F) during the polar winters to highs of up to −5 °C (23 °F) in summers.[8] Such a wide range in temperature is due to the rarefied atmosphere which cannot store much solar heat and the low thermal inertia of Martian soil.[9]

Atmospheric effects

The orbits of planets in the Gliese 581 system as directly compared to those of our own solar system, without any scaling for star mass ratios. G indicates Gliese 581g.

Theoretical models of tidally locked worlds predict that under some conditions, volatile compounds such as water and carbon dioxide, if present, would evaporate in the scorching heat of the sunward side and migrate to the cooler night side, where they would condense to form ice caps. Over time, the entire atmosphere might freeze out on the night side of the planet. Alternatively, an atmosphere massive enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.[10] 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.[11] 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.[12] However, due to the overpowering light of its star, current technology is unable to determine the atmospheric or surface composition of Gliese 581 g.[13]

The greater mass of Gliese 581 g would tend to compress its atmosphere (i.e., reduce its scale height) relative to Earth's.

A new age of discovery

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."[14] This finding foreshadows what Vogt calls a new, second Age of Discovery in exoplanetology:[15]

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.[2]

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.[16]

See also

Notes and references

  1. ^ About 193 trillion kilometres
  2. ^ a b c d e f g h i Cite error: The named reference Vogt was invoked but never defined (see the help page).
  3. ^ Alleyne, Richard (September 30, 2010). "Gliese 581g the most Earth like planet yet discovered". The Daily Telegraph. Retrieved September 30, 2010. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  4. ^ NSF. Press Release 10-172 - Video. Event occurs at 41:25-42:31. See Overbye, Dennis (2010-09-29). "New Planet May Be Able to Nurture Organisms". The New York Times'. Retrieved September 30, 2010. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  5. ^ Borenstein, Seth (2010-09-29). "Could 'Goldilocks' planet be just right for life?". Associated Press. Retrieved September 30, 2010.
  6. ^ Berardelli, Phil (2010-09-29). "Astronomers Find Most Earth-like Planet to Date". ScienceNOW. Retrieved September 30, 2010. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  7. ^ Stephens, Tim (2010-09-29). "Newly discovered planet may be first truly habitable exoplanet". University News & Events. University of California, Santa Cruz.
  8. ^ "NASA, Mars: Facts & Figures". Retrieved 2010-01-28.
  9. ^ "Mars' desert surface..." MGCM Press release. NASA. Retrieved 2007-02-25.
  10. ^ Alpert, Mark (2005-11-07). "Red Star Rising". Scientific American. Retrieved 2007–04–25. {{cite web}}: Check date values in: |accessdate= (help)
  11. ^ Ralph D Lorenz, Jonathan I Lunine, Paul G Withers, Christopher P. McKay (2001). "Titan, Mars and Earth: Entropy Production by Latitudinal Heat Transport" (PDF). Ames Research Center, University of Arizona Lunar and Planetary Laboratory. Retrieved 2007-08-21.{{cite web}}: CS1 maint: multiple names: authors list (link)
  12. ^ Joshi, M. M. (1997). "Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability". Icarus. 129 (2): 450–465. doi:10.1006/icar.1997.5793. Retrieved 2007-08-11. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  13. ^ Shiga, David (2010-09-29). "Found: first rocky exoplanet that could host life". New Scientist. Retrieved September 30, 2010. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  14. ^ Vogt 2010, pp.32-33. For more information, see Turnbull, Margaret C. (Mar., 2003). "Target Selection for SETI: 1. A Catalog of Nearby Habitable Stellar Systems". The Astrophysical Journal. Institute of Physics Publishing. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  15. ^ NSF. Press Release 10-172 - Video. Event occurs at 17:00-17:46.
  16. ^ Vogt 2010, p.2. See Berardelli, Phil (2010-09-29). "Astronomers Find Most Earth-like Planet to Date". AAAS. Retrieved September 30, 2010.
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