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Gliese 581c

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Template:Planetbox begin| Template:Planetbox image Template:Planetbox star Template:Planetbox orbit Template:Planetbox character Template:Planetbox discovery Template:Planetbox end Gliese 581 c (IPA: [ˈgliːzə]) is a "super-earth" extrasolar planet orbiting the red dwarf star Gliese 581.[1] It appears to be the first extrasolar planet discovered in the hypothetical habitable zone surrounding its star, where surface temperatures might maintain liquid water[1][2] and therefore be suitable for life as known on Earth. The planet is astronomically close, at 20.5 light years (194 trillion km or 120 trillion miles) from Earth in the direction of the constellation of Libra.[3] Its star is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; with respect to Earth it is the 87th closest star system.[4]

Gliese 581 c is the first extrasolar planet believed to have a surface temperature similar to that of Earth. It is the smallest extrasolar planet around a main sequence star discovered to date.[1]


Discovery

The discovery of the planet by the team of Stéphane Udry of the Geneva Observatory in Switzerland was announced on April 24, 2007. The team used the HARPS instrument (an echelle spectrograph) on the European Southern Observatory 3.6 m Telescope in La Silla, Chile. The team employed the radial velocity technique. A follow-up study over the next six weeks by the Canadian-built MOST space telescope. As no transit was detected over this time, a direct measurement of the planet has not yet been possible, but a positive sign was found in the star itself that was found to be remarkably stable, changing its brightness only a few tenths of a percent over the six-week period. [5]

The team released a paper dated April 27, 2007 which will be published in an edition of the journal Astronomy and Astrophysics.[6] In the paper they also announce the discovery of another planet in the system, Gliese 581 d, with a minimum mass of 7.7 Earth masses and a semi-major axis of 0.25 astronomical units.

Physical characteristics

Mass

The existence of Gliese 581 c and its mass has been measured by the Radial Velocity Method or the "wobble-effect" method of detecting exoplanets. The mass of a planet is calculated by the small periodical movements around a common centre of mass between the host star Gliese 581 and its planets. Because the "wobbling" of Gliese 581 is a result of all planets in its system, the calculation of the mass of Gliese 581 c depends on the presence of other planets in the Gliese 581 system and on the inclination of the orbital plane with respect to Earth. Using the known minimum mass of the previously detected Gliese 581 b, and assuming the existence of Gliese 581 d, Gliese 581 c has a mass at least 5.03 times that of Earth. The mass of the planet cannot be very much larger than this or the system becomes dynamically unstable.[6] (The citation does not seem to address the stability, in fact)

Radius

If it is a rocky planet with a large iron core, Gliese 581 c has a radius approximately 50% larger than that of Earth, according to Udry's team.[6][7] Gravity on such a planet's surface would be approximately 2.2 times as strong as on Earth.

If Gliese 581 c is an icy and/or watery planet, its radius would still be less than 2 times that of Earth, even with a very large outer hydrosphere, according to density models compiled by Diana Valencia and her team for Gliese 876 d.[8] Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth.

With the Radial Velocity method of detection, it is not possible to measure the radius of an exoplanet. The real value may be anything between the two extremes calculated by density models outlined above.[9] If the planet transits the star as seen from our direction, the radius should be measurable, although with some uncertainty. Udry's team intends to use the Canadian-built MOST space telescope to look for a transit of the planet before its host star. A transit measurement could very well determine whether Gliese 581 c is a primarily rocky or watery object.

Age

The Gliese 581 system is estimated to be around 4.3 billion years old.[10] By comparison, the Solar System is estimated to be 4.57 billion years old.

Orbit

Gliese 581 c has an orbital period ("year") of 13 Earth days[3] and its orbital radius is only about 7% that of the Earth, about 11 million km[11], while the Earth is 150 million kilometres from the Sun[12]. Since the host star is smaller and colder than the Sun—and thus less luminous—this distance places the planet on the "warm" edge of the habitable zone around the star according to Udry's team [6] [7]. This proximity also means that the primary star appears several times larger for an observer standing on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.

Climate and habitability

Scale comparison of the relative sizes of the Earth and Gliese 581 c, assuming Gliese 581 c is a rocky body

Temperature

Using the measured stellar luminosity of Gliese 581 of 0.013 times that of our Sun, it is possible to calculate Gliese 581 c equilibrium surface temperature, which does not take into account a possible atmosphere. According to Udry's team the equilibrium temperature for Gliese 581 c is −3° C / 26.6° F, assuming an albedo (how much light is reflected by the planet's surface) such as Venus (0.64) and 40°C / 104° F for an Earth-like albedo (0.35).[6][3] The actual temperature on the surface also depends on the planet's atmosphere, which remains unknown. Xavier Delfosse of the research team expects that the actual surface temperatures will be hotter; for instance, the corresponding calculation for Earth yields an "effective surface temperature" of 256 K (−17 °C), yet Earth's true surface is 32 K warmer (an average of 288 K (15 °C)) due to the greenhouse effect. Gliese 581c receives more heat from the star than Venus does from our sun.

Liquid water

Gliese 581 c is within the habitable zone where water—a necessary ingredient for life as we know it—could exist.[1][13] However, no direct evidence has been found. Techniques like the one used to measure HD 209458 b could be applied to determine the existence of water vapor in an extrasolar planet's atmosphere, but only in the rare case of a planet with an orbit including transit directly between its star and our planet, which Gliese 581c is not known to do.

Tidal forces

Because of its small separation from Gliese 581, the planet is quite likely to be tidally locked, with one hemisphere always day (facing the star) and the other always night (facing away).[14]. Even then, the planet would undergo violent tidal flexing, because the orbital eccentricity is between 0,09 and 0,23. The planet might also be in a different tidal lock, like Mercury, with eccentricity of just 0,20, which is tidally locked 3:2, so that every point is in sunlight once over 2 orbital periods. The permanently lit hemisphere would be extremely hot and the dark hemisphere extremely cold, while the narrow terminator or "twilight zone" between them might have a moderate climate more suitable for life. In any case, even in case of 1:1 tidal lock, the planet would undergo libration and the terminator would be alternatively lit and darkened during libration.[15]

Theoretical models predict that volatile compounds such as water and carbon dioxide, if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to form ice caps. Over time, the entire atmosphere might freeze into ice caps on the night side of the planet. Alternatively, an atmosphere large enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.[16] For example, although Venus has a small axial inclination, very little sunlight reaches the surface at the poles. A slow rotation rate approximately 117 times slower than Earth's produces prolonged days and nights. Despite the uneven distribution of sunlight cast on Venus at any given time, polar areas and the night side of Venus are kept almost as hot as day by globally circulating winds. However, it remains unknown if water and/or carbon dioxide are even present on the surface of Gliese 581 c.

Greenhouse effects

It has been hypothesized that, due to its strong gravity and proximity to the hotter edge of the habitable zone, Gliese 581 c could be prone to a runaway greenhouse effect, and would not be habitable, thus mimicking what happened to Venus in our solar system.[17]

Further study

Gliese 581 c presents several challenges for study or exploration. It has not been directly observed, and the development of equipment sensitive enough to look for signs of life will take several years.[18] However, according to the research-team member Xavier Delfosse:

Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an X.[18][7]

Several astronomers have suggested that the earthlike properties of Gliese 581 c and its relative proximity (20.5 light-years away) would make it a potential target for any future interstellar probe project.[19][3]

See also

Wikinews has related news:
Wikimedia Commons has media related to Category:Gliese 581 c.

References

  1. ^ a b c d Than, Ker (2007-04-24). "Major Discovery: New Planet Could Harbor Water and Life". space.com. Retrieved 2007-04-29. {{cite web}}: Check date values in: |date= (help)
  2. ^ Than, Ker (2007-02-24). "Planet Hunters Edge Closer to Their Holy Grail". space.com. Retrieved 2007-04-29. {{cite web}}: Check date values in: |date= (help)
  3. ^ a b c d "New 'super-Earth' found in space". BBC News. 25 April 2007. Retrieved 2007-04-25. {{cite news}}: Check date values in: |date= (help)
  4. ^ "The 100 Nearest Stars". RECONS. Retrieved 2007-05-10.
  5. ^ Boring Star May Mean Livelier Planet
  6. ^ a b c d e Udry; et al. (2007). "The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M⊕) in a 3-planet system" (PDF). Astronomy and Astrophysics. preprint: preprint. {{cite journal}}: Explicit use of et al. in: |author= (help)
  7. ^ a b c "Astronomers Find First Earth-like Planet in Habitable Zone". ESO. Retrieved 2007-05-10. Cite error: The named reference "ESOAstronomy" was defined multiple times with different content (see the help page).
  8. ^ Valencia; et al. (2006). "Radius and Structure Models of the First Super-Earth Planet". {{cite journal}}: Cite journal requires |journal= (help); Explicit use of et al. in: |author= (help)
  9. ^ Valencia and Sasselov (2007). "Detailed Models of super-Earths: How well can we infer bulk properties?". ArXiv: 0704.3454v1. preprint: preprint.
  10. ^ "Star : Gl 581". Exoplanets Encyclopedia. Retrieved 2007-04-25.
  11. ^ Overbye, Dennis (2007-04-25). "20 light years away, the most Earthlike planet yet". International Herald Tribune. Retrieved 2007-05-10.
  12. ^ "The Earth Worldbook". NASA. Retrieved 2007-05-10.
  13. ^ Dead Link "Earth-Like Planet Found". Associated Press. Retrieved 2007-05-10.
  14. ^ "Out of our world: Earthlike planet". USA Today. 2007-04-25. Retrieved 2007-05-10. {{cite news}}: |first= missing |last= (help); Unknown parameter |Last= ignored (|last= suggested) (help)
  15. ^ Perlman, David (2007-04-24). "New planet found: It might hold life". San Francisco Chronicle. Retrieved 2007-04-24. {{cite news}}: Check date values in: |date= (help)
  16. ^ Alpert, Mark (2005-11-07). "Red Star Rising". Scientific American. Retrieved 2007-04-25. {{cite web}}: Check date values in: |date= (help)
  17. ^ "The Habitability of Super-Earths in Gliese 581" (PDF). Retrieved 2007-05-29.
  18. ^ a b "Earth-like planet found that may support life". CTV News. Retrieved 2007-04-25.
  19. ^ New Planet Could Be Earthlike, Scientists Say, New York Times.

Selected media articles


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