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Gliese 581 c (Template:PronEng), also cataloged as HO Librae c, Wolf 562 c, and HIP 74995 c, is a "super-earth" or large terrestrial extrasolar planet orbiting the red dwarf star Gliese 581. Assuming the planet's mass is close to the lower limit determined by radial velocity measurements (the true mass is unknown), it would be the smallest known extrasolar planet around a main sequence star to date. Gliese 581 c generated a lot of interest as it was initially reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface and hence, potentially capable of supporting life as we know it. However, subsequent study that took into account the insolation that gives the planet five times the heat the Earth recieves and the greenhouse gas effect of the planet’s atmosphere, with at least five times the greenhouse gases, now appears to make this very unlikely. Gliese 581 d, the third planet in the system, is now regarded as more likely to harbor life. The planet is astronomically close, at 20.4 light years (193 trillion km or 119 trillion miles) from Earth in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give the planet's exact location in our galaxy. Its star is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 87th closest star system to the Sun that we know of.

Detection and Discovery

The discovery of the planet by the team of Stéphane Udry University of Geneva's 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 to identify the planet's influence on the star. The Canadian-built MOST space telescope was used to conduct a follow-up study over the next six weeks. No transit was detected over this time, so a direct measurement of the planet has not yet been possible; however, the star's apparent magnitude changed very little, indicating that it provides a stable source of light and heat to Gliese 581 c.

The team released a paper dated April 27, 2007, published in the July, 2007 journal Astronomy and Astrophysics. In the paper they also announced 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. This inner planet has a red dwarf star and is in the "goldilocks" zone, thus might be habitable.

Physical characteristics

Mass

The existence of Gliese 581 c and its mass have been measured by the Radial Velocity Method or the "wobble" method of detecting exoplanets. The mass of a planet is calculated by the small periodic 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 would be dynamically unstable.

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. Gravity on such a planet's surface would be approximately 2.24 times as strong as on Earth.

If Gliese 581 c is an icy and/or watery planet, its radius would 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. Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth.

It is not possible to measure the radius of an exoplanet using Radial Velocity. The real value may be anything between the two extremes calculated by density models outlined above. 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 in front of its host star. A transit measurement could very well determine whether Gliese 581 c is a primarily rocky or watery object, however, most exosolar planets do not transit their host star from Earth's perspective.

Age

The Gliese 581 system is estimated to be around 4.3 billion years old. 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 and its orbital radius is only about 7% that of the Earth, about 11 million km, while the Earth is 150 million kilometres from the Sun. 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 . A typical radius for an M0 star of Gliese 581's age and metallicity is 0.00128 AU, against the sun's 0.00465 AU. This proximity means that the primary star should appear 3.75 times wider and 14 times larger in area for an observer on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.

Climate and habitability

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's 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 (reflectivity) such as Venus' (0.64) and 40° C/104° F for an Earth-like albedo (0.35). The actual temperature on the surface mainly depends on the planet's atmosphere, of which it is only known the overall mass of the planet is over 5 times that of Earth.

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/−28°F, yet Earth's true surface is 32 K warmer (an average of 288 K/15°C/59°F) due to the greenhouse effect. Gliese 581 c receives more irradiance (4,877.753 W/m average) from its star than Venus does (2,656.70 W/m average) from the sun.

Derivation

See these articles for the relevant astronomical formulae:

• thermodynamic equilibrium
• Climate modeling

Liquid water

Gliese 581 c is outside the habitable zone. No direct evidence has been found for water (an important abundant molecule) to be present, but it is likely not present due to the extremely high irridance at perihelion (see Temperature). Techniques like the one used to measure HD 209458 b may in the future be used to determine the presence of water in the form of vapor in the planet's atmosphere, but only in the rare case of a planet with an orbit aligned so as to transit its star, which Gliese 581 c is not known to do.

Tidal forces

Tidal lock

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).. Even then, the planet would undergo violent tidal flexing, because the orbital eccentricity is between 0.09 and 0.23. Eccentric planets can also be found in a non-synchronous tidal lock, as is Mercury, which is tidally locked in a 3:2 ratio. 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.

Models of the evolution of the planet's orbit over time suggest that tidal heating may play a major role in the planet's geology. It is predicted that tidal heating could yield a surface heat flux about three times greater than Io's, which could result in major geological activity such as volcanoes and plate tectonics.

Theoretical models

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. 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.

For a model of a hypothetical planet like Gliese 581 c, see Aurelia and Blue Moon.

Greenhouse effect

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.

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 years. 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.

Several astronomers have suggested that the earthlike properties of Gliese 581 c and its relative proximity (20.4 light-years away) would make it a potential target for a future interstellar probe project.

One source has produced fiction using Gliese 581 c as a host world, calling it Ymir.

See also

• Planetary habitability
• Planetary human habitability
• Habitable zone
• Goldilocks phenomenon
• Interstellar travel

References

1. ^ 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. Catalog of Nearby Exoplanets - Planets Table
3. 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)
4. Selsis; et al. (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics. 476: preprint. {{cite journal}}: Explicit use of et al. in: |author= (help)
5. ^ "New 'super-Earth' found in space". BBC News. 25 April 2007. Retrieved 2007-04-25. {{cite news}}: Check date values in: |date= (help)
6. "The 100 Nearest Stars". RECONS. Retrieved 2007-05-10.
7. Boring Star May Mean Livelier Planet
8. ^ 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. 469:3: L43–L47. {{cite journal}}: Explicit use of et al. in: |author= (help) Cite error: The named reference "udry" was defined multiple times with different content (see the help page).
9. ^ "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).
10. 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)
11. Valencia and Sasselov (2007). "Detailed Models of super-Earths: How well can we infer bulk properties?". ArXiv: 0704.3454v1. preprint: preprint.
12. "Star : Gl 581". Exoplanets Encyclopedia. Retrieved 2007-04-25.
13. Overbye, Dennis (2007-04-25). "20 light years away, the most Earthlike planet yet". International Herald Tribune. Retrieved 2007-05-10.
14. "The Earth Worldbook". NASA. Retrieved 2007-05-10.
15. Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 M_☉, and from Z=0.0004 to 0.03. Girardi L., Bressan A., Bertelli G., Chiosi C., Astron. Astrophys. Suppl. Ser. 141, 371 (2000).
16. "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)
17. 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)
18. Jackson, Brian (2008). "Tidal Heating of Extra-Solar Planets". ApJ. arXiv:0803.0026. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
19. Alpert, Mark (2005-11-07). "Red Star Rising". Scientific American. Retrieved 2007-04-25. {{cite web}}: Check date values in: |date= (help)
20. "The Habitability of Super-Earths in Gliese 581". Retrieved 2007-05-29.
21. ^ "Earth-like planet found that may support life". CTV News. Retrieved 2007-04-25.
22. New Planet Could Be Earthlike, Scientists Say, New York Times.
23. "The Neighbor: Gliese 581c". The Geochemical Society. Retrieved 2007-12-06.

Selected media articles

• Hazel Muir (April 25 2007). "'Goldilocks' planet may be just right for life". NewScientistSpace. {{cite web}}: Check date values in: |date= (help)
• "Astronomers find first habitable Earth-like planet". Scientificblogging.com. April 24 2007. {{cite web}}: Check date values in: |date= (help)
• "Found 20 light years away:the new Earth". Daily Mail. April 26 2007. {{cite web}}: Check date values in: |date= (help)
• Ian Sample (April 24 2007). "'Second Earth' may mean we're not alone". The Hindu. {{cite web}}: Check date values in: |date= (help)
• J.R. Minkle (April 24 2007). "All Wet? Astronomers Claim Discovery of Earth-like Planet". Scientific American. {{cite web}}: Check date values in: |date= (help)
• "Distant planet judged possibly habitable". World Science. April 23 2007. {{cite web}}: Check date values in: |date= (help)
• ANI (April 23 2007). "First habitable Earth like planet outside Solar System discovered". DailyIndia.com. {{cite web}}: Check date values in: |date= (help)
• Joe Campbell (April 26 2007). "Meet the neighbours". BBC Magazine. {{cite web}}: Check date values in: |date= (help)

External links

• ESO 22/07 Astronomers Find First Earth-like Planet in Habitable Zone
• Space.com: Major Discovery: New Planet Could Harbor Water and Life
• Planet : Gl 581 c
• Red, Willing, and Able: 2001 New Scientist article on types of planets likely to be around red dwarf stars
• New 'Super Earth' found in space
• Artistic Image (APOD)
• Artist conceptions of extrasolar planet Gliese 581 c
• Gliese 581 c (french)
• Speculation about geology/geochemistry of Gliese 581c The Geochemical Society

• Extrasolar planets
• Libra constellation