Gliese 581c

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Gliese 581 c (IPA: [ˈgliːzə]) is an extrasolar planet orbiting the red dwarf star Gliese 581. It appears to be in the habitable zone of space surrounding the star, where the surface temperatures of any planets present might maintain liquid water and therefore be suitable for life as we know it. The planet is relatively close, at 20.5 light years (193.9 trillion kilometres) from Earth in the constellation of Libra. Its star is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars.

Characteristics

Physical

Gliese 581 c may be the first discovered extrasolar planet similar in temperature to Earth. It is the smallest extrasolar planet around a main sequence star discovered to date.

The calculation of its mass depends on the presence of other planets in the Gliese 581 system and on the inclination of the orbital plane with respect to us. Using the already known mass of Gliese 581 b, and assuming the existence of Gliese 581 d, Gliese 581 c would have a mass at least 5.03 times that of Earth.

Under the assumption that it is a rocky planet, Gliese 581 c has a radius at least 50% larger than that of Earth. Gravity on such a planet's surface would be approximately 2.1 times as strong as on Earth. On the other hand, if Gliese 581 c is an icy planet, its radius would be twice as that of Earth, and gravity on the surface would be 1.25 times as strong as on Earth. There is no way to tell based on the data collected thus far whether of these two possibilities is real. Note that these values of radii are only estimations and the reality can be anything between these two extremes.

The Gliese 581 system is estimated to be ~4.3 billion years old.

Orbital

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 km from the Sun. Since the host star is smaller and colder than the Sun – and thus less luminous – this distance places the planet in the habitable zone. This proximity also means that the primary star is several times larger in the planet's sky than the Sun is in Earth's sky.

The free fall accelerations on the day and night sides of the planet would differ by approximately 0.7 m/s² because of the high orbital velocity. For earth, this difference is about 0.01 m/s².

Climatic

Scale comparison of the relative sizes of the Earth and Gliese 581 c

Gliese 581 c has a projected equilibrium surface temperature between 0 °C and 40 °C. However, the actual temperature on the surface 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 of its atmosphere.

Because of its close orbit, the planet would experience tides from its parent star about 400 times as strong as those that the Moon causes on the Earth. It may be tidally locked to the star, with one hemisphere always lit and the other always dark. The lit hemisphere might 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 similar to Earth's.

A theoretical model predicts that volatile compounds such as water and carbon dioxide 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 could become frozen as ice caps on the night side of the planet. Alternatively, if it has an atmosphere large enough to be stable, it should circulate the heat more evenly, allowing for a wider habitable area on the surface.

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 on the European Southern Observatory 3.6 m Telescope in La Silla, Chile. The team employed the radial velocity technique. The team now intends to use the Canadian-built MOST space telescope to conduct follow-up studies of the planet as it passes in front of the star.

File:Gliese 581 c in the habitable zone.svg

The range of theoretical habitable zones for stars of different mass, and the approximate position of Gliese 581 c. Our Sun is the yellow star to the left of the diagram.

Liquid water

Gliese 581 c is within the habitable zone where liquid water – a necessary ingredient for life as we know it – could exist.

Although the potential for liquid water is predicted by the habitable-zone model, 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 this method requires the rare coincidence of a planet whose orbit causes it to transit directly between its star and our planet; this is something Gliese 581 c is not known to do.

Difficulty of exploration

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. 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 extra-terrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an X.

According to Alison Boyle, the curator of astronomy at London's Science Museum,

It's 20 light-years away and so we won't be going there anytime soon, but with new kinds of propulsion technology that could change in the future. And obviously we'll be training some powerful telescopes on it to see what we can see.

With current propulsion technology, it would take approximately 8 million years to reach Gliese 581 c. For the difficulties and speculative possibilities of sending even an uncrewed probe to another star, see interstellar travel. Then any signals transmitted back to Earth would take another 20.5 years to receive.

See also

• Planetary habitability
• Habitable zone
• Goldilocks phenomenon
• Aurelia and Blue Moon

References

• 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)
• 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)
• 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