Gliese 581 c

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**Gliese 581 c**
Parent star
Extrasolar planet		List of extrasolar planets
Artist's impression of the planetary system around the red dwarf star Gliese 581, with Gliese 581 c in the foreground.
Star		Gliese 581
Constellation		Libra
Right ascension	(α)	15^h 19^m 26^s
Declination	(δ)	−07° 43′ 20″
Spectral type		M2.5V
Orbital elements
Semimajor axis	(a)	0.073 AU
Eccentricity	(e)	0.16±0.07
Orbital period	(P)	12.93 d
Inclination	(i)	?°
Longitude of periastron	(ω)	?°
Time of periastron	(τ)	? JD
Physical characteristics
Mass	(m)	> 5 M_E
Radius	(r)	~1.5 R_E
Density	(ρ)	? kg/m³
Temperature	(T)	~290 K
Discovery information
Discovery date		2007-04-04
Discoverer(s)		Udry et al.
Detection method		Radial Velocity
Discovery status		published

This article documents a current event.
Information may change rapidly as the event progresses.

Gliese 581 c (IPA: [ˈgliːˌzə]) is an extrasolar planet orbiting the red dwarf star Gliese 581^[1]. 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^[1]^[2]. The planet is relatively close, at 20.5 light years (190 trillion kilometres) from Earth in the constellation of Libra^[3]. Its star is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars.

[edit] Characteristics

[edit] 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^[1].

The calculation of its mass depends on the presence of other planets in the Gliese 581 system. Using the already known mass of Gliese 581 b, and assuming the existence of Gliese 581 d, Gliese 581 c would have a mass 5.03 times that of Earth.^[4]

Under the assumption that it is a rocky planet, rather than an icy planet, Gliese 581 c has a radius 50% larger than that of Earth^[5]. Gravity on such a planet's surface would be approximately 2.1 times as strong as on Earth.

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

[edit] Orbital

Gliese 581 c has an orbital period ("year") of 13 Earth days and its orbital radius is only about 7% of that of the Earth,^[3] about 11 million km (6.8 million miles), while the Earth is 150 million km (93.2 million miles) 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 0.7m/sec² because of the high orbital velocity. For earth, this difference is about 0.01m/sec².

[edit] 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^[3]. However, the actual temperature on the surface depends on the planet's atmosphere, which remains unknown. Research team member Xavier Delfosse expects that the actual surface temperatures will only 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. ^[6]

Because of its close orbit around its parent star, the planet would experience tides 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^[7]. 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.^[8]

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.^[9]

[edit] 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.

Diagram showing 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.

[edit] Liquid water

Gliese 581 c is within the habitable zone where liquid water—a necessary ingredient for life as we know it—could exist.^[1]^[10]

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.

[edit] 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.^[11] 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. ^[11]^[5]

If a probe were sent from Earth to explore Gliese 581 c, it would take 20.5 years to reach it if it could travel at the speed of light; scientists observing Gliese 581 c are seeing what happened to it 20.5 years ago because Gliese 581 c is 20.5 light years away^[12]^[3].