# Gliese 581 C

Gliese 581 C is a Exoplanète in Orbite around the star Gliese 581, located at 20,5 light-years of the Ground. It was discovered the April 4th 2007 by a team of Astronome S French, Portuguese and Swiss. Of a diameter of approximately 1,5 times that of the Ground, it would be 5 times more massive. It revolves in the livable Zone of Gliese 581 and its average temperature lies between 0 and 40 °C, which authorizes the presence of Eau to the state Liquide. To date (April 2007), it is the known exoplanète which resembles our Ground more. Gliese 581 C belongs to a system of 3 planets, whose first (Gliese 581 B) was already known. Gliese 581 C was discovered, like its colleagues, with the ultrastable Spectrographe HARPS, installed with the Télescope of 3,6 m of the Observatoire of Silla (ESO), with the Chile.

## Characteristics

### Physics

Gliese 581 C would be the first planet extrasolaire discovery similar to the Earth from the point of view of the temperature. It is the smallest known planet extrasolaire to date turning around a star of the principal Séquence. According to Professor Michel Mayor, member of the team which discovered the Planet, Gliese 581 C is not a hot Jupiter, because it is too not very massive to be made up of Gaz. It is probably about a telluric Planet, perhaps entirely covered with water, a Planet ocean. Measurements of radial Speed which allowed the discovery of planets of this system, also make it possible to measure their mass minimum, since the slope of the orbital plan compared to the observer remains unknown to date. The article of the discovery indicates that Gliese 581 C would have a mass of with more the 5,03 times that of the Earth (in fact $5.03 * \ sin^3 \left(I\right)$, where $i$ is the angle of inclination of the orbital plan). The Gravité on the surface of such a planet would be roughly 2,2 times that on Earth. On the other hand, if Gliese 581 C is a frozen planet, its ray would be twice more important than that of the Earth and gravity on the surface would be approximately 1,25 times there higher. The age of the planetary system of Gliese 581 was estimated roughly at 4,3 billion years.

### Orbital

Gliese 581 C has one orbital period (“year”) 13 days terrestrial and its orbital ray accounts for only 7% of that of the Earth, i.e. it measures only 11 million km whereas the Earth is to 150 million km of the Sun. Since its sun is smaller and more cold that ours (and thus less luminous), this distance places planet in the livable zone. This proximity also means that their sun appears much more important in the sky of planet than is to it the Sun in the terrestrial sky.

### Climatic

Because of its local Orbit, the planet knows a Force of tide on behalf of its sun 400 times more important than that which the Moon exerts on the Earth.

Gliese 581 C could be in synchronous Rotation with its star. For an observer located close to star, the planet would always present the same face. On this assumption, the differences in temperature between the continuously lit face and the hidden side would be extreme. In spite of that, a zone “hinge” between the enlightened face and the hidden side could profit from a more moderate climate, favourable with the life. Moreover, the appearance of the life could be opposed by the violence of the solar eruptions common to the dwarf reds, even if Gliese 581 seems “as calm as the Sun”, according to Xavier Delfosse, of the Université Joseph-Fourier with Grenoble.

## Outdistance and direct observation

The star Gliese 581, with a score of light-years of the Earth, is a star near known as “in the solar vicinity”. Its Standard spectral, m2, indicates that it is a not very massive and not very luminous star. This type of star is very promising for search for exoplanètes of the terrestrial type since while being not very massive, it is easier to detect the gravitational effect induced by the presence of light planets (see Méthodes of detections of the exoplanètes). Moreover, the livable zone of these stars is very close to star. These two effects make that it is possible already with the current instruments to detect planets of the terrestrial type. For such planets close to stars truly similar to our Sun, it will be necessary to await the next generation of Télescopes, like OWL.

The direct observation of Gliese 581 C in the search of signs of life on this planet is still out of reach current instruments. In spite of that, according to Xavier Delfosse, member of the team at the origin of the discovered one:

Because of its temperature and its relative proximity, this planet Gl 581 C will be probably a very important target for the future missions in space devoted in the extraterrestrial search of life. On the chart of the treasures of the universe, one would be tempted to mark this planet with large X. ,

It is not possible to send a probe with current technologies to it, since this planet is located at 20,5 light-years of the Earth (its light thus spends 20,5 years to reach us). No current technology makes it possible to travel at such speeds and with current technologies of propulsion, it would roughly take 350.000 years to reach Gliese 581 C and 20,5 years for retransmettre the first data with the Earth.

## References

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