http://en.wikipedia.org/wiki/Gliese_581_d wrote:
An artist's impression of Gliese 581 d and speculative moons.
<<Gliese 581 d (pronounced /ˈɡliːzə/) or Gl 581 d is an extrasolar planet approximately 20 light-years away in the constellation of Libra. Because of its mass, nearly 8 times that of Earth, the planet is classified as a super-Earth. In late April 2009 new observations by the original discovery team concluded that the planet is within the habitable zone where liquid water, and therefore, life, could exist.
The planet was discovered by the team of Stéphane Udry of the Geneva Observatory in Switzerland using the HARPS instrument on the European Southern Observatory 3.6 meter telescope in La Silla, Chile on 24 April 2007. Udry's team employed the radial velocity technique, in which the size and mass of a planet are determined based on the small perturbations it induces in its parent star’s orbit via gravity.
The team is confident that the planet exists but recognizes that unlikely events could mimic its existence. They believe the issue will be settled by upcoming studies. Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets are coplanar show that the system becomes unstable if the masses of the planets exceed 1.6 – 2 times the minimum values. The upper mass limit for Gliese 581 d is 13.8 Earth masses.
It was originally thought that Gliese 581 d orbits outside the theoretical habitable zone of its star. Further studies released in April of 2009 confirmed that the planet is within the habitable zone where liquid water could exist. According to Stephan Udry,
"d could be covered by a 'large and deep ocean'; it is the first serious Ocean planet candidate." Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star." Scientists originally believed that Gliese 581 d would be too cold for liquid water to exist, and therefore could not support life in forms as existing on Earth. However, since Earth's temperature would be about -18°C without any greenhouse gases, and due to a theorized greenhouse effect of Gliese 581 d, research now suggests that atmospheric conditions on the planet could create temperatures at which liquid water can exist, and therefore the planet may be capable of supporting life.
On 21 April 2009, ESO announced, together with the discovery of Gliese 581 e, that they had refined the models of Gliese 581 d's orbit, discovering it lay well within the habitable zone. Calculations by Barnes et al. suggest, however, that tidal heating is too low to keep plate tectonics active on the planet, unless radiogenic heating is somewhat higher than expected.>>
