The exoplanet Gliese 581d is the fourth planet (that explains the letter d) of a planetarium system that is located at 20 light years from us.
The listenings done via the Doppler method per a franco-switzerland astronomers team show that this planet, the closest of this system that seems to contain 6, would have a size twice as the Earth and turns around a stable star that is 3 times less big than the Sun.
From the group of scientists Pierre Simon Laplace in Paris, this planet would be in an inhabitable zone, and even if it receives only a little energy from its star, its atmosphere made with CO2 could maintain a surface temperature over 0°C.
Gliese 581e, another planet of this system, that is estimated to be 1,4 times less than Earth, could have water (about 25% of its surface) and its atmosphere would be made with hydrogen and helium.
A spectroscopic analysis of these planets could confirm these observations.
Right now, the astronomical tools don't allow this approach but progress lets expect this it will be possible not far in the future.