Earth like planet Kepler-22b

First observed transit by Kepler space telescope in May 12, 2009. It is located about 600 light-years from Earth. Kepler-22b's size is roughly twice that of Earth. Its mass and surface composition are unknown.
Scientists can estimate the possible surface conditions as follows:

In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately −11 °C.
If the atmosphere provides a greenhouse effect similar in magnitude to the one on Earth, it would have an average surface temperature of 22 °C.