Guardian.co.uk: Nasa discovers planet that is about 2.4 times the size of our own and lies in the ‘Goldilocks zone’ of its solar system.
A new planet outside Earth’s solar system has been identified with many similarities to our own – making it the latest best potential target for life. Kepler 22-b, which is about 2.4 times the size of Earth and lies in the so-called “Goldilocks zone”, has a relatively comfortable surface temperature of about 22C (72F) and orbits a star not unlike Earth’s sun.
Wikipedia: The discovery was announced December 5, 2011. The planet’s radius is roughly 2.4 times the radius of Earth; it is 600 light years away from Earth, in orbit around the G-type star Kepler 22.
The distance from Kepler-22b to its star is about 15% less than the distance from Earth to the Sun. Its orbit is about 85% as large as Earth’s orbit. One orbital revolution around its star takes 289.9 days.
The light output of Kepler-22b’s star is about 25% less than that of the Sun.
The combination of a shorter distance from the star and a lower light output are consistent with a moderate surface temperature. Scientists estimate that in the absence of atmosphere, the equilibrium temperature would be approximately -11°C. If the greenhouse effect caused by the atmosphere is Earthlike, this corresponds to approximately 22 °C (72°F) average surface temperature.