On 5 December 2011 the NASA released a press statement to the effect that it had found a planet in the centre of the 'Goldilocks Zone' of a star 587 light years from Earth in the constellation of Cygnus. The Goldilocks Zone is the area in a star system where there is likely to be liquid water on an Earth-like planet; it is neither too hot, nor too cold, like the Baby Bear's porridge in the folk-tale. The system has been dubbed Kepler-22 (i.e. the 22nd planetary system discovered by the Kepler Space Telescope), with the star named Kepler-22A and the planet Kepler-22b (the first object in a system is 'a', the second 'b' etc.; stars are capitalized, planets are not).
A comparison of the habitable zones of our solar system and the Kepler-22 system.
Kepler-22A is a very sun-like star, 97% of the mass of our sun and 98% its volume. Kepler-22b obits this star every 290 days, at 85% of the distance at which the Earth orbits the sun. Keppler-22b planet has 2.4 times the radius of the Earth, though its mass is unknown; it was detected as it transits it's star; it does not exert enough gravitational pull to cause the star to wobble detectably (the other means by which we commonly discover new planets), which is how the mass of exoplanets can be determined. This is not greatly surprising, most planets that cause measurable wobbles in their stars are large planets orbiting small stars. It is estimated that if Kepler-22b was Earth-like then that its average surface temperature would be 22°C (the average surface temperature on Earth is 14°C).
This discovery has provoked a great deal of interest in the popular press, and has been widely hailed as a potential home for extra-terrestrial life, something NASA has done little to downplay. However the estimations of the habitability of Kepler-22b are based upon the assumption that it might be an earthlike planet. At 2.4 times the volume of the Earth Keppler-22b is likely to have a much thicker atmosphere, leading to a greater greenhouse effect, thereby raising the planet's surface temperature. This can make a considerable difference to a planet's temperature; without the greenhouse effect Venus would probably have a mild, pleasant, climate and Earth would be a frozen snowball. Even on Earth changes in the atmosphere are thought to have caused spectacular changes in climate over geological timescales, from an average surface temperature of -57°C during the Cryogenian, 700-750 million years ago to 21°C in the Cambrian 542-488 million years ago. Worst case scenarios for anthropogenic global warming suggest that the Earth's average surface temperature could rise by 6°C within the next hundred years, which would be devastating for human life (and much non-human life), but which would not make the planet uninhabitable. On Kepler-22b a much thicker atmosphere could potentially make the planet to hot for liquid water, and therefore unsuitable for life as we understand it.
NASA intend to follow up the initial observation with studies of the system using the Spitzer Space Telescope and ground-based observatories.