For your entertainment and edification, a short note on Kepler-22b, the exoplanet that has caused a flurry of 'habitable planet' stories in the mass media.
Is it actually habitable? The short and accurate answer is that we don't know. All we really know directly about Kepler-22b are its orbital period and size. But the first of these - not quite 290 days - yields an average orbital distance (semi-major axis) of 0.849 AU. In the Solar System that would fall close to midway between Venus and Earth.
The G5 parent star, however, is a shade smaller, cooler, and dimmer than Sol. Its estimated luminosity is 0.79 solar, meaning that the planet, at its average distance, receives just about 10 percent more light - and therefore heat - than Earth does. The planet must shed that heat by radiation, and a quick-and-dirty temperature estimate, based on the 4th power law of radiation to temperature, makes it 2.3 percent warmer than Earth - about six or seven degrees C.
In fact, the paper announcing the discovery comes up with an equilibrium temperature of 262 K for Kepler-22b, as compared to 255 K for Earth.
Radiative equilibrium is only a starting point for planetary conditions. How much heat a planet actually absorbs depends on how much light is reflected, or its albedo (0.29 for Earth). And its radiative temperature, in the IR, is measured at the top of any greenhouse layer in the atmosphere. The radiative equilibrium temperature is only a starting point.
But - assuming similar albedo and greenhouse heating to Earth - the surface temperature would also be a few degrees warmer than Earth. Which is easily inside the habitable range.
Pause here to note that most of what makes global warming problematic for our civilization is abrupt change in climate, not a somewhat warmer climate per se. An Earthlike planet with stifling tropics but mild subarctic zones would still be eminently habitable for humans.
Now for the caveats. For one, in spite of the cool illustration above (via Sky & Telescope) we do not know Kepler-22b's orbital eccentricity. Most extrasolar planets, unless so close in that their orbits have been tidally circularized, have notably more eccentric orbits than Earth does; this one could be searing hot at periastron, freezing cold at apoastron.
And we do know Kepler-22's size, 2.38 Earth radii, so it cannot really be very Earthlike at all. The radius corresponds to a volume 13.5 times Earth's. If it is primarily rocky its mass is proportional, with surface gravity well above 2g. On the other hand it could be a 'water giant,' with roughly earth-sized rocky core and a hydrosphere thousands of miles deep. Ocean is far too weak a term.
Earthlike it is not, whatever its composition. But it is hard not to speculate. Suppose that a rocky core has a radius of 1.38 x Earth's (and probably a bit denser), with a hydrosphere above.
Planetary mass is then close to 5 Earth masses. Surface gravity is about 0.9 g, and low-orbit velocity is around 11 km/s - similar to Earth's escape velocity. Getting back off after 'landing' on the hydrosphere would be tough, but not as tough as getting there in the first place, since it is 600 light years away.
So ... speculate!
Bonus science destructiveness news: Thanks to regular commenter 'Thucydides,' five cataclysmic events to think about.