NASA’s Kepler mission is specifically designed to search for habitable planets. Among the thousands of candidate planets found in the past few years is Kepler-22b, the first non-solar planet confirmed to orbits within the habitable zone of its star.
Let’s be clear about what this means. This does not mean that Kepler-22b contains life, or even that it could sustain life. All it means is that the planet orbits at the correct distance from its star so that liquid water could form on its surface. We don’t yet know if there’s any water actually there. In fact, we don’t even know whether the planet is rocky or gaseous, though at only 2.4 times the radius of the Earth, it could well have a solid surface.
This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission.
Image credit: NASA/Ames/JPL-Caltech
Because NASA requires three transits (views of the planet passing in front of its star) of an exoplanet to confirm its existence and position, it can take years to verify a finding. In this case, Kepler-22b was first seen by William Borucki and his team at NASA Ames Research Center just three days after the launch of the Kepler mission, back in 2009. The third transit was not observed until the end of 2010.