Kepler-22b was the first exoplanet found to orbit in the habitable zone of a Sun-like star. NASA’s Kepler mission announced the discovery on December 5, 2011, making it a landmark in the search for potentially Earth-like worlds.
The planet circles Kepler-22, a G5-type star in the constellation Cygnus about 640 light-years away. Kepler-22b was detected by the transit method: each passage across its star caused a tiny, regular dip in starlight. Its orbit lasts about 290 days, placing it within the star’s broadly defined habitable zone.
“Habitable zone” does not mean that scientists have found life, liquid water, or a surface suitable for humans. It is simply the region where temperatures could permit liquid water under the right atmospheric conditions. Kepler-22b’s size and composition remain uncertain; it may be a rocky super-Earth, a water-rich world, or a planet with a thick gaseous envelope.
Kepler-452b is a common mix-up because it also orbits within the habitable zone of a Sun-like star, but it was announced later, in 2015.