In December 2011, Kepler-22 b became the first planet NASA's Kepler mission confirmed in the habitable zone of a Sun-like star. The habitable zone is the band of distances where a planet could, in principle, keep liquid water on its surface. Kepler found it by the transit method, catching the small dip in starlight each time the planet crossed in front of its star. Kepler needed three transits to verify a signal, and the result was checked with ground-based telescopes and the Spitzer Space Telescope. The discovery paper, led by William Borucki, called it the first confirmed planet with a measured size to orbit in the habitable zone of any star other than the Sun.
Its star is a G-type star, slightly smaller and cooler than the Sun, about 640 light-years away. The planet takes about 290 days to go around it, at about 0.81 times Earth's distance from the Sun. It is roughly 2.1 times Earth's width. The discovery paper gave 2.4, and later analyses revised the size down. Its equilibrium temperature, the temperature it would have with no atmosphere trapping heat, is estimated at about 279 K, a little above the freezing point of water.
Size is where certainty ends. When it was announced, NASA said scientists did not know whether Kepler-22 b is mostly rocky, gaseous or liquid. That is still true. Knowing its mass would help, because mass and size together give density. Mass is usually measured with radial velocity: tracking the star's tiny back-and-forth wobble as the planet tugs on it. The discovery team's measurements could only cap the mass at 124 Earth masses.
A 2023 study led by Aldo Bonomo used nearly ten years of measurements from the HARPS-N spectrograph across 38 Kepler and K2 planetary systems. It still did not detect the wobble from Kepler-22 b. Instead it set an upper limit of about 9.1 Earth masses. That is a ceiling, not a weight. The planet could be much lighter, so a rocky world, a water-rich world or a planet with a thick gas envelope all remain possible.
TerraForge's model takes that 9.1 Earth-mass ceiling as the planet's mass and, with its 2.1 Earth-radius size, classifies it as an ice giant with a low habitability score. If the true mass is well below the limit, the planet could be a denser, more Earth-like world, so the simulation shows one possibility rather than a measured answer.