K2-288 B b is a small, temperate planet found by members of the public. It orbits the fainter star of K2-288, a pair of red dwarfs about 214 light-years away, every 31.39 days. It is about 1.9 times Earth's width, with an estimated equilibrium temperature, set by starlight alone, of about 226 kelvin.
Volunteers spotted it while classifying light curves from NASA's K2 mission on Exoplanet Explorers, a citizen-science project on the Zooniverse platform. A team led by Adina Feinstein followed it up and validated it as a planet in 2019. A transit shows how much a planet dims the light reaching us, which gives its size only in proportion to the star it crosses. Because K2-288 is two stars too close together to separate in K2's images, the team had to work out which one the planet orbits. They concluded it most likely orbits the secondary, smaller star, K2-288 B, which is why the planet carries a B in its name.
That places it in or near the habitable zone of its star, the range of distances where liquid water could last on a surface. The two stars are about 55 times Earth's distance from the Sun apart, and K2-288 B b orbits more than 0.1 times Earth's distance from the Sun from its own star. The team called the arrangement unique. Its size also falls in the gap in the planet radius distribution, a range of sizes where planets are less common, which may mean it is in a transitional stage of its evolution. It was the third planetary system found through the Exoplanet Explorers program. Its mass has not been measured.