K2-251 is an orange K-type dwarf (about 0.52 solar masses, 3,717 K, 0.49 solar radii), 147 light-years from Earth. K2-251 b is a planet with 2.4 Earth radii, with an orbital period of 9.3 days, at 0.069 AU from its star, and an equilibrium temperature near 437 K, discovered in 2018 via transit. The planet was found when it crossed the face of its star, producing periodic dips in brightness that reveal its size and orbital period to space-based photometry. K2-251 b orbits well inside Earth's distance from the Sun.