GJ 1252 is a cool M-dwarf (about 0.38 solar masses, 3,458 K, 0.39 solar radii), 20.4 light-years from Earth. GJ 1252 b is a 1.3-Earth-mass planet, with an orbital period of 12.4 hours, discovered in 2020 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. At 1.2 Earth radii, GJ 1252 b is a compact world comparable in size to rocky planets in our Solar System.