K2-21 is an orange K-type dwarf (about 0.68 solar masses, 4,222 K, 0.65 solar radii), 83.6 light-years from Earth. K2-21 b is a planet with 1.8 Earth radii, with an orbital period of 9.3 days, at 0.076 AU from its star, on an eccentric orbit (e ≈ 0.10), discovered in 2015 via transit. K2-21 c is a planet with 2.5 Earth radii, with an orbital period of 15.5 days, at 0.107 AU from its star, on an eccentric orbit (e ≈ 0.21), discovered in 2015 via transit.