K2-121 is an orange K-type dwarf (about 0.71 solar masses, 4,526 K, 0.68 solar radii), 169 light-years from Earth. K2-121 b is a 0.16-Jupiter-mass companion, with an orbital period of 5.2 days, and an equilibrium temperature near 716 K, discovered in 2017 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. Repeated transit observations constrain the planet's orbital inclination and stellar density, anchoring the system's physical scale.