K2-39 is an orange K-type dwarf (about 0.66 solar masses, 4,881 K, 2.97 solar radii), 307 light-years from Earth. K2-39 b is a 0.09-Jupiter-mass companion, with an orbital period of 4.6 days, discovered in 2016 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. Its low mass makes K2-39 b a Neptune- or super-Earth-class world rather than a gas giant. Repeated transit observations constrain the planet's orbital inclination and stellar density, anchoring the system's physical scale.