GJ 3942 is a cool M-dwarf (about 0.63 solar masses, 3,867 K, 0.61 solar radii), 16.9 light-years from Earth. GJ 3942 b is a 7.1-Earth-mass planet, with an orbital period of 6.9 days, at 0.061 AU from its star, and an equilibrium temperature near 590 K, on an eccentric orbit (e ≈ 0.12), discovered in 2017 via radial velocity. The planet betrayed itself through the star's reflex motion — a technique that first revealed worlds around Sun-like stars in the 1990s and remains a workhorse of exoplanet discovery.