GJ 436 is a cool M-dwarf (about 0.47 solar masses, 3,586 K, 0.46 solar radii), 9.8 light-years from Earth. GJ 436 b is an eccentric hot Neptune with a huge comet-like tail of escaping hydrogen and helium. GJ 436 b is a 0.07-Jupiter-mass companion, with an orbital period of 2.6 days, at 0.029 AU from its star, on an eccentric orbit (e ≈ 0.14), discovered in 2004 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. The planet's marked eccentricity suggests past scattering events or perturbations from unseen companions.