SPECULOOS-3 is a cool M-dwarf (about 0.10 solar masses, 2,800 K, 0.12 solar radii). SPECULOOS-3 b is an Earth-sized planet on a 17-hour orbit around a nearby ultracool dwarf, likely a bare roasted rock. SPECULOOS-3 b is a planet with 1.0 Earth radii, with an orbital period of 17.3 hours, at 0.007 AU from its star, and an equilibrium temperature near 553 K, discovered in 2024 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. SPECULOOS-3 b skims its star closer than Mercury orbits the Sun.