About this world
TRAPPIST-1 d is an Earth-sized planet orbiting TRAPPIST-1, a cool red dwarf (type M8.0V) 40.5 light-years from Earth. It is about 0.79 times the size of Earth and holds 0.39 Earth masses. It completes an orbit every 4.05 days at 0.022 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 286 K (13 °C), which places it in the optimistic habitable zone: the band where liquid water could exist on a rocky surface. By the "cosmic shoreline", its star has probably stripped its atmosphere away, leaving bare rock. Its atmosphere has already been studied: 3 spectroscopic observations, 2 of them with the James Webb Space Telescope.
It was discovered in 2016 by transit with La Silla Observatory. A planet crosses its star and dims it by a tiny, periodic fraction. TRAPPIST-1 has 7 known planets; the others are TRAPPIST-1 b, TRAPPIST-1 c, TRAPPIST-1 e, TRAPPIST-1 f, TRAPPIST-1 g and TRAPPIST-1 h.