Proxima Centauri d, also known as Proxima d, or Alpha Centauri Cd, is an extrasolar planet orbiting the red dwarf star Proxima Centauri, approximately 4.24 light years away from Earth in the constellation of Centaurus. Like its counterparts Proxima Centauri b and c, it is one of the closest extrasolar planets ever found, only 4 light years away.
Astronomers discovered the planet using radial velocity data from the European Southern Observatory's Very Large Telescope, originally to study Proxima Centauri b's mass.[1] The radial velocity signal of Proxima d was later confirmed by Faria et al. in a follow up analysis in February 2022.
The planet is a sub-Earth that has at least one-quarter of the mass of the Earth (twice that of Mars), orbiting around 0.029 AU (4.3 million million km; 2.7 million mi) away from Proxima Centauri every 5.1 days. It is the least massive and innermost planet of the Proxima Centauri system, and is currently the least massive extrasolar planet detected with the radial velocity method.
The planet's equilibrium temperature is around 360 K (87 °C; 188 °F). Assuming Earth-like reflectivity, it is theoretically possible for Proxima d to possess polar regions that contain habitable temperatures.