| Extrasolarplanet: | yes |
| Proxima Centauri d | |
| Apsis: | astron |
| Discovery Site: | VLT-ESPRESSO |
| Discovered: | 2020 |
| Discovery Method: | Radial velocity |
| Star: | Proxima Centauri |
| Single Temperature: | 360K |
Proxima Centauri d (also called Proxima d) is a candidate exoplanet orbiting the red dwarf star Proxima Centauri, the closest star to the Sun and part of the Alpha Centauri triple star system. Together with two other planets in the Proxima Centauri system, it is the closest known exoplanet to the Solar System, located approximately 4.2ly away in the constellation of Centaurus. The first signs of the exoplanet emerged as a weak 5.15-day signal in radial velocity data taken from the Very Large Telescope during a 2020 study on Proxima b's mass. This signal was formally proposed to be a candidate exoplanet by Faria et al. in a follow-up paper published in February 2022.
Proxima d is a sub-Earth at least one-quarter of the mass of Earth (or twice the mass of Mars), orbiting at roughly 0.029AU every 5.1 days. It is the least massive and innermost known planet of the Proxima Centauri system. It is the least massive exoplanet detected with the radial velocity method . Its proximity to the star and short orbital period of 5.1 days suggest that it is likely tidally locked due to strong tidal forces. Although Proxima d orbits too close to its star to have a habitable equilibrium temperature (which likely reaches 360K from about 190% of Earth's irradiation—assuming an Earth-like reflectivity, it is theoretically possible that Proxima d possesses polar regions with habitable temperatures.
Proxima d is considered a candidate exoplanet by its discoverers and the NASA Exoplanet Archive, because it has not been independently confirmed by more than one observatory. However, some astronomers regard Proxima d as confirmed because it could be detected via different methods of measuring the same radial velocity data from which Proxima d was discovered. As of 2022, Proxima d has been detected by the ESPRESSO spectrograph on the Very Large Telescope, and also at a 2σ level (which does not constitute confirmation) by the HARPS spectrograph at the La Silla Observatory.