Proxima Centauri b is an exoplanet orbiting within the habitable zone of the red dwarf star Proxima Centauri in the constellation Centaurus. It can also be referred to as Proxima b, or Alpha Centauri Cb. The host star is the closest star to the Sun, at a distance of about 4.2 light-years (1.3 parsecs) from Earth, and is part of the larger triple star system Alpha Centauri. Proxima b and Proxima d, along with the currently disputed Proxima c, are the closest known exoplanets to the Solar System. Proxima Centauri b orbits its parent star at a distance of about 0.04848 AU (7.253 million km; 4.506 million mi) with an orbital period of approximately 11.2 Earth days. Its other properties are only poorly understood, but it is probably a terrestrial planet with a minimum mass of 1.06 M🜨 and a slightly larger radius than that of Earth. The planet orbits within the habitable zone of its parent star; but it is not known whether it has an atmosphere, which would impact the habitability probabilities. Proxima Centauri is a flare star with intense emission of electromagnetic radiation that could strip an atmosphere off the planet. Announced on 24 August 2016 by the European Southern Observatory (ESO), Proxima Centauri b was confirmed via several years of Doppler spectroscopy measurements of its parent star. The detection of Proxima Centauri b was a major discovery in planetology, and has drawn interest to the Alpha Centauri star system as a whole. As of 2023, Proxima Centauri b is believed to be the best-known exoplanet to the general public. The exoplanet's proximity to Earth offers an opportunity for robotic space exploration.
Discovery
Proxima Centauri had become a target for exoplanet searches already before the discovery of Proxima Centauri b, but initial studies in 2008 and 2009 ruled out the existence of larger-than-Earth exoplanets in the habitable zone. Planets are very common around dwarf stars, with on average 1–2 planets per star, and about 20–40% of all red dwarfs have one in the habitable zone. Additionally, red dwarfs are by far the most common type of stars. Based on observations with instruments at the European Southern Observatory in Chile prior to 2016, motion anomalies were identified in Proxima Centauri that could not be satisfactorily explained by flares or chromospheric activity of the star. This suggested that Proxima Centauri may be orbited by an exoplanet. In January 2016, a team of astronomers launched the Pale Red Dot project to confirm this hypothetical exoplanet's existence. On 24 August 2016, the team led by Anglada-Escudé proposed that a terrestrial exoplanet in the habitable zone of Proxima Centauri could explain these anomalies and announced Proxima Centauri b's discovery. In 2022, the exoplanet Proxima Centauri d, which orbits even closer to the star, was confirmed. An exoplanet candidate named Proxima Centauri c was reported in 2020, but its existence has since been disputed due to potential artifacts in the data. The claimed existence of a dust belt around Proxima Centauri remains unconfirmed.
Physical properties
Distance, orbital parameters and age Proxima Centauri b is the closest exoplanet to Earth, at a distance of about 4.2 ly (1.3 parsecs). It orbits Proxima Centauri every 11.185 Earth days at a distance of about 0.048 AU, over 20 times closer to Proxima Centauri than Earth is to the Sun. As of 2021, it is unclear whether it has a significant eccentricity but Proxima Centauri b is unlikely to have any obliquity. The age of the planet is unknown; Proxima Centauri itself may have been captured by Alpha Centauri and thus not necessarily of the same age as the latter pair of stars, which are about 5 billion years old. Proxima Centauri b is unlikely to have stable orbits for moons.
Mass, radius and composition As of 2025, the estimated minimum mass of Proxima Centauri b is 1.055±0.055 M🜨; other estimates are similar, but all estimates are a minimum because the inclination of the planet's orbit is not yet known. Assuming an inclination of 47°, coplanar with its host star's rotation, its true mass would be 1.44±0.21 M🜨. This makes it similar to Earth, but the radius of the planet is poorly known and hard to determine—estimates based on possible composition give a range of 0.94 to 1.4 R🜨, and its mass may border on the cutoff between Earth-type and Neptune-type planets, if that value is lower than previously estimated. Depending on the composition, Proxima Centauri b could range from being a Mercury-like planet with a large core—which would require particular conditions early in the planet's history—to a very water-rich planet. Observations of the Fe–Si–Mg ratios of Proxima Centauri may allow a determination of the composition of the planet, since they are expected to roughly match the ratios of any planetary bodies in the Proxima Centauri system; various observations have found Solar System-like ratios of these elements. Little is known about Proxima Centauri b as of 2021—mainly its distance from the star and its orbital period—but a number of simulations of its physical properties have been done. A number of simulations and models have been created that assume Earth-like compositions and include predictions of the galactic environment, internal heat generation from radioactive decay and magnetic induction heating, planetary rotation, the effects of stellar radiation, the amount of volatile elements the planet consists of and the changes of these parameters over time. Proxima Centauri b likely developed under different conditions from Earth, with less water, stronger impacts and an overall faster development, assuming that it formed at its current distance from the star. Proxima Centauri b probably did not form at its current distance to Proxima Centauri, as the amount of material in the protoplanetary disk would be insufficient. Instead, the planet, or protoplanetary fragments, likely formed at larger distances and then migrated to the current orbit of Proxima Centauri b. Depending on the nature of the precursor material, it may be rich in volatiles. A number of different formation scenarios are possible, many of which depend on the existence of other planets around Proxima Centauri and which would result in different compositions.
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