Proxima Centauri b


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 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 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 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, 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 . It orbits Proxima Centauri every Earth days at a distance of about, over 20 times closer to Proxima Centauri than Earth is to the Sun., 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

, the estimated minimum mass of Proxima Centauri b is ; 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. 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, 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 species 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.

Tidal locking

Proxima Centauri b is likely to be tidally locked to the host star, which for a 1:1 orbit would mean that the same side of the planet would always face Proxima Centauri. It is unclear whether habitable conditions can arise under such circumstances as a 1:1 tidal lock would lead to an extreme climate with only part of the planet habitable.
However, the planet may not be tidally locked. If the eccentricity of Proxima Centauri b was higher than 0.1–0.06, it would tend to enter a Mercury-like 3:2 resonance or higher-order resonances such as 2:1. Additional planets around Proxima Centauri and interactions with Alpha Centauri could excite higher eccentricies. If the planet is not symmetrical, a capture into a non-tidally locked orbit would be possible even with low eccentricity. A non-locked orbit, however, would result in tidal heating of the planet's mantle, increasing volcanic activity and potentially shutting down a magnetic field-generating dynamo. The exact dynamics are strongly dependent on the internal structure of the planet and its evolution in response to tidal heating. A non-locked planet may experience ocean tides much more intense than Earth.

Host star

Proxima Centauri b's parent star Proxima Centauri is a red dwarf, radiating only 0.005% of the amount of visible light that the Sun does and an average of about 0.17% of the Sun's energy. Despite this low radiation, due to its close orbit Proxima Centauri b still receives about 70% of the amount of infrared energy that the Earth receives from the Sun. Proxima Centauri is also a flare star with its luminosity at times varying by a factor of 100 over a timespan of hours, its luminosity averaged at.
Proxima Centauri has 12.2% of the Sun's mass and 15.4% of the radius of the Sun. With an effective temperature of, it has a spectral type of M5.5V, making it an M-type main-sequence star that is fusing hydrogen at its core to generate energy. The magnetic field of Proxima Centauri is considerably stronger than that of the Sun, with an intensity of ; it varies in a seven-year-long cycle.
It is the closest star to the Sun, hence the name "Proxima", with a distance of. Proxima Centauri is part of a multiple star system, whose other members are Alpha Centauri A and Alpha Centauri B which form a binary star subsystem. The dynamics of the multiple star system could have caused Proxima Centauri b to move closer to its host star over its history. The detection of a planet around Alpha Centauri B in 2012 was considered questionable. Despite its proximity to Earth, Proxima Centauri is too faint to be visible to the naked eye, except during superflares.

Surface conditions

Climate

Proxima Centauri b is located within the classical habitable zone of its star and receives about 65% of Earth's irradiation. Its equilibrium temperature is estimated to be about. Various factors, such as the orbital properties of Proxima Centauri b, the spectrum of radiation emitted by Proxima Centauri and the behaviour of clouds and hazes influence the climate of an atmosphere-bearing Proxima Centauri b.
There are two likely scenarios for an atmosphere of Proxima Centauri b: in one case, the planet's water could have condensed and the hydrogen would have been lost to space, which would have only left oxygen and/or carbon dioxide in the atmosphere after the planet's early history. However, it is also possible that Proxima Centauri b had a primordial hydrogen atmosphere or formed farther away from its star, which would have reduced the escape of water. Thus, Proxima Centauri b may have kept its water beyond its early history. If an atmosphere exists, it is likely to contain oxygen-bearing gases such as oxygen and carbon dioxide. Together with the star's magnetic activity, they would give rise to auroras that could be observed from Earth if the planet has a magnetic field.
Climate models including general circulation models used for Earth’s climate have been used to simulate the properties of Proxima Centauri b's atmosphere. Depending on its properties such as whether it is tidally locked, the amount of water and carbon dioxide a number of scenarios are possible: A planet partially or wholly covered with ice, planet-wide or small oceans or only dry land, combinations between these, scenarios with one or two "eyeballs" or lobster-shaped areas with liquid water, or a subsurface ocean with a thin ice cover that may be slushy in some places. Additional factors are: