Proxima Centauri b
Proxima Centauri b (or Proxima b) is the closest, potentially habitable exoplanet to Earth. It is orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the triple star system Alpha Centauri. It is approximately from Earth in the constellation Centaurus, making it, Proxima c, and Proxima d the closest known exoplanets to the Solar System. Proxima Centauri b orbits its star at a distance of roughly with an orbital period of approximately 11.2 Earth days. Its other properties are only poorly understood, but  it is believed to be a possibly Earth-like planet with minimum mass of . The planet orbits within the habitable zone of its star, it is, however, unknown whether it has an atmosphere. Proxima Centauri is a flare star with intense emission of electromagnetic radiation that could rip an atmosphere off the planet. The planet's proximity to Earth offers an opportunity for robotic space exploration, for example, with the Breakthrough Starshot project. 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 types of stars. Before 2016, observations with instruments at the European Southern Observatory in Chile had identified anomalies in Proxima Centauri which could not be satisfactorily explained by flares or chromospheric activity of the star. This suggested that Proxima Centauri may be orbited by a planet. In January 2016, a team  of astronomers launched the Pale Red Dot project to confirm this hypothetical planet's existence. On 24 August 2016, the team led by Anglada-Escudé proposed that an terrestrial exoplanet in the habitable zone of Proxima Centauri could explain these anomalies and announced Proxima Centauri b's discovery. In 2020, another planet named Proxima Centauri c was discovered, and a third planet Proxima Centauri d was confirmed in 2022, while the existence of a dust belt around Proxima Centauri is, , unconfirmed. The discovery of Proxima Centauri b, a planet at habitable distances from the closest star to the Solar System , was a major discovery in planetology and has drawn interest to the Alpha Centauri star system that Proxima is a member of. Physical properties. 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 , over 20 times closer to Proxima Centauri than Earth is to the Sun. it is unclear if it has an 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, which are about 5 billion years old. Proxima Centauri b is unlikely to have stable orbits for moons. , the estimated minimum mass of Proxima Centauri b is ; other estimates are similar but all estimates are dependent on the inclination of the planet's orbit and may be underestimates. This makes it similar to Earth, but the radius of the planet is poorly known and hard to determine and the mass borders on the cutoff between Earth-type and Neptune-type planets. Depending on the composition, Proxima Centauri b could either be 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 these of the planets; various observations have found Solar System-like ratios of these elements. Relatively little is known about Proxima Centauri b —mainly its distance from the star and its orbital period—but a number of simulations of its 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 than 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, it or fragments 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 if 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 isn't symmetrical (triaxial), 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. Star. Proxima Centauri is a red dwarf with a mass equivalent to solar masses and it has a radius of solar radii. With an effective temperature of , it has a spectral type of M5.5V and a luminosity of % that of the Sun. Proxima Centauri is a flare star  and its luminosity varies by a factor of 100 over a timespan of hours. The magnetic field of Proxima Centauri is considerably stronger than that of the Sun, with an intensity of ; it varies in a 7-year long cycle. It is the closest star to the Sun, 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 in 2012 is considered questionable. Despite its proximity to Earth, Proxima Centauri is too faint to be visible to the naked eye with the exception of an instance where a flare made it visible to the naked eye. Surface conditions. Proxima Centauri b is located within the classical habitable zone of its star; it receives about 65% of Earth's irradiation. Its equilibrium temperature is 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 was 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 compounds such as oxygen and carbon dioxide. Together with the star's magnetic activity, they would give rise to aurorae that could be observed from Earth if the planet has a magnetic field. Climate models including general circulation models used for Earth 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 or scenarios with one or two "eyeballs" or lobster-shaped areas with liquid water. Additional factors are: Stability of an atmosphere. The stability of an atmosphere is a major issue for the habitability of Proxima Centauri b: Even if Proxima Centauri b lost its original atmosphere, volcanic activity could rebuild it after some time. A second atmosphere would likely contain carbon dioxide, which would form a more stable atmosphere than an Earth-like atmosphere. In the case of Earth, the amount of water contained within the mantle might approach  that of one Earth ocean. Additionally, impacts of exocomets could resupply water to Proxima Centauri b, if they are present. Delivery of water to Proxima Centauri b. A number of mechanisms can deliver water to a developing planet; how much water Proxima Centauri b received is unknown. Modelling by Ribas "et al." 2016 indicates that Proxima Centauri b would have lost no more than one Earth ocean equivalent of water but later research suggested that the amount of water lost could be considerably larger and Airapetian "et al." 2017 concluded that an atmosphere would be lost within ten million years . The estimates are strongly dependent on the initial mass of the atmosphere, however, and are thus highly uncertain. Life. In the context of exoplanet research, "habitability" is usually defined as the possibility that liquid water exists on the surface of a planet. As normally understood in the context of exoplanet life, liquid water on the surface and an atmosphere are prerequisites for habitability—any life limited to the sub-surface of a planet, such as in a subsurface ocean like in Europa in the Solar System, would be difficult to detect from afar although it may constitute a model for  life in a cold ocean-covered Proxima Centauri b. The habitability of red dwarfs is a controversial subject, with a number of considerations: On the other hand, red dwarfs like Proxima Centauri have a lifespan much longer than the Sun, up to many times the estimated age of the Universe, and thus give life plenty of time to develop. The radiation emitted by Proxima Centauri is ill-suited for oxygen-generating photosynthesis but sufficient for anoxygenic photosynthesis although it is unclear how life depending on anoxygenic photosynthesis could be detected. One study in 2017 estimated that the productivity of a  Proxima Centauri b ecosystem based on photosynthesis may be about 20% that of Earth's. Observation and exploration. , Proxima Centauri b has not yet been directly imaged, as its separation from Proxima Centauri is too small for that. It is unlikely to transit Proxima Centauri from Earth's perspective; all surveys have failed to find evidence for any transits of Proxima Centauri b. The star is monitored for the possible emission of technology-related radio signals by the Breakthrough Listen project which in April-May 2019 detected the BLC1 signal; later investigations however indicated it is probably of human origin . Future large ground-based telescopes and space-based observatories such as the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope could directly observe Proxima Centauri b, given its proximity to Earth, but disentangling the planet from its star would be difficult. Possible traits observable from Earth are the reflection of starlight from an ocean, the radiative patterns of atmospheric gases and hazes and of atmospheric heat transport. Efforts have been done to determine what Proxima Centauri b would look like to Earth if it has particular properties such as atmospheres of a particular composition. Even the fastest  spacecraft built by humans would take a long time to travel interstellar distances; Voyager 2 would take about 75,000 years to reach Proxima Centauri. Among the proposed technologies to reach Proxima Centauri b in human lifespans are solar sails that could reach speeds of 20% the speed of light; problems would be how to decelerate a probe when it arrives in the Proxima Centauri system and collisions of the high-speed probes with interstellar particles. Among the projects of travelling to Proxima Centauri b are the Breakthrough Starshot project, which aims to develop instruments and power systems that can reach  Proxima Centauri in the 21st century. View from Proxima Centauri b. From Proxima Centauri b, the binary stars Alpha Centauri would be considerably brighter than Venus is from Earth. and the Sun would appear as a bright star with an apparent magnitude of 0.40 in the constellation of Cassiopeia. The brightness of the Sun would be similar to that of Achernar or Procyon from Earth.