Planetary system

Confirmed planetsedit

Proxima Centauri bedit

Proxima Centauri b is a terrestrial planet discovered in 2016 by astronomers at the European Southern Observatory. It has a minimum mass of 1.17 M_⊕ (Earth masses) and orbits approximately 0.049 AU from Proxima Centauri, placing it in the star's habitable zone.

Proxima Centauri cedit

Proxima Centauri c was formally discovered and confirmed in 2020 and is a likely super-Earth or mini-Neptune. It has a mass of roughly 7 M_⊕ and orbits about 1.49 AU from Proxima Centauri with a period of 1,928 days (5.28 yr). In June 2020, a large ring system encircling the planet was possibly detected.

Controversial and hypothetical planetsedit

Alpha Centauri Bbedit

In 2012, a planet around Alpha Centauri B was announced, Alpha Centauri Bb, but in 2015 a new analysis concluded that it almost certainly does not exist and was just a spurious artefact of the data analysis.

Unconfirmed discoveryedit

Whilst ruling out the existence of Alpha Centauri Bb, a possible transit of a separate exoplanet in 2013 was observed. The transit event could correspond to a planetary body with a radius around 0.92 R_⊕. This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less, with only a 5 percent chance of it having a longer orbit. The median of the likely orbits is 12.4 days with an impact parameter of around 0–0.3. Its orbit would likely have an eccentricity of 0.24 or less. Like the probably spurious Alpha Centauri Bb, it likely has lakes of molten lava and would be far too close to Alpha Centauri B to harbour life.

Around Proxima Centauri, a small spike with a periodicity of 5.15 days was found while revisiting Proxima Centauri b. Were it a planetary companion, it would have to be at most 0.29 Earth masses, but it could also be random noise found in the data.

Hypothetical planetsedit

Additional planets may exist in the Alpha Centauri system, either orbiting Alpha Centauri A or Alpha Centauri B individually, or in large orbits around Alpha Centauri AB. Because both stars are fairly similar to the Sun (for example, in age and metallicity), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have used various radial velocity or star transit methods in their searches around these two bright stars. All the observational studies have so far failed to find evidence for brown dwarfs or gas giants.

In 2009, computer simulations showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star. Certain special assumptions, such as considering that the Alpha Centauri pair may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster), would permit an accretion-friendly environment farther from the star. Bodies around Alpha Centauri A would be able to orbit at slightly farther distances due to its stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around Alpha Centauri make the likelihood of terrestrial planets greater than otherwise. A theoretical study indicates that a radial velocity analysis might detect a hypothetical planet of 1.8 M_⊕ in Alpha Centauri B's habitable zone.

Radial velocity measurements of Alpha Centauri B made with the High Accuracy Radial Velocity Planet Searcher spectrograph were sufficiently sensitive to detect a 4 M_⊕ planet within the habitable zone of the star (i.e. with an orbital period P = 200 days), but no planets were detected.

Current estimates place the probability of finding an Earth-like planet around Alpha Centauri at roughly 75%. The observational thresholds for planet detection in the habitable zones by the radial velocity method are currently (2017) estimated to be about 50 M_⊕ for Alpha Centauri A, 8 M_⊕ for Alpha Centauri B, and 0.5 M_⊕ for Proxima Centauri.

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B,note but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets difficult. Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet.

In the Solar System, Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System, providing the inner planets with a source of water and various other ices. In the Alpha Centauri system, Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming, enriching the area around Alpha Centauri with volatile materials. This would be discounted if, for example, Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or vice versa), or if Alpha Centauri A and B themselves were able to perturb comets into each other's inner system as Jupiter and Saturn presumably have done in the Solar System. Such icy bodies probably also reside in Oort clouds of other planetary systems. When they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars, many of these icy bodies then travel star-wards. Such ideas also apply to the close approach of Alpha Centauri or other stars to the Solar System, when, in the distant future, the Oort Cloud might be disrupted enough to increase the number of active comets.

To be in the habitable zone, a planet around Alpha Centauri A would have an orbital radius of between about 1 and 2 AU so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone is between about 0.7 and 1.2 AU.

With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri AB were among the listed "Tier 1" target stars for NASA's Space Interferometry Mission (SIM). Detecting planets as small as three Earth-masses or smaller within two AU of a "Tier 1" target would have been possible with this new instrument. The SIM mission, however, was cancelled due to financial issues in 2010.

Circumstellar discsedit

Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24 µm (mid/far-infrared) band surrounding α Centauri AB, which may be interpreted as evidence for a sparse circumstellar disc or dense interplanetary dust. The total mass was estimated to be between 10−7 to 10−6 the mass of the Moon, or 10–100 times the mass of the Solar System's zodiacal cloud. If such a disc existed around both stars, α Centauri A's disc would likely be stable to 2.8 AU, and α Centauri B's would likely be stable to 2.5 AU. This would put A's disc entirely within the frost line, and a small part of B's outer disc just outside.