4.37 light-years away in the southern constellation of Centaurus, the Alpha Centauri triple star system is located. The system consists of the three stars Rigil Kentaurus, Toliman and Proxima Centauri. However, only Rigil Kentaurus and Toliman are visible from Earth with the unaided eye, but appear only as one star. The system is widely known, and can be found in multiple cultures, such as the old Greeks or the Inca. The first flag of Australia for example, featured, besides the constellation Crux and the star Beta Centauri also Alpha Centauri. But not only in our real world is the system very popular, also in the Science Fiction Universe. The star system appears in multiple films, books and video games, where it is often populated by either aliens or humans. In this article we will present you the “real” Alpha Centauri system and we try to answer the question if it is possible to colonize it.
The Alpha Centauri system
The Alpha Centauri system is 4.37 light-years, or in the case of Proxima Centauri 4.244 light-years away from our solar system. This makes it the closest star system and Proxima Centauri the nearest star to the Sun.
Rigil Kentaurus and Toliman are in an orbit around a common barycenter, while the third star is orbiting the two other members of the system once every 550,000 years.
In the night sky, Alpha Centauri can be found in the southern constellation of Centaurus near the star Beta Centauri. Alpha Centauri is the brightest star in the constellation and the third brightest in the night sky, after Sirius and Canopus. Neighbouring constellations are for example Hydra, Carina or Crux.
The system once was also visible from the norther hemisphere too, however, due to the axial precession of the Earth, Alpha Centauri migrated south.
Rigil Kentaurus, or commonly known as Alpha Centauri A is the biggest star in the Alpha Centauri system. The star is a main-sequence star like our Sun, which means that Alpha Centauri A generates power through nuclear fusion of the element hydrogen into helium. Rigil Kentaurus has around 10% more mass than the Sun, which makes the star 22 percent larger and a bit more luminous.
So far no exoplanets have been found in orbit around Alpha Centauri A.
Toliman, or Alpha Centauri B is the second largest star, and the second member of the binary system. The star is also a main-sequence star like the Sun or Rigil Kentaurus, however, Toliman is slightly smaller than the Sun, with 90% of its mass. The star is magnetically more active than its partner and shows a 8.2 year long activity cycle, which can be compared to the 11 year long cycle of our star.
In 2012 and 2013, two potential exoplanets have been observed, but more about that in the paragraph about the exoplanets.
Proxima Centauri or Alpha Centauri C is the third member of the triple star system. The star is 4.244 light-years or 1.301 parsec away from our star, which makes Proxima Centauri our closest known neighbour.
Alpha Centauri C is a main-sequence star as well, however, because of its small size it is classified as a red dwarf. Proxima Centauri has only 12 percent of the mass of the Sun. Due to the low mass, the star is only 0.17% as luminous as our star.
The star is a M-type star, which means it emits most of its light in infrared wavelengths.
Due to its low luminosity and its spectrum, the star was not found until 1915.
In 2017 a cold dust belt was found around Proxima Centauri. The belt lies in a distance of one to four astronomical units (one astronomical unit (AU) is the distance between the Sun and the Earth. Around 150,000,000 km) away from the star, and seems to be as cold as -230°C, which can be compared to the Kuiper belt in our solar system. The scientist also found evidence for another belt 30 astonomical units away from Proxima Centauri, as well as a compact emission source at a distance of about 1.2 arscec from the star.
Exoplanets in the Alpha Centauri system
Alpha Centauri Cb
Alpha Centauri Cb also called Proxima Centauri b or Proxima b is the first planet discovered around Proxima Centauri.
Its existence was announced on August 24, 2016, after the planet was discovered using the HARPS spectograph on the 3.6 m telescope at the La Silla Observatory and the UVES spectrograph on the 8 m Very Large Telescope on the Cerro Paranal in Chile. However, the first indications for Proxima Centauri b were found in archival observation data back in 2013.
Proxima Centauri b is an earth-size planet, slightly larger than Earth and orbits Proxima Centauri inside its habitable zone, the region, where water could be liquid on a planets surface.
It takes Alpha Centauri Cb around 11 days for one complete orbit around its parent star.
If Proxima Centauri b may habour life is the topic of many discussions since its discovery in 2016. One big issue is the planets close proximity to Alpha Centauri C.
Red dwarf stars are in comparision to stars the size of our Sun much more active. Proxima b experiences stellar wind pressures of more than 2,000 times those experienced by Earth. This stellar wind and the increased radiation is capable of destroying a planets atmosphere in a relatively short time.
Another issue, the exoplanet might be tidally locked to its star. This means, one side of the planet always faces Proxima Centauri. The star facing side gets extremely hot and the space facing side extremely cold, which makes it very difficult for life to evolve.
Proxima Centauri c
A few months ago, scientists annouced the possible discovery of a second world around Proxima Centauri.
The new planet, named, Proxima Centauri c or Proxima c, seems to be a relatively large planet with around six times the mass of Earth. This would make Proxima c either a super-Earth or a small gas giant.
The planet orbits outside the habitable zone around 1.5 astonomical units away from the star, and needs around 5.2 years for one orbit.
Only further observation will show if Proxima Centauri c really exists.
Planets around Alpha Centauri B
In 2012, scientist found evidence for a potential exoplanet around the star Toliman. However a paper, published by astronomers in 2015 disproved the existence of Alpha Centauri Bb. The planet was most likely a false artefact in the used data.
In 2013, a second potential exoplanet was found, which would be slightly smaller than Earth and would orbit its parent star in less than 20.4 days. If this planet really exists, it would definitely be to hot to habour life.
In the end, it is hard to say if the Alpha Centauri system is populated by some sort of alien life. With more observations of the system, many questions about the current exoplanets could be answered, and mabey more planets will be found, which could provide the right enviroment for life to evolve or to be colonized by humanity in the future.