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Alpha Centauri system is a system comprising of four planets and three stars, located 4.3 light-years away in the constellation Centaurus in the caelregio Simianus. It is the nearest star system to our solar system.

Stellar components[]

The primary star of this system is Alpha Centauri A, which is the sun-like star, though bigger and brighter than our Sun. Alpha Cen A is richer in metals and older than our own star. The secondary star is Alpha Centauri B, which is an orange K-type star, meaning it is smaller, dimmer and cooler than our Sun. Separation between two stars range from 11.2 AU to 35.6 AU (1.67 to 5.30 Tm) throughout the 80-year orbit. The ternary star in this system is Alpha Centauri C, well known as Proxima Centauri, which is the nearest individual star to our solar system. It orbits at approximately 15,000 AU or 0.24 light-years from the AB pair, however it is possible that it is not bound to the system if is moving in a hyberbolic path. Alpha Cen C is a dim red dwarf of spectral class M5V-VIe massing one-eighth that of the Sun. This star gives off 600 times less radiation than our sun emits.

Planetary system[]

Class Semimajor axis (AU) Eccentricity Orbital period (yr) Radius (R) Mass (M) Density (g/cm3) Gravity (g) Rotation period (d) Surface temp (K)
Ab? ? 1.1 ? ~1 ~3.3-7 ~9-35 ? ? ? ?
Bc? LEcB ~0.1415122 ≤0.24 ≤0.05589041 0.92 ? ? ? ≤20.4 861
Cd LEcB 0.02885 0.04 0.014032877 0.81 ≥0.26 2.69 0.396 5.122 ~360?
Cb OEbB 0.04857 0.109 0.030641589041 1.30 1.60 4.01 0.946 11.18418 234
Cc INM 1.489 0.04 5.282192 ? 7 ? ? ? 39

Alpha Centauri system has six planets, possibly one around Alpha Cen A, possibly one around Alpha Cen B, and three, possibly four, around Proxima Cen. One planet was detected in a very close orbit around Alpha Cen B in October 2012, but in 2015 it was discovered to be an artifact in the data, and therefore it is now disproven.

Around Alpha Cen A, both planets are rocky/metallic, one is a mid-Earth and one is a super-Earth. Around Alpha Cen B, all four are rocky, one sub-Earth, one mid-Earth, and one super-Earth.

Their orbital periods range from seven days to nine months.

Alpha Centauri Bb[]

Alpha Centauri Bb (Ixionidae) was the only planet detected in the Alpha Centauri system that is now refuted. This was one of the least massive confirmed planet detected by radial velocity survey. It massed 1.28 M and sizes 1.18 R, corresponding to its density of 4.32 g/cm3. The surface gravity was slightly less than Earth's, at 92% the Earth's strength.

Alpha Centauri Bb was tidally locked to the parent star, meaning one hemisphere always faces the star while the other never sees it. The surface temperature was hot, at 1400°F, because the planet orbits so close to the star at just 124 the Earth–Sun distance. The surface of this planet would mostly be covered in lava due to strong tidal heating caused by parent star's gravitational forces, making it volcanically active.

Alpha Centauri Bc[]

Alpha Centauri Bc is a candidate sub-Earth in a 20 day orbital period. The planet masses a third of that of Earth. This planet is tide-locked to the star with a 20.4-day period. Its density is low for a rocky world, just half that of Earth's, meaning this planet has only a tiny core made mostly of iron. In response to its lower mass and size, Alpha Cen Bc has lot lower gravity than Earth. If a person weighs 150 lbs. on Earth, a person of same mass would weigh just 62 lbs on this exoplanet.

Its surface temperature when facing the star is 1091°F and much of that surface would be covered in lava, but on the side facing away the star, the surface is icy with the temperature of about 300 degrees below zero. So Alpha Cen Bc is a world of fire and ice, since it has no atmosphere capable of redistributing heat.

Alpha Centauri Cb[]

Main article: Proxima Centauri b

Alpha Centauri Cb (more commonly known as Proxima Centauri b) is the first planet that was discovered around the star Proxima Centuari. Despite the fact that it orbits 20 times closer to the star than Earth is to the Sun, the planet is in the habitable zone, though it is tidally locked, meaning its rotation and orbital periods are identical at 11.186 days or 268.5 hours. Because of the tidal locking, only one side ever faces the star, which causes the night and day sides to have extreme temperature variations, especially if it doesn't have an atmosphere to redistribute the heat. It is 49% more massive than Earth, but its density and gravity are less than Earth's. The planet is barren with small ponds of liquid water on and around the terminator line.

Alpha Centauri Cc[]

Main article: Proxima Centauri c

Alpha Centauri Cc (more commonly known as Proxima Centauri c) is the second planet found orbiting Proxima. It has a mass of 7 earths and it may have large rings, because direct imaging showed that Proxima c was far too bright for a planet of that size and distance from its star, but it was already confirmed not to be a background star, so it was theorised that the planet may have a ring system with a size of around 5 RJ. Proxima c orbits at a distance of around 1.5 AU from Proxima, which makes the planet is far too cold to be habitable. Another limiting factor of its habitability is the fact that it is probably a gas dwarf, a gaseous planet with a mass lower than Neptune's (17.15 M⊕).

Alpha Centauri Cd[]

Main article: Proxima Centauri d

In 2020, astronomers discovered a third planet around Proxima Cen using radial velocity data from the Very Large Telescope, initially part of a study on Proxima b's mass. The RV signal was later confirmed in a follow-up analysis in February 2022. It has a mass of ≥0.26±0.05 M🜨 and a radius of 0.81±0.08 R🜨, which means it is a sub-Earth rocky planet, however it is likely not capable of supporting life thanks to its proximity to the star. The planet takes 5.122 days to orbit, and probably takes the same amount of time to rotate thanks to its proximity to its star, meaning it is probably tidally locked. This could give the planet a few lava lakes on the day side, and probably mostly ice on the night side if it doesn't have an atmosphere. If it does have an atmosphere, then it can distribute heat across its surface, and it also raises the chances of it being habitable. Its orbit has a low eccentricity and a semi-major axis of 0.02885 +0.00019 −0.00022. It is probably either a Venusian, Martian, or Mercurian-type planet.

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