Alpha Centauri system is a system comprising of five 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 | OEbH | 0.4077 | 0.20167 | 0.2483 | 1.3342 | 0.7689 | 1.785 | 0.432 | 45.3377 | 469 |
| Ac | DEaG | 0.8706 | 0.13066 | 0.7746 | 1.1549 | 3.4760 | 12.444 | 2.606 | 1.0063 | 552 |
| Bc | LEcB | 0.0718 | 0.03847 | 0.0199 | 0.8517 | 0.3133 | 2.796 | 0.432 | 7.2771 | 861 |
| Bd | SBEbH | 0.4227 | 0.09911 | 0.2843 | 1.1656 | 0.9958 | 3.468 | 0.733 | 41.5379 | 406 |
| Be | OEaC | 0.7801 | 0.11338 | 0.7129 | 2.5969 | 5.1396 | 1.618 | 0.762 | 1.8899 | 401 |
Alpha Centauri system has five planets, two around Alpha Cen A, three around Alpha Cen B, and zero around Proxima Cen. One of those planets was detected in October 2012, the innermost planet around Alpha Cen B, but now the existence is refuted.
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.
All five planets orbit within one astronomical unit of their respective star. Their orbital periods range from seven days to nine months.
Alpha Centauri Ab
Alpha Centauri Ab is the inner planet of Alpha Cen A. It is bigger than the Earth, but less massive, so the planet's density is 1.8 g/cm3, telling that this planet's main composition are water and rock. Its thick atmosphere is chiefly hydrogen, helium and carbon dioxide. The planet orbits in a distance similar to Mercury and eccentricity similar to Mercury.
Alpha Cen Ab is the only planet in the six-planet star system to have an axial tilt greater than 20°. In this case, it tilts at 26.16°, slightly more tilted than Earth's (23.44°) and Mars' (25.13°). Due to this, the planet has seasons just like these two planets, but its eccentricity would make seasonal effects blurred. The planet's rotation is very long, taking 451⁄3 days to complete one spin on its axis.
Alpha Centauri Ac
Alpha Centauri Ac is the outer planet of Alpha Cen A. It orbits more than twice the distance of the inner planet and takes more than three times longer to orbit the star as the inner planet. However, the planet is hotter than the inner planet due to greenhouse effect of thick atmosphere. An average surface temp is 534°F, compared to 385°F for the inner planet.
Alpha Cen Ac takes just 13 minutes longer to rotate once on its axis than Earth is. It is 31⁄2 times more massive than Earth, but just 15% bigger in size than Earth, making this planet extremely dense. The planet has no core as the bulk is made mostly of heavy metals such as iron, lead, platinum, and tin. The planet's strength of surface gravity is similar to Jupiter and 21⁄2 times that of Earth's.
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 1⁄24 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 sub-Earth in a one-week orbital period. The planet masses a third of that of Earth. This planet is tide-locked to the star with a 71⁄4-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 Bd
Alpha Centauri Bd is a planet almost identical to Earth in mass, but it is a little bit larger. It orbits at 0.42 AU and takes 104 days to revolve once around the parent star. It's surface and atmosphere are sulfur-rich, classifying the world as sulfur planet with the surface temp of 271°F. The atmosphere contains significant amounts of sulfur dioxide, hydrogen sulfide, and sulfuric acid, and the clouds are made of sulfuric acid.
Alpha Centauri Be
Alpha Centauri Be is the outermost planet of the Alpha Cen B planetary system; it orbits at 0.78 AU, three quarter the Earth–Sun distance and takes 260 days to orbit once around the orange star. However, the planet is not always found at 0.78 AU, it varies from 0.69 AU to 0.87 AU. The rotation period is 451⁄3 hours, almost twice as long as Earth's.
The planet masses over five Earth masses and is 160% bigger. The planet's mean density is 1.6 grams per cubic centimeter and surface gravity 76% that of Earth's.