Alpha Centauri

Alpha Centauri System

Factual

Most people — at least, science fiction fans — know that Alpha Centauri is our nearest stellar neighbor beyond the Sun. It’s actually a triple star system, comprising a sun-like G star, Alpha Centauri A, a slightly cooler K star, Alpha Centauri B, and orbiting both at a much greater distance, a red dwarf, Alpha Centauri C. Because the latter is currently at a place in its orbit that is much closer to Earth than the others, it is also known as Proxima Centauri. Another classical name for the system as a whole is Rigil Kentaurus.

Multiple star systems are problematic for the formation of planets stable enough to evolve life, because the complex gravitational interactions could tear a planet loose from one star, and to orbit both (unless the stars are very close together) the planet would be too far out to be warm enough. The scene in the first Star Wars movie, of Tatooine’s twin suns setting, shows the suns only a few diameters apart; this is very close as binary stars go, they’d be orbiting each other at perhaps a tenth of Mercury’s distance from the Sun. It turns out, though, that Alpha Centauri isn’t that bad.

Alpha Centauri B orbits A (actually they both orbit a common center of gravity, but that is much closer to A than to B) in an elliptical path whose distance varies from about 11 AU (astronomical units, the average distance between the Earth and the Sun) to about 36 AU, or roughly from Saturn’s orbit to Pluto’s orbit. It takes about 79.91 Earth years for a complete orbit. It turns out that orbits within the habitable zone of both A and B are far enough from the other star (B and A, respectively) to be stable even at closest approach. This means we can have habitable planets around both stars, with travel between them roughly equivalent to a voyage to Saturn, Pluto or points between, depending on where the stars are in their orbit.

Proxima, at 12,000 to 13,000 AU (about 0.2 light years), is too small and far away to affect the planetary systems of A or B. Indeed, it’s possible that it isn’t properly part of the multiple system, and is just passing by.

Fictional

In my T-space stories, Sawyers World orbits Alpha Centauri A, and Kakuloa orbits Alpha Centauri B. See Alpha Centauri: First Landing and Alpha Centauri: Sawyers World for initial exploration of the system and first landings.

System

Designation	Planet	Diameter (mi)	Day	Gravity (g)	Density	Ve (km/s)	Vl (km/s)
α Cen A	star A	(G-type star)
α Cen A I	TBD - rocky, Mercury-like
α Cen A II	Sawyers World	8320	25:44:17	0.986	5.23	11.49	7.68
α Cen A III	TBD - gas subgiant?
α Cen B	star B	(K-type star)
α Cen B I	TBD - rocky
α Cen B II	Kakuloa	7880	~23	1.028	5.76	11.35	7.55
α Cen B III	if any? Mars like?
α Cen C	Proxima	(red dwarf star)

Ve is escape velocity, Vl is launch velocity to low orbit ignoring drag but adjusting for planetary rotation at the equator.