AB Mass Center0.0........................
Alpha Centauri A10.779.90.5279.2360,000130......1.3-2.3
Center of H.Z. A1.251.4079.2...............
Alpha Centauri B12.879.90.5279.2300,00090......1.3-2.3
Center of H.Z. B0.740.66079.2...............

NOTE: This animation attempts to relate the orbits and possible habitable zones of Stars A and B in the Alpha Centauri AB system to their common center of mass. The initial display does not show the system's actual orbital tilt (at an inclination of 79.2) from the visual perspective of an observer on Earth. Indeed, the orbital inclination of any planet that may be discovered some day around either star would likely be different from those of the habitable zone orbits depicted here. At about a fifth of a light-year away, Proxima Centauri is too far away to be depict here.

The distance separating Alpha Centauri A from its companion star B averages 23.4 AUs (of a semi-major axis), but swings between 11.2 and 35.6 AUs away in a highly elliptical orbit (e= 0.52) that takes almost 80 years to complete (Heintz Orbit Table, 12/1997; and Worley and Heintz, 1983). As viewed from a hypothetical planet around either star, the brightness of the other increases as the two approach and decreases as they recede. However, the variation in brightness is considered to be insignificant for life on Earth-type planets around either star. At their closest approach of 11.2 AU, the distance between A and B is somewhat closer than the distance Earth and Saturn, while their farthest separation is a little greater than the average distance between the orbits of the Earth and Neptune.

In a binary system, a planet must not be located too far away from its "home" star or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. Recent numerical integrations, however, suggest that stable planetary orbits exist: within three AUs (four AUs for retrograde orbits) of either Alpha Centauri A or B in the plane of the binary's orbit; only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for planets circling both stars (Weigert and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like the Solar System: Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU), and Mars (1.5 AUs).


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