AB Mass Center0.0........................
Alpha Centauri A10.979.90.51979.231.101.23......1.3-2.3
Inner H.Z.-A Edge?1.171.20079.23...............
Outer H.Z.-A Edge?2.333.39079.23...............
Alpha Centauri B12.879.90.51979.230.930.86......1.3-2.3
Inner H.Z.-B Edge?0.560.43079.23...............
Outer H.Z.-B Edge?1.101.20079.23...............

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. Although the initial display shows the system's actual orbital tilt (at an inclination of 79.2) from the visual perspective of an observer on Earth, the orbital inclination of any planet that may be discovered someday around either star would likely be different from those of the habitable zone orbits shown here. At an estimated separation of about 13,000 AUs, Proxima Centauri is too far away to be depicted.

The distance separating Alpha Centauri A from its companion star B averages 23.7 AUs (of a semi-major axis), but swings between 11.4 and 36.0 AUs away in a highly elliptical orbit (e= 0.519) that takes almost 80 years to complete (see Dimitri Pourbaix, 2000 in the new Sixth Catalog of Orbits of Visual Binaries; 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, Stars A and B are about two AUs farther apart than the average orbital distance of Saturn around the Sun, while their widest separation is still about six AUs farther the average orbital distance of 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).

On February 25, 2008, a team of astronomers released a paper on simulation results which indicate that multiple-planet systems could have formed in close orbits around both Alpha Centauri A and B. Their simulations suggest that at least one planet in the one to two Earth-mass range could have formed within orbital distances of 0.5 to 1.5 AUs around both heavy-element-rich stars; of particularly note, the simulations frequently generated a Earth-like planet in or near Star B's habitable zone (where liquid water could exist on the planet's surface). Additional simulation work presented in the paper also indicates that long-term telescopic observations may detect wobbles from such planets using the radial velocity method. Star B, the chromospherically calmer, orange-red dwarf, is an easier target for detecting wobbles from terrestrial planets, possibly within only three years of "high cadence" observations for a 1.8 Earth-mass planet (more from New Scientist and Guedes et al, 2008).


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