At a distance of only 4.36 light years, the two components of the Alpha Centauri binary system are the closest Sun-like stars to Earth. Alpha Centauri A is slightly larger and hotter than our Sun; Alpha Centauri B is slightly smaller and cooler. Marginally closer to us is a dim, lightweight red dwarf known as Proxima Centauri, which may or may not be gravitationally bound to the binary (Wertheimer & Laughlin 2006). No planets have been detected around any of these stars.

three stars

Alpha Centauri A is the system’s primary, a bright yellow star of spectral type G2 – exactly the same type as our Sun. Its mass is measured at 1.1 MSOL, with Pourbaix et al. favoring 1.105 MSOL and Thevenin et al. favoring 1.100 MSOL (Pourbaix et al. 2002, Thevenin et al. 2002). Within error margins these two values are in perfect agreement. The star’s radius is about 1.22 Rsol; its effective temperature is about 5750 K; and its metallicity is about 0.20 (Thevenin et al. 2002, Kervella et al. 2003). The star is half again as luminous as our Sun, with a luminosity of about 1.5 LSOL (Thevenin et al. 2002). Its habitable zone – the orbital space in which liquid water can persist on the surface of an Earth-mass planet – extends from 1.1 to 1.3 AU (Barbieri et al. 2002).

Alpha Centauri B is the secondary, an orange star of spectral type K1. Its mass is less well constrained than that of Star A, with Pourbaix and colleagues calculating 0.934 MSOL and Thevenin et al. arguing for 0.907 MSOL. Thevenin’s group provide a radius of 0.86 Rsol, an effective temperature of 5250 K, and a metallicity of 0.23 (Thevenin et al. 2002, Kervella et al. 2003). The slight difference in the two stars’ metallicity seems odd, since both formed in the same chemical environment. Star B is only half as luminous as our Sun, with a luminosity of about 0.5 LSOL (Thevenin et al. 2002). Its habitable zone extends from 0.5 to 0.9 AU (Guedes et al. 2008).

No consensus has yet emerged regarding the binary’s age, except that all recent estimates make it older than our Solar System. Thevenin’s group calculate 4.85 billion years, a little more than the Sun’s age, while Guedes et al. accept a value of about 5.9 billion years and Eggenberger et al. argue for 6.52 billion years (Thevenin et al. 2002, Guedes et al. 2008, Eggenberger et al. 2004). All agree that Alpha Centauri A and B are mature main-sequence stars.

They are a close pair, following an elliptical orbit (e = 0.52) with a semimajor axis of 23.4 AU and a period of 79.9 years (Pourbaix et al. 2002, Barbieri et al. 2002). At periastron they are only 11.2 AU apart; at apastron the separation increases to 35.6 AU.

Proxima Centauri is a small, dim M5 dwarf with a mass of only 0.1 MSOL. It is currently separated from Stars A and B by about 15,000 AU (that is, about 500 times the average distance of Neptune from our Sun). Robust proof of Proxima’s association with or detachment from the Alpha Centauri binary remains elusive (Wertheimer & Laughlin 2006). Conceivably, the M dwarf may be dynamically associated with the binary at this stage of its orbit around the Galactic Center without sharing a common origin with them. In any case, Proxima’s distance from Stars A and B suggests that it has played no significant role in their history.

potential planets

As our nearest neighbor, Alpha Centauri has long been a favored setting for fictional interstellar narratives. Classic examples include “Far Centaurus,” by A.E. van Vogt (1944); Revolt on Alpha C, by Robert Silverberg (1955); and Alpha Centauri or Die! by Leigh Brackett (1964). A more recent and better known instance is James Cameron's film Avatar (2009), which is set on a moon of an imaginary gas giant planet orbiting somewhere in the Alpha Centauri system.

Unfortunately, both theory and observation rule out gas giants around either of the system's bright stars. Marzari & Scholl (2000) note that the original circumstellar disk around Alpha Centauri A would have been truncated by its companion star at about 2.7 AU, well within star A's primordial ice line. Thus no giants could have formed (see Evolution of Planetary Systems). Although the ice line of the cooler star, Alpha Centauri B, would have been smaller and thus more giant-friendly, continuing radial velocity observations have returned no sign of such objects (Endl et al. 2001).

In fact, since the late 1990s, the Alpha Centauri system been most popular among astronomers who wish to model terrestrial planet formation around members of a close binary pair. At least six peer-reviewed articles have reported on sophisticated numerical simulations to assess the likelihood of Earthlike planets (Wiegert & Holman 1997, Marzari & Scholl 2000, Barbieri et al. 2002, Quintana et al. 2002, Quintana 2003, Guedes et al. 2008). The current consensus is that rocky planets could have formed within 2 AU of either star, on orbits in the same geometrical plane as the shared orbit of Alpha Centauri A and B. As Elisa Quintana concludes, “The binary companion has a similar effect on terrestrial planet formation around the central star as Jupiter and Saturn have around the sun; i.e., the companion determines an outer boundary for the terrestrial region but doesn’t prevent the formation of terrestrial planets” (Quintana 2003).

A key factor in favor of planet formation is the system’s high metallicity, with [Fe/H] in excess of 0.2 (Kervella et al. 2003). Stellar metallicities of 0.2 and higher are strongly associated with the assembly of massive planets (Butler et al. 2006, Udry & Santos 2007). Further, The truncation of the primordial disk around Alpha Centauri A would not have affected its habitable zone (Barbieri et al. 2002). Various simulation studies demonstrate that terrestrial-mass planets can readily form within 2 AU of the star, with many runs producing an ensemble of 2 to 5 planets with masses analogous to Mars, Venus, and Earth (Barbieri et al. 2002, Quintana et al. 2002). Barbieri’s group noted a trend toward greater orbital eccentricity at semimajor axes larger than 1 AU (Barbieri et al. 2002).

Simulations focused on Alpha Centauri B have produced very similar results (Quintana 2003, Guedes et al. 2008). Each run produced 1 to 4 planets with masses ranging from half that of Mercury to twice that of Earth. (The wider spread in masses around Star B may simply reflect variations in simulation algorithms and parameters, rather than any inherent likelihood of a greater diversity of masses in this environment.) Eccentricities ranged from zero to above 0.3, though these studies noted no tendency for eccentricity to increase with semimajor axis.

The next step in the assessment of planet formation in Alpha Centauri will involve state-of-the-art radial velocity observations of extreme sensitivity (Guedes et al. 2008). If sufficient resources are devoted to these searches, we might see results within a decade.

Among star systems located in the immediate Solar neighborhood (within 10 parsecs), a handful resemble Alpha Centauri insofar as they consist of two Sun-like stars with a periastron separation of at least 10 AU. This is probably wide enough to permit planet formation (Quintana 2003, Guedes et al. 2008). The systems are 70 Ophiuchi, Eta Cassiopeiae, Xi Bootis, Gliese 66, and Gamma Leporis. Unlike Alpha Centauri, however, most have sub-Solar metallicity, a deficit that may have impeded planet formation.

Last update December 2009