James Cameron’s new movie Avatar depicts a gas giant with a habitable moon, Pandora, around it. Could there be real habitable planets orbiting among the three stars of the Alpha Centauri system? What are the odds that a "Pandora," really exists?
With the diminishing odds that our solar system supports advanced forms of life, the nearest stars become ever more attractive candidates for discovery. And among the stars with possible habitable planets is the Alpha Centauri triple star system, the closest star system at only 4.37 light years from our Sun, which hangs above the horizon of Saturn in the image above. Both Alpha Centauri A and B -- stars very similar to our own Sun -- are clearly distinguishable in the image. (The third star in the Alpha Centauri system, the red dwarf Proxima Centauri, is not visible here.)
From the orbit of Saturn, light (as well as Cassini's radio signal) takes a little more than an hour travel to Earth. The distance to Alpha Centauri is so great that light from these stars takes more than four years to reach our Solar System. Thus, although Saturn seems a distant frontier, the nearest star is almost 30,000 times farther away.
The image was taken in visible red light with the Cassini spacecraft narrow-angle camera on May 17, 2008. The view was obtained at a distance of approximately 534,000 kilometers (332,000 miles) from Saturn. Image scale on Saturn is about 3 kilometers (2 miles) per pixel.
While Cameron was creating a fictional moon orbiting a gas giant circling Alpha Centauri A, several very real astronomers have focused in on Alpha Centauri as a potential zone for Earthlike planets. Professor Debra Fischer of Yale University, currently working at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, is aiming the CTIO 1.5 meter telescope at both Alpha Centauri “A” and “B” as a part of a 5 year observation that will hopefully reveal planets as small as Mars.
Fisher is using the radial velocity method, which uses spectral measurements to detect variations in the speed that a star moves towards or away from Earth (any star with planets will move in its own small orbit around a common center of gravity).
In addition to Fischer’search, Michel Mayor’s team at the La Silla Southern Sky extrasolar Planet search Programme, is using instruments sensitive enough to detect a Centauri planet down to Mars size, depending on the stability of the stellar atmospheres. We may well find one or more small, rocky worlds, at which point the question becomes whether or not such a planet could have oceans — we don’t know what close binaries may do to water delivery from asteroids and comets. Whatever the case, though, a Centauri planet in the habitable zone would be a potent stimulus to earth-bound human imagination and an obvious target for interstellar probes of the future.
To the two ongoing hunts for planets around the Alpha Centauri stars the Centauri Dreams blog reports that we can now add a third. John Hearnshaw (University of Canterbury, Christchurch) reports that the university’s Mt. John Observatory has begun a program to search for Earth-mass planets around Centauri A and B. Although the observatory is heavily invested in microlensing technologies, the new efforts will put radial velocity methods to work using the Hercules spectrograph.