Luke Skywalker's home planet, which orbits a binary star system, could be out there, according to Ryosuke Kita of Northwestern University, who presented this morning at the Astrobiology Science Conference 2008. Of the 200 or so star systems where we've found planets, about 20 percent are binary systems, and that number should go up as we find more, since about half of all main-sequence stars are binaries. Kita's calculations show that gravity from a second star will perturb an Earth-like planet's orbit to the point of making its climate uninhabitable. The key, he says, is that the planet can't be alone — it needs a nearby gas giant to help stabilize its orbit, and allow a climate that could support life.
In his simulations, Rasio assumed that the Earth-like planet was orbiting at the same distance we orbit our Sun (1Astronomical Unit, or AU, or around 93 million miles), and he calculated orbital eccentricities in cases where the second star was orbiting at distances of 250, 500, and 1000 AU.
Left by itself the lonely Earth-twin would probably oscillate between a highly elliptical orbit, and a fairly circular one. If it's too oval (an eccentricity greater around 0.7 say) the planet's climate goes haywire and turns into a runaway greenhouse, superheating the surface. At around an eccentricity of 0.4, most of the water would evaporate, but would hover in the atmosphere...and Vaporators would sell like hotcakes.
Things got a little better when Kita added a Jupiter-mass planet to the system. When placed just a few AU from the Earth-like planet, the eccentric orbit stabilized. In that case, the rocky planet's climate would be much like ours.
So far there are two systems that are our best bets for finding the real Mos Eisley — 55 Cancri and Epsilon Andromeda. Both have multiple planets, though Kita says we're not sure yet whether any are rocky or if they're all gas giants.