A recent discovery by the SETI Institute using NASA Kepler data has revealed an indisputable planet orbiting both stars in a binary star system. This planet, the first of its type to be definitively proven to exist, is in the Kepler-16 system and is known as Kepler-16b.
The Kepler-16 system is about 200 light-years from Earth. The primary star is only about 69% as massive as Sol, and the secondary star is about 20% as massive. These stars happen to orbit each other on a plane that is parallel to our vantage point, thus appearing to eclipse each other regularly. In addition to these alternative eclipses -- the smaller one partially eclipsing the larger, and the larger fully eclipsing the smaller -- lesser eclipses were also detected across both stars and when they were in varying relative positions, which led to the discovery of this planet.
Unfortunately, Kepler-16b is inhospitable. While it is closer to the stars than Earth is to Sol, the stars' relatively lower mass and size causes the hospitable zone to be much closer to the two stars. Kepler-16b orbits the binary system in about 229 days and is likely to be composed of about half rock and half gas.
This planet, even though it's not hospitable, has been compared to Tatooine because of its status as circumbinary -- that is, orbiting both stars in a binary system. While we haven't discovered one yet, it's now more possible to discover a planet similar to Tatooine -- hospitable (arguably) and circumbinary.
For more details, read the NASA release about the discovery, and the SETI Institute blog post by Dr. Franck Marchis, Kepler-16: Exoplanets around binary star systems DO exist.
Image credit: NASA/JPL-Caltech/R. Hurt