In the Kepler 47 system, however, the two stars' orbits of one another are nearly circular, and the stars are very close together. The length of their orbits is about 7.5 days, and they are only about 13 million kilometers apart. (For perspective, that is about a quarter of the average distance between the sun and Mercury.) The researchers determined these parameters using Doppler spectroscopy, which measures emitted light to determine how a smaller body exerts gravitational force on a larger body.
To model the planetary orbits, the researchers used numerical models to find the orbits that best fit the planetary transit data collected by Kepler. The inner planet takes about 49.5 days to orbit the stars and is only about 44 million kilometers away from them; the outer planet takes about 303 days and is about as far away from the pair as Earth is from the sun. That puts Kepler 47's outer planet in the habitable region, where liquid water could exist. With such a location, the outer planet might call to mind the Star Wars world Tatooine, but the researchers say that this planet is probably a gas giant, which is not a hospitable environment for Earth-like life. But Fabrycky notes that if you were in a hot air balloon looking through the outer planet's atmosphere, the dual sunset might look like something out of Star Wars, with the two suns positioned very close together at the horizon. And if the planet has a moon, there's a tiny chance that it could support life.
Ellipses are good approximations of the planets' paths, but they are not perfect because the gravitational forces acting on the worlds are not coming directly from the two stars' center of mass. Fabrycky says, "They're almost going on an ellipse, but they wiggle around it a little bit due to the stars dancing around each other." Over time these forces will cause the orbits of the planets—and thus the shape of this distant solar system to evolve. Fabrycky says that it will take only about a century for the system to change substantially. (In our solar system, by contrast, those kinds of changes take tens of thousands of years.)
Going forward, astrophysicists are continuing to use the telescope to search for potentially habitable planets, applying Kepler's laws to describe their motion. Fabrycky expects that in the next few years researchers will be announcing many more exoplanetary discoveries in both single- and multiple-star systems. Astronomers hope that Kepler will help them find answers to big questions about our universe. Are all binary star systems planar? What different processes give rise to planets? And of course, is there anyone else out there?