(Related pictures: "The Worlds of Star Wars.")
NASA's Kepler spacecraft uncovered the new planet, dubbed Kepler 16b, as it transited—or crossed in front of—both its parent stars, causing the brightness of each star to dim periodically.
Kepler data first allowed scientists to see that the stars are what's known as an eclipsing binary system—a pair of stars that orbit in such a way that they eclipse each other, causing them to dim, as seen from Earth.
Based on the eclipses, the team calculates that the binary stars are just 20 percent and 69 percent the mass of our sun.
Sometimes, however, the system's overall brightness dipped even when the stars were not eclipsing each other—hinting at the presence of a third body orbiting the binary pair.
"By timing the stellar eclipses, we could determine how much the third body was perturbing the two inner stars," said study leader Laurance Doyle, an astronomer at the SETI Institute in Mountain View, California.
The extra, irregular dimming "turned out to be no stronger than a planetary gravitational pull would be."
Next: An Earthlike Planet With Two Suns?
Kepler's data suggest that the new planet is a Saturn-like gas giant without a solid surface.
Traveling on a nearly circular, 229-day orbit around both host stars, the planet lies outside the system's habitable zone—the region where liquid water, and thus life as we know it, could exist.
In fact, the new planet likely receives about the same amount of sunshine as Mars, which means that, even if it had a solid surface, the world would be far too cold to support life.
Still, as on Tatooine, "from Kepler 16b one would see a double sunset, but with the stars shifting position [and moving in relation to each other] while setting."
And while Kepler 16b may not have any sand dunes, it's theoretically possible for Earthlike planets to exist in similar binary star systems—an arrangement that Doyle says may be quite common. (See "Many Planets Could Circle Twin Suns, NASA Says.")
"I estimate that there may be about two million such systems in our galaxy," he said.
The new planet with two suns is described in this week's issue of the journal Science.