A mournful French horn blows. An angsty Luke Skywalker stomps out of his aunt and uncle's sand hut and peers up at Tatooine's double sunset, his hair blowing in the breeze. It's a memorable scene from Star Wars—but now, a precedent for such a sky with two suns has been found in our universe.
Using data from the Kepler space observatory, scientists at the Harvard-Smithsonian Center for Astrophysics and SETI have discovered for the first time a planet orbiting a binary star system, passing in front of both its parent stars along its orbit.
The planet, Kepler-16b, resembles Saturn in its mass and gaseous makeup. That mostly rules out the possibility of any living beings being present to enjoy the double sunset view, although chances are good Kepler-16b has an icy, non-gaseous satellite or two, as Saturn does.
The two stars in the system are 20% and 69% as massive as our sun, respectively. The planet orbits at a distance analogous to Venus's orbit in our solar system, which typically would place it within the "habitable zone" of planets that could support life. But since the combined mass of the two stars is still less than our sun, Kepler-16b's Venus-like orbit is most likely a cold one.
An animation of Kepler-16b's orbit:
Binary star systems, first cataloged at length by English astronomer William Herschel in the early 19th century, are key to our understanding of distant stars, since it's easy to derive each star's mass by studying their linked orbit (the two stars in a binary system both orbit around their shared center of mass). But whether or not such systems, which by some estimates account for about half of the stars in the known universe, could form and support orbiting planets has been a contentious topic--making today's finding significant not just for Star Wars fans.
"It's been pretty much a split vote amongst the theorists," said Alan Boss, a theoretical astrophysicist at the Carnegie Institution for Science, and a co-author of the Kepler 16-b paper. "Some say 'Yeah, we think it's possible to make a Saturn-mass object [in a binary star system].' Other papers say 'Well, no, we don't think it's going to work at all, because those changing gravitational forces from that central binary are going to screw up the process of trying to get little bodies to run into each other and grow bigger and bigger.'"
"One of the exciting things about this is: Kepler, as usual, has answered the question for us," said Boss.
The Kepler observatory's mission is to find and analyze potential Earth-like exoplanets throughout the universe. Today's discovery now significantly expands the working set of stars that could potentially harbor orbiting planets. That means more work for Kepler as it continues what has so far been an extremely successful mission.
The paper, authored primarily by Laurence Doyle of the SETI institute, appears in the journal Science today.
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