Double sunsets on a distant world

Astronomers discover for the first time a planet circling two stars

If you lived on a planet that circled two stars, sunsets might look something like this. NASA/Ames Research Center/Kepler Mission

If you were stranded on a recently discovered planet called Kepler-16b and looked up during the day, you’d see an unusual light show: two glowing orbs making their way across the sky.

Here on Earth, we’re used to seeing only one sun rise in the morning and set in the evening. But on far-off Kepler-16b, there would be two sunsets and two sunrises, one for each star the planet orbits. And those two stars also orbit each other, which means they’d appear to change position in the sky.

“You’d get a different sunset every day!” Laurance Doyle told Science News. Doyle is an astronomer at the SETI Institute in Mountain View, Calif., and was part of the team of scientists that discovered the doubly lit world.

The two stars are locked together by gravity in a binary, or double, star system. Astronomers say that binary stars aren’t unusual. In fact, most stars in the universe have a stellar companion they spin around. Solitary stars, like our sun, are more unusual.

Doyle told Science News that Kepler-16b is the first planet to be found orbiting a binary, but it won’t be the last.

“Now that we know how to find them, what they look like, and the things to look for, we’re probably going to find a few more,” he said.

In recent years, scientists have discovered hundreds, possibly thousands, of planets outside our solar system. Since binary star systems are so common, it seemed to be just a matter of time before a planet was found orbiting a star pair.

“This is a really important discovery,” astronomer Andrew Howard, of the University of California, Berkeley, told Science News. Howard was not part of the discovery team. He says that when scientists try to predict what kinds of things exist in the universe, “this is one of them.”

Astronomers make these kinds of predictions by using existing information to build computer simulations, or models, of events that happen in the universe. Howard said the discovery seemed amazing, even though the models predicted that some binary star systems should have planets nearby.

“It’s still so new and different that it still shocks at least me,” he told Science News.

Kepler-16b is about the size of Saturn, and a year on Kepler-16b lasts about 226 days on Earth. Its two host stars are small, and if you smooshed them together, they’d still have less heft than the sun. To find the planet, astronomers used NASA’s Kepler space telescope, which follows the Earth around the sun and is always pointed at the same swath of sky in search of planets. Since it was launched in 2009, the Kepler telescope has beamed back information about more than 1,200 potential candidates.

The telescope is designed to find a planet when it transits, or passes, in front of its star(s). A star shines, but a planet doesn’t. So, as the planet transits, light from the star will dim a little bit. Kepler’s instruments can detect this tiny dimming of starlight, and scientists can study Kepler’s data to see if a planet caused the decrease. Though a planet like Kepler-16b is too far away to be seen directly, scientists can study the starlight from its stellar twins to measure the planet’s size.


exoplanet A planet that orbits a star outside the solar system.

orbit The curved path of a planet around a star.

binary star A system of two stars in which one revolves around the other, or they both revolve around a common center.

transit The passing of a planet across the face of a star, or of a moon or its shadow across the face of a planet.

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