September 23, 2011
Last week, the science world was abuzz with the news that scientists from the Harvard-Smithsonian Center for Astrophysics had discovered a distant planet with an unusual quality. Like the fictional Tatooine from Star Wars, the planet, known as Kepler-16b, is what scientists call a circumbinary: It orbits around two stars. It’s the first confirmed circumbinary planet astronomers have found.
The discovery comes from NASA’s Kepler Mission, which aims to gather information on habitable planets in the Milky Way. Josh Carter, who worked on the team that made this discovery, says they detect the presence of distant planets by a mechanism known as planetary transit. “What happens is a planet in its orbit passes in front of a star,” says Carter. “When it does that, it blocks a little light from it, just like an eclipse of the sun by the moon.”
“Of course, we can’t see the individual object, all we see is the total light coming from it,” Carter says. The light is detected by the Kepler space-based telescope. When a planet transits in front of the star during its orbit, says Carter, “you see a very small little dip in the total light from the system, and then we infer based upon its shape and basic properties that it’s an object transiting.”
This process has been used to find 21 confirmed planets so far, with thousands more potential planet candidates still being researched. But over the summer, the team noticed that one system showed dimming at irregular intervals. They realized that the multiple transits corresponded to a planet crossing in front of each star, as well as two stars crossing in front of each other. “When you see one transit in the light curve, you can guess it is the planet crossing one of the stars, but until we had three, we weren’t sure it was a circumbinary,” says Carter.
The attempt to find a circumbinary planet had been in the works for some time, Carter says. “Laurence Doyle had been looking through the collection of eclipsing stars in the catalog, and he was looking specifically for transits of a planet in a circumbinary,” says Carter. “We already had known this system had shown a single transit, but this summer with new data, we saw that there’s a total of three.”
The team further refined their understanding of Kepler-16b by using a trace spectrograph. Initially, just from studying the pattern of light emitted, they were able to establish the sizes of the stars and planet in the system, but only relative to each other. By using the spectrograph—a device that separates the light into a frequency spectrum—they could go further. “From the spectrograph, we see the velocity of the big star in the system,” says Carter. “That gives us an absolute scale with which we can learn the masses and the radii of all three objects in the system.”
Armed with this data, the researchers could then infer the planet’s composition. “We say, ‘well, it’s got this radius, it’s got this mass, what could it possibly be comprised of, what’s its structure?’” says Carter. The planet, roughly 200 light-years away from earth, is a gas giant, similar to Saturn in both size and mass.
Carter says he and his colleagues will continue searching for more circumbinary planets as they survey the wide diversity of planets in our galaxy. If they’re out there, the team will do their best to find them. “In fact,” Carter says, “we already have a few more candidate systems that we’re investigating right now.”
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