Incredible discoveries from NASA’s Kepler space telescope just keep coming! This week, scientists found not one but two planets orbiting a binary star, that is, a pair or stars that orbit a point between them called a barycenter. Incredibly, one is even in the habitable zone. But don’t get excited about moving to a real life Tatooine just yet. Both planets aren’t themselves habitable, but that doesn’t make the discovery any less interesting or important.
It’s been less than a year since Kepler found the first circumbinary planet, that is a planet orbiting a binary star. That planet, Kepler-16b, was discovered last September. Finding another one in such short order suggests that circumbinary planets aren’t a rare phenomenon, and proves that more than one planet can form and persist in this stressful realm.
The newly discovered planets orbit the Kepler-47 system. This system’s two stars eclipse each other every 7.5 days from our vantage point on Earth. One star is similar to the sun in size, though only 84 percent as bright. The second star is diminutive, measuring just one-third the size of the sun and less than 1 percent as bright. The system is some 4,900 light-years from us in the constellation Cygnus.
For one planet to exist in this kind of system – let alone many planets – demands they deal with a whole different set of circumstances compared to a planet orbiting a single star. “In contrast to a single planet orbiting a single star, the planet in a circumbinary system must transit a ‘moving target.’ As a consequence, time intervals between the transits and their durations can vary substantially, sometimes short, other times long,” says Jerome Orosz, associate professor of astronomy at San Diego State University.
Scientists find exoplanets by measuring the dips in light from stars. The irregular transit pattern of circumbinary planets is how scientists find these bodies. “The intervals were the telltale sign these planets are in circumbinary orbits,” says Orosz.
The inner planet, currently called Kepler-47b, orbits the binary star in less than 50 days. Astronomers can’t see the planet directly, but they’ve inferred that it’s likely a sweltering world where methane is super-heated in the atmosphere to form a thick blanket of haze – almost like a methane version of Venus. Kepler 47-b is about three times the radius of Earth, making it the smallest known transiting circumbinary planet.
The outer planet is currently called Kepler-47c. It orbits the host stars every 303 days. That orbit puts the planet in the system’s so-called “habitable zone,” the region where liquid water could exist on the surface. But astronomers aren’t getting ready to migrate just yet. Kepler 47-c is thought to be a gaseous giant slightly larger than Neptune with an atmosphere of thick bright water-vapor clouds.
But even if we haven’t found a habitable world around a binary star, this discovery is significant. “Unlike our sun, many stars are part of multiple-star systems where two or more stars orbit one another. The question always has been – do they have planets and planetary systems? This Kepler discovery proves that they do,” said William Borucki, Kepler mission principal investigator at NASA’s Ames Research Center. “In our search for habitable planets, we have found more opportunities for life to exist.”
Greg Laughlin, professor of Astrophysics and Planetary Science at the University of California in Santa Cruz, adds that “the presence of a full-fledged circumbinary planetary system orbiting Kepler-47 is an amazing discovery. These planets are very difficult to form using the currently accepted paradigm, and I believe that theorists, myself included, will be going back to the drawing board to try to improve our understanding of how planets are assembled in dusty circumbinary disks.”
If more circumbinary systems are found, we might have a whole new model to follow in the ongoing search for habitable exoplanets.