Dark matter and Big Bangs may not be easy to visualize, but planets orbiting twin stars is something many of us have envisioned from the moment the London Symphony Orchestra accompanied Luke Skywalker’s wistful gaze at Tatooine’s binary suns. The one hitch: it was pure imagination. (Sorry, Star Wars fans.) Until recently, astronomers had little evidence suggesting such ornate cosmic architectures could actually exist in this galaxy, if not one far, far away.
Now new research into a tantalizing binary star system may trump an entire generation’s collective imagination. Using information collected by NASA’s Kepler Space Telescope, astronomers have discovered the equivalent of a cosmic poker hand: a pair of planets orbiting a pair of suns. And there’s even a chance for a full house since the researchers have also found another object that, while still unconfirmed, may turn out to be a third planet orbiting the same two stars.
“This shows us that close binaries can host full-fledged planetary systems,” says Jerome Orosz, an astronomer at San Diego State University and the lead author of a paper on the new binary star system, labeled Kepler-47, that was just published in the journal Science. “If we get more data that confirms a third planet, then the sky’s the limit.”
Until now, astronomers were aware of about a dozen examples of single planets that are gravitationally tied to “eclipsing binary” stars—stars orbiting each other, with one occasionally blocking the other’s light when viewed from Earth. In addition, there are another four recently discovered cases where single planets are orbiting “close binaries,” meaning stars that are no more than a few of our sun’s diameter apart—equivalent to the neighboring cubicle in the galactic office. But Kepler-47 marks the first time astronomers have found evidence that more than one world can orbit the same close binary star system.
“There are lots of binary stars with planets, but the stars are widely separated,” says Orosz. “Imagine the sun and then a companion star where Pluto is, or even farther out. In cases like that, it’s basically as if the other star’s not there.” With Kepler-47, the two stars are just nine solar diameters away from one another and the so-called “circumbinary planets” are very much there. The inner planet is three times Earth’s diameter, yet orbits the two stars—one roughly the size of the Earth’s sun and a companion about one-third that size—in just 49.5 days. The outer planet, likely a gas giant, is 4.6 times the size of Earth—about the same size as Uranus—but circles the two stars every 303 days. In effect, both planets are close enough to circle two stars in less time than it takes Earth to circle one.
Orosz and his colleagues discovered the planets using Kepler, an orbiting observatory with an ultra-sensitive photometer designed to detect Earth-sized planets. (The photometer converts incoming starlight into electrons, and measures the accumulated charge of those electrons to produce high-resolution data for planet searches.) The effort was part of an ongoing, systematic search for circumbinary planets among more than 1,000 of 2,100 known binary stars. For all the high-tech hardware, the search method is quite simple. When a planet orbits a star, the orientation of its orbit can be at any angle. In some lucky instances, like the case of Kepler-47, that orbit is edge-on with Earth. The serendipitous result: the planet passes directly between the star and the Kepler’s view, creating a tiny dip in starlight for a brief period of time. “It’s like a fly walking across the headlight of your car,” says Orosz. “We’re talking fractions of a percent in change.”
The orbiting telescope’s photometer is able to detect that change. At Kepler-47, the research team found that the small star eclipsed the large star every 7.45 days. They then noticed two other objects were also eclipsing the large star and realized they were seeing planets. In a span of nearly three years they detected the outer planet three times, the inner planet 18 times and the mysterious third object once—which is why they can’t yet confirm that it wasn’t merely random debris passing through the system. By recording the amount of time between eclipses the scientists were eventually able to calculate the planets’ bumpy, “quasi-elliptical” orbits.
But a wobbly ride takes nothing away from the larger planet’s intriguing location within the two stars’ so-called “habitable zone,” where liquid water and Earth-like climate conditions are theoretically possible. “If you were to stand on that planet, the amount of sunlight is very much like you would get here on Earth, 86 to 90%,” Orosz says. “So an Earth-like planet in that orbit would have the necessary conditions for liquid water and the temperature wouldn’t be too cold or too hot.” While neither of Kepler-47’s planets are Earth-like, both could easily have moons which the Kepler telescope isn’t able to detect. “Everybody’s got a moon, even the asteroids,” Orosz says. “If these planets have no moons, it would be shocking.”
Still, astronomers say you’ll want to think twice before visiting any of Kepler-47’s circumbinary planets or moons since the gravitational tug-of-war from two vastly different stars probably makes for an unstable ride. Viewed from afar, however, they’re just perfect—and a rare, cinema-free chance to let the imagination and the science intertwine.