Most of the planets astronomers have found orbit only one star 鈥� a few orbit two, but so far we鈥檝e never spotted an exoplanet that orbits three stars simultaneously. Now, researchers have spotted hints of a young world with three host stars.
These hints come from a stellar system called GW Orionis, which consists of two stars orbiting one another at about the same distance that Earth sits from the sun, and a third circling those two about eight times further away. Encircling this system is a disc of dust and gas separated into three rings.
There is a huge gap 鈥� about 15 billion kilometres across 鈥� between the innermost ring and the other two. Some astronomers have interpreted this gap as having formed when the torque from the three stars鈥� gravity ripped the disc apart, but through a series of simulations, at the University of Nevada, Las Vegas, and his colleagues have determined that it鈥檚 more likely a planet carved it out.
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They performed a set of 11 simulations of the stars, including the gas in the disc 鈥� the dust in the disc is expected to follow the structure of the gas. Without a giant planet, torque from the stars didn鈥檛 rip the disc apart into separate rings, but inserting a planet about the size of Jupiter reproduced the observed gap. When the gap filled in over time, the planet simply carved a new one.
鈥淲e haven鈥檛 observed the planet directly, but there is some evidence that a planet is shaping this gap,鈥� says Smallwood. If it is there, this would be the first world ever found that orbits three stars, and there may even be a second, smaller planet between the outer two rings.
However, confirming this will be difficult, he says. The larger purported world would be far away from the stars and have a tilted orbit, making it hard to detect using the usual method, which relies on a planet passing in front of a star and blocking out some of its light. The best way to confirm the planet鈥檚 existence would be taking a direct image of it, but the system is more than 1000 light years from Earth, so that would be tough, too.
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