A binary system is a solar system in which two stars orbit one another, locked in a dance around their centres of gravity. Astronomers estimate that about half the stars in the universe are found in pairs, but not long ago, we were unsure whether these systems could actually host planets—but in the past couple of years, we’ve found over sixteen binary systems with planets orbiting them. One of these planets, PH1, is particularly interesting. Last year, volunteers on the citizen science website Planet Hunters, Kian Jek of San Francisco and Robert Gagliano of Cottonwood, Arizona, discovered an exoplanet in a system of not one, not two, but four stars. This quadruple star system is named KIC 4862625 and is about 3,200 light-years from Earth. Its planet, named PH1, is thought to be a gas giant the size of Neptune, with about half the mass of Jupiter, and the radius of its orbit is 1000 times bigger than Earth’s. But it’s not orbiting four stars; rather, the planet is orbiting a pair of binary stars, which are then being orbited by another pair of binary stars. So from PH1, the sky would have two suns (imagine a double sunset!), then there would be also be two very bright stars in the night sky, wandering along against the backdrop of the universe. Finding exoplanets in binary systems is both incredibly fascinating and incredibly important, because it sends astronomers back to the drawing board with their models of planetary formation, trying to figure out how planets could evolve in such a dynamic environment.
