There may be a day in the far-off future when saying a planet is a two-star planet implies it has, maybe, a tepid, stale Galactic breakfast waiting in the lobby — but certainly not a full-service restaurant, luxury spa, and fax machine in the business center like your higher-end, four- and five-star planets. (The infinity pool at the black hole in the next galaxy over, is getting some good reviews.)
But barring the development of an interplanetary rating system, when we talk about two-star planets, we're talking about a planet in orbit around a binary star system — two stars linked together by gravity, orbiting a common point called a barycenter.
While double stars may seem like an exotic astral feature to us here on Earth, they're actually fairly common. More than half of all known stars are part of a binary system, or are in multiple-star systems that feature binary stars. The two suns Luke Skywalker saw when looking up from the dusty surface of Tatooine is a more likely view, it seems, than that of our sole Sol.
Adding to the recent spate of planetary finds, astronomers from NASA's Goddard Space Flight Center and San Diego State University (SDSU) announced yesterday they've discovery the largest-known planet to orbit two stars, confirming theories about large planets around binary systems. The findings have been accepted for publication in the Astrophysics Journal.
Spotted with the Kepler Space Telescope, the new planet Kepler-1647b is 3,700 light-years away. And at approximately 4.4 billion years old, it's about the same age as our sun. It takes 1,107 Earth days, or slightly more than three of our years, to orbit its suns — collectively named Kepler-1647. The planet Kepler-1647b is a gas giant, and about the same size and mass as Jupiter, the largest gas giant in our solar system, and researchers say that, like Jupiter, it probably has multiple moons.
Planets like Kepler-1647b in orbit around binary stars are known as circumbinary planets, and planet hunters spot them by looking for a dimming in the light from a star as the planet transits, or passes in front of the star from our perspective.
"Finding circumbinary planets is much harder than finding planets around single stars,” said SDSU astronomer William Welsh, one of the paper's coauthors, in a press release announcing the discovery. “The transits are not regularly spaced in time and they can vary in duration and even depth.”
For the casual astronomer, you won't be able to spot the planet or its stars, as they're too faint to be spotted with the naked eye. But if you look towards the constellation Cygnus, you're staring in the right direction.