This diagram shows newly-discovered Kepler-1647b alongside other known circumbinary planets, revealing it as the biggest yet discovered. Credit: Lynette Cook
The Kepler Space Telescope has discovered the largest planet orbiting two stars ever found.
Kepler-1647, as the binary star system is called, is 3,700 lightyears away and is about 4.4 billion years old, which is roughly the age of Earth.
One of the stars is slightly bigger than the Sun, and one is slightly smaller.
The orbiting planet has a mass and radius similar to Jupiter, making it the largest transiting circumbinary ever discovered.
Astronomers are able to discover planets by searching for dips in the brightness of stars that show a planet could be passing in front of it, or ‘transiting’.
But in the case of circumbinary planets, these transits are harder to spot as they are not as regular and can vary in duration and depth.
The newly-discovered planet, Kepler-1647b, might have been an easy discovery considering its size, but took longer to confirm than expected because its orbital period is so long.
It takes 1,107 days to orbit both stars; the longest orbital period of any transiting exoplanet confirmed so far.
It is also further away from its stars than any other known circumbinary, contradicting the tendency for such planets to have closer orbits.
This puts Kepler-1647b in the habitable zone, which is the distance from its star required for liquid water to pool on the surface.
However, like Jupiter, the planet is a gas giant, and therefore unlikely to host life.
“Habitability aside, Kepler-1647b is important because it is the tip of the iceberg of a theoretically predicted population of large, long-period circumbinary planets,” says William Welsh, astronomer at San Diego State University and coauthor of the paper.