Planets that orbit two stars may seem like science fiction, but they have been a rare yet constant finding since the beginning of exoplanetary astronomy in the 1990s. Now, a Jupiter-sized gas giant has been observed orbiting two stars, making it the largest planet ever found in a so-called circumbinary orbit.
The findings are reported in a paper accepted for publication in the Astrophysical Journal, available online at arXiv.
“It’s a bit curious that this biggest planet took so long to confirm, since it is easier to find big planets than small ones,” said SDSU astronomer Jerome Orosz, co-author of the study, in a statement. “It took so long to confirm because its orbital period is so long.”
The planet, called Kepler-1647 b, has actually got the longest orbital period (year) for a circumbinary planet discovered so far, as it orbits a pair of Sun-sized stars in 1,107 days. The two stars are eclipsing binaries that orbit each other every 11 days.
The discovery was possible thanks to the Kepler Space Telescope, which looks for periodical dips in the light of stars. The longer the period, the longer it takes to be confirmed.
“But finding circumbinary planets is much harder than finding planets around single stars,” said SDSU astronomer William Welsh, another of the paper’s co-authors, in the statement. “The transits are not regularly spaced in time and they can vary in duration and even depth.”
Kepler-1647 b is located about 3,700 light-years from Earth and it is believed to be slightly younger than Earth, with an age of 4.4 billion years. The planet is located in the habitable zone of the system, and while it is unlikely that it hosts life (because it has 1.5 times the mass of Jupiter), it might have moons that do.
“Habitability aside, Kepler-1647 b is important because it is the tip of the iceberg of a theoretically predicted population of large, long-period circumbinary planets,” Welsh added.
To end up orbiting two stars, planets most likely migrate inward after they form, and the discovery of Kepler-1647 b provides more evidence that this is indeed the case. Planet migration could be a common feature of any young star system, so studying circumbinary planets in detail could allow us to understand how the Solar System formed.