It’s not yet Tatooine or Magrathea, but we are getting closer to the science fiction dream of a habitable planet with two suns. The Kepler spacecraft has identified a planet, called Kepler-453b, that orbits two stars while maintaining a distance compatible with liquid water, and therefore life. The fly in the ointment is that the planet is a gas giant, 60% larger than Neptune.
Planets that orbit two stars are known as “circumbinary.” It was once thought it might be impossible for an object to maintain a stable orbit over billions of years in these conditions, but this is the 10th such example Kepler has found. Unlike most predecessors, Kepler-453b receives enough light to put it in the “Goldilocks zone,” neither too hot nor too cold for liquid water at the surface (were it a rocky planet instead of a gas giant).
Kepler detects planets as they transit across their parent star’s face. Since close-in planets transit more often, most of the objects found so far have been in short orbits and consequently uncomfortably hot. Kepler-453b takes 240 days to go around its twin stars, astronomers announce in a forthcoming paper in The Astrophysical Journal, available in pre-publication on arXiv.
One of the parent stars is slightly smaller and fainter than the Sun, while the other has just 19% of the Sun's mass, and provides just 0.3% of the system's light. The two stars orbit each other every 27 days.
Kepler-453b's orbit is close to circular, so it does not experience wild temperature swings that might make life impossible. Its mass is unknown, being too small to measurably perturb the stars at such a distance.
Co-author Dr. Stephen Kane of San Francisco State University sees the most exciting part of Kepler-453b as what its discovery says about finding terrestrial planets with similar orbits. Most planets orbiting a single star either always or never transit as seen from Earth. However, Kane compares Kepler-453b's orbital precession to “a spinning top,” which causes it to transit the larger star on just 8% of orbits.
"It's amazing how fortunate we were in catching it at the right time," Kane says. "It's a good reminder that there's always a value in checking again." The next transit will not occur until 2066, while transits of the secondary star are too faint to be detected.
The fact that we got lucky this time suggests we miss 12 circumbinary systems like this for every one we see. "We didn't know circumbinary systems could exist until Kepler came along, and since then we've been finding them in larger numbers," says Kane. Despite having made over 100 exoplanetary finds, Kane says, "Being involved in these discoveries never gets old."
The paper notes, “Such a planet could host a large moon capable of sustaining life.” Imagine the sunsets on a world that orbits a large planet circling two stars.
