A Sky with Two Suns: Tenth Planet Orbiting Two Stars Discovered

September 9, 2015 | Sarah Tse

A planet rotates around two suns
Photo credit: A illustration of a Kepler-47, the first circumbinary system to be discovered. (copyright: Mark Garlick)

Once an improbable, if not impossible, idea, why are circumbinary planets—those that orbit two stars at once—being discovered left and right?

A planet that orbits two suns sounds like something out of a science-fiction movie—like the planet Tatooine from Star Wars, to be exact. For years, scientists thought that such a system would be too unstable to exist. But four years ago, the first two-star system was discovered by the Kepler mission, and the list of circumbinary planets has grown since then. Astronomers recently discovered the tenth addition to that list, called Kepler-453b.

Its discovery, published in the Astrophysical Journal, was a bit of a fluke: the planet’s orbit tilts in a process called precession, since it experiences gravitational pulls from two suns. This reduces the chances of astronomers detecting the planet’s transit between us and its host star to only 9 percent.

"If we had observed this planet earlier or later than we did, we would have seen nothing and assumed there was no planet there," said Stephen Kane, an assistant professor at San Francisco State University and one of the team-members who made the discovery. In fact, if researchers had missed Kepler-453b this time around, they would not have gotten another chance to see it until 2066.

The lower visibility caused by their erratic orbit also makes scientists think they may be missing many other circumbinary planets. "The low probability for witnessing transits means that for every system like Kepler-453 we see, there are likely to be 11 times as many that we don't see," said Jerome Orosz, another professor of astronomy at San Diego State University.

Kepler-453b orbits within the habitable zone of its host stars, which means it would be capable of supporting life. While the planet itself is ruled out by its composition as a gas giant, it could have rocky moons that would offer a more comfortable environment. Looking up from the planet’s surface, one would see two suns that orbit each other every 27 days. One sun is about 94 percent as large as ours, while the other is only 20 percent as large and therefore much cooler. A year on Kepler-453b would last 240 days.

Circumbinary planets continue to fascinate astronomers for their unpredictability, in terms of orbit as well as arrangement. They are also more likely to be in their stars’ habitable zones, since the presence of two stars keeps the system compact while the planets must maintain a far enough distance to stabilize their orbit.

Researchers studying a different circumbinary system, Kepler-34, have also modeled the gravitational effects of two stars on collisions. Two stars exerting gravitational pulls is not unlike a pair of petulant toddlers fighting over planetary building blocks, and such conditions would be too volatile for a planet to grow. Unlike planets in single-star systems, these would have formed much farther away, and then migrated to their current locations.

The list of circumbinary planets will only keep growing as astronomers continue to study the wealth of data collected by the Kepler mission, which concluded in 2012. Planet-hunting is thrilling enough, and the possibility of discovering these complex and diverse systems is like icing on the cosmic cake.


Based on materials provided by San Francisco State University and San Diego State University.

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