Planet hogs two stars in astronomical tug-of-war

When the inhabitants of Tatooine from Star Wars and Gallifrey from Doctor Who look up at the sky, they have a very different view from what we’re used to — they see two suns.

The planets of Star Wars and Doctor Who are called “circumbinary planets” because they orbit two stars. While Tatooine and Gallifrey are just popular science fiction examples, circumbinary planets are thought to be fairly common in the universe. In fact, eighteen such systems have been confirmed to date.

The origins of circumbinary planets are complicated. Until recently it wasn’t entirely clear how they were able to form. A major logical roadblock for astronomers was the gravitational pull from both stars. Because of the intensity of this pull, any planet attempting to form within a binary star system would likely be beaten down by collisions with the rocky bits that, under less hostile conditions, would slowly merge to form the planet.

So scientists have been posing the same question since the 2003 discovery of the first circumbinary planet: How do these planets even form?

Research published in Astrophysical Journal Letters by a team from the University of Bristol found that the planets of circumbinary systems likely form far away from the binary stars and then migrate to their current locations, where they then settle into steady orbits. The scientists used computer simulations to recreate the early stages of planet formation. They modeled the collisions between one million individual rocky building blocks and the effects of gravity on both the behavior of the objects in the system and the system as a whole.

“Our simulations show that the circumbinary disk is a hostile environment even for large, gravitationally strong objects,” Zoe Leinhardt, one of the study’s authors, said in a statement.

Kepler-34(AB)b, a circumbinary planet discovered in 2012 by NASA’s Kepler space telescope, was used as a test subject in the study. The Bristol group found that the size of the planet ruled out the possibility of it forming inside the binary star system.

“Taking into account date on collisions as well as the physical growth rate of planets, we found that Kepler 34(AB)b would have struggled to grow where we find it now,” Leinhardt said in her statement.

The team extrapolated its results and concluded that all of the confirmed circumbinary planets must have formed at a great distance from their current locations. The only possible exception, they said, may be Kepler-47(AB)c, which is stably situated farther from its stars than any of the other circumbinary planets.

The number of identified circumbinary planets and other exoplanets has skyrocketed since the launch of the Kepler spacecraft in 2009. Kepler uses a photometer to monitor the brightness of over 145,000 stars and transmits the data for analysis on Earth. Effects like microlensing, the bending of light due to a gravitational field, and partial eclipses of the stars give hints to the presence of planets that cannot be seen directly.

As of July 2013, Kepler has found 134 confirmed exoplanets in 76 stellar systems in its search for Earth-size planets in or near the “habitable zone” that supports liquid water. Along the way, though, its discoveries have shed light on the such mysteries as the origins of circumbinary planets.

“Circumbinary planets have captured the imagination of many science-fiction writers and film-makers — our research shows just how remarkable such planets are,” Stefan Lines, the lead author of the study, said. “Understanding more about where they form will assist future exoplanet discovery mission in the hunt for earth-like planets in binary star systems.”