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Single-exoplanet systems may be the fault of their stars.

Exoplanets are abundant in the galaxy, but the systems they belong to seem to come in two distinct configurations. Some stars have multiple planets orbiting in the same plane, much like the solar system. But some appear to have only one world crossing their face.

Recent studies have shown an overabundance of these singleton planets among the thousands seen by the Kepler space telescope, compared with the numbers computer models predict.

This excess is often interpreted to mean that, although only one planet passes in front of the star, others may orbit at different inclinations relative to Earth and so be invisible to us.

Now, and at Caltech in Pasadena, California, argue that the rapid rotation of young stars may explain this situation. The spinning makes each star bulge in the middle, altering its gravitational field and tweaking the planets’ orbits, especially inner ones, Spalding says.

Stars are also often slightly out of alignment with their planets – that’s true of the sun, whose axis perpendicular to the plane of the planets is offset by about six degrees. This tilt can happen due to a passing star aeons ago, or interactions between the primordial dusty disc and the stars’ magnetic fields. Such tilts can further destabilise vulnerable planets.

“If the star is a perfect sphere, the orbital plane will not change: your planet will follow a circle and that circle will not change over time,” he says. “But if you make the star a little oblate, the planets will not stay in that plane if the star is not in that plane. The closer you are to the star, the faster this effect occurs.”

Rockette planets

There are other factors, however: giant planets lurking beyond the star system’s influence, or even companion stars nearby, could also cause disarray. These tagalongs could tilt a star’s planets so they all line up, like Rockettes dancing in synchrony. In that case, that are aligned in the same plane, say and at Cornell University, New York.

“The inner planets all want to be aligned to each other, but they also want to be aligned to the giant companion, so they are in competition,” Pu says.

Both giant exterior planets and bulging stars can cause a game of gravitational billiards, says Spalding.

“One of the big results we find is that this process doesn’t just misalign the orbits, but it will misalign them in such a way that the planetary orbits interact in an unstable manner,” he says. “They will throw other planets out of the star system.”

To test this theory, Spalding and Batygin built a computer model of a well-studied system called Kepler-11, which has six known planets. The star system is about 8 billion years old and 2000 light years from Earth. In reality, the star and planets are neatly aligned, which allowed astronomers to observe all six worlds.

Flung into space

In the simulation, the star was tilted slightly and spun more rapidly. The simulation showed that, after 1 million years, between two and five of the planets would have been flung into interstellar space, never to be seen again.

Others have argued that massive planets close to their stars would have , and they have always been singletons.

However, there are so many single-planet systems that both mechanisms might be involved, say Pu and Lai. In some systems, a larger planet may be interfering with the smaller planets, while in others, it could be an inside job and the star’s fault.

“There’s probably not a single explanation to account for all of these systems,” Pu says.