The hypothetical ninth planet at the fringes of our solar system might have been captured from a star passing by long ago

The Pleiades star cluster: it might once have been the sun’s nursery, with planets ripe for the taking NASA

Our solar system might harbour an alien interloper. The proposed Planet Nine lurking at the edge of the solar system could have been stolen from a passing star.

In January, Konstantin Batygin and Michael Brown announced evidence for an unseen planet around 10 times Earth’s mass lurking in the fringes of the solar system. Other astronomers immediately came forward with suggestions for how so-called Planet Nine might have migrated from the inner solar system towards its outer edges.

But one team now suggests just the opposite: that it was captured from a nearby star.


The idea isn’t all that far-fetched. The sun was born in a reasonably large stellar cluster with roughly 1000 or maybe even 10,000 stars, says Alexander Mustill from the Lund Observatory in Sweden. In such a dense cluster, the sun would have had quite a few close encounters with other stars, potentially letting them swap planets from time to time.

“It would be pretty wild – to pick up an alien planet and bring it along for the ride,” says Greg Laughlin at the Lick Observatory in California.

Good odds

To check just how wild, Mustill and his colleagues ran simulations of encounters between the solar system and any passing planetary systems. They found that if that a system happened to have a wide-orbit planet, the likelihood it would be captured by the sun is about 50 per cent.

Those are pretty good odds, but they dropped when the team took into account whether the passing planetary system would have a wide-orbit planet in the first place. Also, it wasn’t enough to just capture a planet – their simulations only worked if they captured one that was exactly like Planet Nine. Overall, Mustill and his colleagues think the chance that Planet Nine is an exoplanet ranges from 0.1 to 2 per cent.

“Although these probabilities seem low, you have to compare them to each other, and not absolutely,” says Mustill. “Because ultimately any very specific outcome is very unlikely.” The probability that evidence for Planet Nine’s existence is random chance is just 0.007 per cent at present, so the fact that the odds of it being an exoplanet are 15 to 300 times higher than that actually bodes well for the exoplanet scenario.

Capture or exile?

A fugitive on the run is just one way to explain Planet Nine, however. Batygin and Brown initially thought it was likely to be the core of a gas giant ejected from the inner solar system early in its formation. “My pet theory is it happened early and there was a lot of gas around in the solar nebula and that gas sort of slowed it down and kept it from being completely removed,” Brown says.

This theory is relatively straightforward, says Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. The array of Jupiter-like exoplanets that orbit dangerously close to their host stars suggest that massive planets regularly migrate inward. “Whenever you scatter something inward, to conserve energy, you’re likely to scatter something else outward,” Kenyon says.

And there’s one final option: the planet could have formed where we find it now. Although some have speculated that there wouldn’t be enough material to form a planet in the outer fringes of the solar system, Kenyon found that there could be enough icy pebbles to form something as small as Planet Nine in a couple of hundred million years.

“I think it’s premature to say what’s most likely,” says Kenyon. A definitive answer will likely hinge on actually finding the unseen planet.

References: http://arxiv.org/abs/1603.07247, http://arxiv.org/abs/1603.08008