Planet Nine is the hypothesized ninth planet of our Solar System, predicted earlier this year by the same scientist responsible for Pluto’s demotion from planet to dwarf planet. But what if Planet Nine was not originally part of our Solar System? New research has suggested that it may be an imposter.

A team of scientists led by Alexander Mustill from the Lund Observatory in Sweden is proposing that Planet Nine might actually be an exoplanet (a world beyond our Solar System) captured by our Sun during a close passing of another star. In their paper, a pre-print of which is available on Arxiv, they admit that the chances of this are low, 0.1 to 2 percent, but they think the possibility is significant enough to warrant further attention.

“Although these probabilities seem low, you have to compare them to each other, and not absolutely,” Mustill told New Scientist. “Because ultimately any very specific outcome is very unlikely.”

Evidence for this theory comes from simulations run by Mustill and his colleagues on how the Sun has interacted with other stars. Our Sun was thought to have been born in a stellar cluster containing thousands of other stars, so it’s possible that early in its life it underwent numerous close encounters.

The paper notes that, in order for a capture to be successful, the Sun must have passed a star at a distance of more than 150 AU (astronomical units, 1 AU is the Earth-Sun distance) to avoid significantly disturbing the region of comets at the edge of the Solar System known as the Kuiper Belt. The encounter star’s planet must also have been in an orbit of more than 100 AU.

If these two requisites are met, then the chance of a capture event is estimated at 50 percent by the researchers.

^{An artist's impression of what Planet Nine might look like. Caltech/R. Hurt (IPAC)}

Of course, this is very much all speculation for now. The original theory proposed by Michael Brown and Konstantin Batygin was that Planet Nine could be a failed gas planet almost the size of Neptune ejected to the outer Solar System. Their evidence for its existence comes from the observed eccentric orbits of objects in the Kuiper Belt, backed up by a recent discovery, which can be explained by the gravitational effect of Planet Nine.

Another theory for the formation of Planet Nine, also published recently on Arxiv, suggests it may simply have formed in the same manner as other bodies in the Solar System, the pebble accretion model, without requiring one of these more dramatic scenarios.

While we still have no direct evidence for Planet Nine’s existence, we are starting to narrow down where it could be. And this latest evidence will add to this growing mystery. Mustill and his colleagues note that their theory could be confirmed by studying the orbits of distant trans-Neptunian objects (TNOs), those that orbit beyond Neptune, and calculating which possible history of Planet Nine best describes them.

For now, the mystery of Planet Nine’s existence continues.

(H/T: New Scientist)