Why is Planet Nine so far from us?

Evidence thus far may be pointing to the existence of Planet Nine, a Neptune-mass planet in an elliptical orbit 10 times farther from our Sun than Pluto, but the question as to how it ended up in such a distant orbit has baffled theorists.

New research by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) examines a number of scenarios and finds that most of them have low probabilities. Therefore, the presence of Planet Nine remains a bit of a mystery.

"The evidence points to Planet Nine existing, but we can't explain for certain how it was produced," says lead author Gongjie Li.

In the new study, the team conducted millions of computer simulations to consider three possibilities for the creation of Planet Nine, the first and most likely being that a passing star is tugging Planet Nine outward. This would also create an elliptical orbit, making it the most likely explanation thus far. Even still, the probability of this scenario was only determined to be 10 percent.

The other two possibilities are even less likely: that Planet Nine is an exoplanet that originates from a passing star system, or a free-floating planet that was captured when it came into the proximity of our solar system. The probability of both of these scenarios is less than 2 percent.

Another explanation proposed by alternative research is that Planet Nine formed closer to the sun before interacting with other gas giants such as Jupiter and Saturn. Over time, a series of gravitational pushes sent the planet into the larger, more elliptical orbit.

If Planet Nine is eventually discovered, further observations will help determine which of the above scenarios is correct.

"The nice thing about these scenarios is that they're observationally testable," astronomer Scott Kenyon said. "A scattered gas giant will look like a cold Neptune, while a planet that formed in place will resemble a giant Pluto with no gas."

The study is published online in the journal Astrophysical Journal Letters.