Why Is Mysterious Planet Nine On The Outer Fringes Of The Solar System?

Planet Nine exists. It's a Neptune-sized planet in the outer fringes of the solar system, with an elliptical orbit that is 10 times further away from the sun than Pluto. It has puzzed scientists.

Researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) have studied various scenarios that can explain the creation of Planet Nine, but they also disclose that most of them do not seem too probable.

"The evidence points to Planet Nine existing, but we can't explain for certain how it was produced," said Gongjie Li, an astronomer from the CfA and lead author of the study.

Planet Nine keeps revolving around the sun at a distance of 40 billion to 140 billion miles, which puts it at the outer edge of the solar system. However, how was this formed before it landed in this unusual orbit much later?

After undertaking millions of computer simulations scientists narrowed down the possibilities of the planet formation to three or four.

Firstly, a passing star is pulling it outward and creates an elliptical orbit. But even though this explanation was taken as the most likely so far, its probability is just 10 percent.

Secondly, Planet Nine may be an exoplanet originating from a passing star system.

Thirdly, it could have been a free-floating planet that joined the solar system when it came near.

The second and third probabilities are not really likely---and their chances may be less than 2 percent.

Alternative research speculates that Planet Nine was formed closer to the sun before it interacted with alternative gas giants such as Jupiter and Saturn. However, with time, a number of gravitational pushes sent it into the larger and more elliptical orbit.

"Think of it like pushing a kid on a swing," said CfA astronomer Scott Kenyon, an author of two other papers that also examine the mystery of Planet Nine's orbit. "If you give them a shove at the right time, over and over, they'll go higher and higher. Then the challenge becomes not shoving the planet so much that you eject it from the solar system."

One of these observations will finally turn out to be correct.

"The nice thing about these scenarios is that they're observationally testable," 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 findings are available online and will be published in the journal Astrophysical Journal Letter.