Although evidence thus far points to the existence of Planet Nine, a Neptune-sized planet in the outer reaches of our solar system, its elliptical orbit - 10 times farther from our sun than Pluto - has baffled scientists. Now, a new study by researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) examines numerous scenarios to explain the creation of Planet Nine and reveals that most of them have low probabilities.
"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 circles our sun at a distance of approximately 40 billion to 140 billion miles, placing it at the outer edge of our solar system, far past the other planets. However, the question is now if it formed here or elsewhere in the solar system before landing in its unusual orbit at a later date.
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.
"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."
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," 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 Letters.
