It has likely had a really rough life.
It was not too long ago that scientists presented evidence for the existence of a hypothetical Planet Nine, a Neptune-sized planet in an elliptical orbit ten times farther from the sun than Pluto. Since that time, theorists have puzzled over how this planet could end up in such a distant and bizarre orbit.
Now, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) have examined a number of scenarios to explain how Planet Nine could have landed itself so far from the sun.
"The evidence points to Planet Nine existing, but we can't explain for certain how it was produced," said CfA astronomer Gongjie Li, lead author on a paper accepted for publication in the Astrophysical Journal Letters, in a news release.
Planet Nine orbits the sun at a distance of about 40 billion to 140 billion miles, placing it far beyond all the other planets in the solar system. But how the heck did it get there? Did it form there, or did it originate somewhere else and end up in this strange orbit later?
Li and co-author Fred Adams of the University of Michigan, as well as CfA astronomer Scott Kenyon and Benjamin Bromley of the University of Utah, conducted millions of computer simulations to test three likely scenarios, as well as investigate two more extreme possibilities.
The first and most likely explanation involves a passing star tugging Planet Nine outward. This type of interaction would not only nudge the planet into a wider orbit, but also make the orbit much more elliptical. Since the sun originally formed in a star cluster with several thousand neighbors, these stellar encounters were likely more common in the early history of the solar system.
However, the researchers note that a star is more likely to pull Planet Nine away completely and eject it from the solar system, knocking this scenario’s probability to 10 percent.
Second, in two papers submitted to the Astrophysical Journal, Kenyon and co-author Bromley propose that Planet Nine formed much closer to the sun and then interacted with other gas giants, particularly Jupiter and Saturn. Then through a series of gravitational kicks, the planet was shoved into a larger more elliptical orbit over time.
"Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they'll go higher and higher," explained Kenyon in the release. "Then the challenge becomes not shoving the planet so much that you eject it from the solar system."
Third, Kenyon and Bromley examined the possibility that Planet Nine actually formed at this exceptional far distance from the sun to begin with. They found that the right combination of the initial mass of the solar system’s protoplanetary disk and the disks lifetime could have created Planet Nine in time to be nudged by Li’s passing star.
"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."
Finally, Li and Adams looked at two other “extreme” scenarios: that Planet Nine is an exoplanet that was captured from a passing star system, or that it’s free-floating planet that was captured when it drifted too close to the solar system. However, they concluded that the chances of either of these possibilities are less than 2 percent.
Nevertheless, considering the probability of intelligent evolution on any given planet is less than 0.01 percent over 4 billion years, these scenarios really don’t sound that unrealistic. But clearly, the presence, or absence, of Planet Nine still remains a bit of a mystery.
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