Questioning the existence of Planet Nine that was discovered earlier this year, new research by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) has found that most of the scenarios indicating its presence actually have low probabilities.
In January, astronomers from the California Institute of Technology claimed to find evidence of a new giant planet that has a mass about 10 times that of Earth and can bring the tally of planets in our solar system back to nine.
The Neptune-mass planet was claimed to be in an elliptical orbit 10 times farther from our Sun than Pluto.
Since then, theorists have puzzled over how this planet could end up in such a distant orbit. But the presence of Planet Nine remains a bit of a mystery till date.
“The evidence points to Planet Nine existing but we can't explain for certain how it was produced,” said astronomer Gongjie Li, lead author on a paper forthcoming in the Astrophysical Journal Letters.
Planet Nine circles our Sun at a distance of about 400-1500 astronomical units. An astronomical unit is the average distance of the Earth from the Sun or 93 million miles. This places it far beyond all the other planets in our solar system.
Li and her co-author Fred Adams from the University of Michigan conducted millions of computer simulations in order to consider three possibilities.
The first and most likely possibility involves a passing star that tugs Planet Nine outward.
Since the Sun formed in a star cluster with several thousand neighbors, such stellar encounters were more common in the early history of our solar system.
However, an interloping star is more likely to pull Planet Nine away completely and eject it from the solar system. Li and Adams found only a 10 percent probability at best of Planet Nine landing in its current orbit.
The team proposes that Planet Nine formed much closer to the Sun and then interacted with the other gas giants, particularly Jupiter and Saturn. A series of gravitational kicks then could have boosted the planet into a larger and more elliptical orbit over time.
They also examine the possibility that Planet Nine actually formed at a great distance to begin with.
Finally, Li and Adams looked at two wilder possibilities - that Planet Nine is an exoplanet that was captured from a passing star system or a free-floating planet that was captured when it drifted close by our solar system.
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However, the chances of either scenario are less than two percent, they noted.