Earlier this year California Institute of Technology scientists Konstantin Batygin and Mike Brown presented strong evidence for Planet Nine, a hypothesized massive ice giant that may orbit in the far outer Solar System. Since then astronomers have puzzled over how this planet could end up in such a distant orbit.
Planet Nine is thought to be gaseous, similar to Uranus and Neptune; hypothetical lightning lights up the night side. Image credit: R. Hurt, IPAC / Caltech.
New research by two teams of scientists from the United States examines a number of scenarios and finds 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 Dr. Gongjie Li from the Harvard-Smithsonian Center for Astrophysics, lead author on a paper accepted for publication in the Astrophysical Journal Letters.
Planet Nine circles our Sun at a distance of 400 to 1,500 AU (astronomical units). This places it far beyond all the other planets in the Solar System.
The question becomes: did it form there, or did it form elsewhere and land in its unusual orbit later?
Dr. Li and her colleague, Dr. Fred Adams from the University of Michigan, conducted millions of computer simulations in order to consider three possibilities.
The first and most likely involves a passing star that tugs Planet Nine outward.
Such an interaction would not only nudge the planet into a wider orbit but also make that orbit more elliptical. And 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.
The scientists find only a 10% probability, at best, of Planet Nine landing in its current orbit.
Moreover, the ice giant would have had to start at an improbably large distance to begin with.
Dr. Scott Kenyon from the Harvard-Smithsonian Center for Astrophysics and Dr. Benjamin Bromley from the University of Utah believe they may have the solution to that difficulty.
In two papers submitted to the Astrophysical Journal, they use computer simulations to construct plausible scenarios for the formation of Planet Nine in a wide orbit.
“The simplest solution is for the Solar System to make an extra gas giant,” Dr. Kenyon said.
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.
The scientists also examine the possibility that Planet Nine actually formed at a great distance to begin with.
They find that the right combination of initial disk mass and disk lifetime could potentially create Planet Nine in time for it to be nudged by a passing star.
“The nice thing about these scenarios is that they’re observationally testable. 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,” Dr. Kenyon said.
The work by Dr. Li and Dr. Adams also helps constrain the timing for Planet Nine’s formation or migration.
The Sun was born in a cluster where encounters with other stars were more frequent.
Planet Nine’s wide orbit would leave it vulnerable to ejection during such encounters.
Therefore, Planet Nine is likely to be a latecomer that arrived in its current orbit after the Sun left its birth cluster.
Finally, Dr. Li and Dr. 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.
However, they conclude that the chances of either scenario are less than 2%.
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Gongjie Li & Fred C. Adams. 2016. Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine. ApJL, accepted for publication; arXiv: 1602.08496
Scott J. Kenyon & Benjamin C. Bromley. 2016. Making Planet Nine: Pebble Accretion at 250 – 750 AU in a Gravitationally Unstable Ring. ApJ, submitted for publication; arXiv: 1603.08008
Benjamin C. Bromley & Scott J. Kenyon. 2016. Making Planet Nine: A Scattered Giant in the Outer Solar System. ApJ, submitted for publication; arXiv: 1603.08010
