The presence of a new planet on the outskirts of the Solar System was proposed by Caltech astronomers Konstantin Batygin and Mike Brown in 2016 to explain the intricate orbital structure of the Kuiper Belt, a field of icy bodies orbiting the Sun beyond Neptune. Since then, astronomers have been busy gathering evidence of its existence. Now Batygin, Brown and two University of Michigan astronomers, Juliette Becker and Fred Adams, have reviewed this evidence in a pair of papers.
Artist’s impression of Planet Nine. Image credit: Tom Ruen / ESO.
The Planet Nine hypothesis is founded on evidence suggesting that the clustering of Kuiper Belt objects is influenced by the gravitational tugs of the unseen planet.
It has been an open question as to whether that clustering is indeed occurring, or whether it is an artifact resulting from bias in how and where Kuiper Belt objects are observed.
To assess whether observational bias is behind the apparent clustering, Professor Brown and Dr. Batygin developed a method to quantify the amount of bias in each individual observation, then calculated the probability that the clustering is spurious.
That probability, the researchers found, is around one in 500. The results were published in a paper in the Astronomical Journal.
“Although this analysis does not say anything directly about whether Planet Nine is there, it does indicate that the hypothesis rests upon a solid foundation,” Professor Brown said.
Orbits of the distant Kuiper Belt and Planet Nine: orbits rendered in purple are primarily controlled by Planet Nine’s gravity and exhibit tight orbital clustering; green orbits, on the other hand, are strongly coupled to Neptune, and exhibit a broader orbital dispersion; updated orbital calculations suggest that Planet Nine is a 5 to 10 Earth mass planet that resides on a mildly eccentric orbit with a period of about 10,000 years. Image credit: James Tuttle Keane / Caltech.
The second paper, published in the journal Physics Reports, provides thousands of new computer models of the dynamical evolution of the distant Solar System and offers updated insight into the nature of Planet Nine, including an estimate that it is smaller and closer to the sun than previously suspected.
Based on the new models, Dr. Batygin and Professor Brown — together with Becker and Professor Adams — concluded that Planet Nine has a mass of between 5 and 10 times that of Earth and has an orbital semimajor axis in the neighborhood of 400 AU, making it smaller and closer to the Sun than previously suspected — and potentially brighter.
“At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar super-Earth,” Dr. Batygin said.
“It is the Solar System’s missing link of planet formation. Over the last decade, surveys of exoplanets have revealed that similar-sized planets are very common around other Sun-like stars. Planet Nine is going to be the closest thing we will find to a window into the properties of a typical planet of our Galaxy.”
“The strongest argument in favor of Planet Nine is that independent lines of evidence can all be explained by a proposed new planet with the same properties,” Professor Adams said.
“In other words, there are multiple reasons to believe that Planet Nine is real, not just one.”
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Michael E. Brown & Konstantin Batygin. 2019. Orbital Clustering in the Distant Solar System. AJ 157, 62; doi: 10.3847/1538-3881/aaf051
Konstantin Batygin et al. The Planet Nine Hypothesis. Physics Reports, published online February 10, 2019; doi: 10.1016/j.physrep.2019.01.009
