Last year scientists presented evidence for Planet Nine, a Neptune-mass planet in an elliptical orbit 40 billion to 140 billion miles from our sun – or roughly 10 times farther from our sun than Pluto. Yet theorists are puzzled over how Planet Nine could end up in such a distant orbit. Two possibilities are suggested by two computer-simulation studies by University of Utah astrophysicist Ben Bromley and his colleague Scott Kenyon at the Harvard-Smithsonian Center for Astrophysics. They propose that Planet Nine formed much closer to the sun and then got kicked into a larger and more elliptical orbit by gravity from other giant gas planets, particularly Jupiter and Saturn. Bromley and Kenyon also examine the possibility that Planet Nine initially formed in place at a great distance. “We are finding more and more planets around other stars,” says Bromley, a professor of physics and astronomy. “To understand what they’re like and where they came from, we look to our own solar system. The discovery of a Planet Nine would help us understand how planet formation works. If it turns out to be icy like Pluto, on a nice, circular orbit, then we’d learn that big planets could form surprisingly far from the sun. If Planet Nine followed a more elliptical path and was a gas giant like Neptune, we’d suspect it was flung out by Jupiter or Saturn.” The studies by Bromley and Kenyon have been submitted to the Astrophysical Journal and are available online here and here. A Harvard-Smithsonian news release about those and a third, related study is here.

Ben Bromley, office 801-581-8227, cell 801-230-7297, bromley@physics.utah.edu