Over the last few years, there's been growing evidence for a huge, ninth planet lurking on the very edges of the Solar System. The dwarf planets and other icy bodies out there move in mysterious ways that suggest an unseen world is pulling on them, but new calculations suggest that there is no Planet Nine – these distant objects might just be jostling each other like bumper cars.
Ten years after Pluto was demoted to dwarf planet, the possibility of a new Planet Nine was put forward by a Caltech team in January 2016. The key evidence was the highly eccentric orbits of some Kuiper Belt objects, which are tilted 30 degrees off-kilter from the rest of the planets. Plus, their sheer distance from the Sun and the eight known planets doesn't really make sense yet.
In the years since, more evidence of a ninth planet has turned up in the orbits of trans-Neptunian objects and the wobble of the Sun, while other astronomers have modeled its composition and even floated the idea of a Mars-sized 10th planet. But since Planet Nine has yet to be directly observed, some scientists are naturally questioning its existence. Can the quirks of these distant objects be explained some other way?
Researchers at the University of Colorado Boulder started by developing computer simulations of the orbits of "detached objects" – distant dwarf planets like Sedna, as well as icy comets, moons and other bodies, that seem to be separated from the rest of the Solar System. Their interactions may just hold the answers.
"There are so many of these bodies out there," says Anne-Marie Madigan, co-author of the new study. "What does their collective gravity do? We can solve a lot of these problems by just taking into account that question."
The simulations revealed that the orbits of these objects would influence each other gravitationally, producing the weird motions we see in real life. The team compared the movements to the hands of a clock – smaller objects like asteroids move fast like minute hands, while the larger objects like Sedna move relatively slower, like hour hands. At certain points these "hands" line up, and their gravity pushes the objects into unusual orbital paths.
"You see a pileup of the orbits of smaller objects to one side of the sun," says Jacob Fleisig, lead author of the study. "These orbits crash into the bigger body, and what happens is those interactions will change its orbit from an oval shape to a more circular shape."
These bumper car-like interactions can explain many of the anomalies out there, without needing to invent a huge Planet Nine. Of course, the discussion is far from settled, and this new research will probably be rebutted with other evidence. The question likely won't be answered until we actually see the planet for ourselves.
The research was presented at the meeting of the American Astronomical Society this week.
Source: University of Colorado Boulder
