A rogue planet twice the size of Earth may have pushed asteroids into the outer reaches of the early solar system before it was ejected. This could solve the mystery of how some of the most distant objects in the Kuiper belt got there, which cannot be explained by most models of solar system formation.
The Kuiper belt is a huge disc of cold rocks and ice that extends out beyond the orbit of Neptune. It contains three populations of objects: scattering objects, which typically orbit the sun relatively close to Neptune with their paths determined by its gravity; resonant objects, which follow particular orbits that are tied to Neptune’s orbit and are often further from the sun; and detached objects, which follow non-resonant orbits and never come closer to the sun than Neptune.
Astronomers have long thought that the scattering objects and some of the others probably got there when the giant planets migrated to their current positions in the solar system more than 4 billion years ago. But both resonant objects with high perihelion – meaning they never come close to the sun – and detached objects don’t fit the same explanation.
Advertisement
“Jupiter and Saturn were very good at basically rapidly chucking everything out into the outer solar system,” says at the University of British Columbia in Canada. “But the efficiency of that process fails to make enough of these objects with high perihelion.”
Gladman and his colleagues used a set of simulations to study the possibility that another object, a primordial planet with double Earth’s mass, was thrown into the outer solar system early on and pushed those distant objects onto their orbits. Their simulations began with this object already in the Kuiper belt and followed how it would affect the other bodies out there.
They found that it worked perfectly, but not simply by gravitationally dragging the objects out with it. In the simulations, most of the Kuiper belt objects didn’t interact directly with the planet. “It’s jiggling the orbits of everything in this area just a little bit and kicking things into and out of the resonances,” says Gladman.
This object would have been unrelated to the giant planet that may reside in the outer reaches of the solar system – dubbed Planet Nine – as if it exists, its orbit takes it far beyond the Kuiper belt. But the newly suggested planet could explain both how the high-perihelion objects got there and the existence of some particularly large Kuiper belt objects on unexpectedly distant orbits.
Eventually, the planet would move so far from the sun that it was no longer bound by its gravity and floated away into interstellar space, becoming what astronomers call a rogue planet. “When the rogue goes away, those oscillations in the orbits suddenly damp,” says at the University of Arizona, who was not involved in this work. “Anything that happens to be on a really high perihelion will get frozen into the orbit it’s on. They get stranded.”
Those objects stranded in orbits that are particularly distant from the sun would then just stay there, without any other huge objects to perturb them. “The comparison to astronomical observations for this particular model looks pretty favourable,” says Volk. The next step may be a more detailed model beginning with the formation of the rogue planet, she says.
Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every Friday
Reference:
