The dwarf planet Sedna lurks in the outer Solar System.Credit: NASA/JPL-Caltech

Denver, Colorado

Hundreds of Moon-sized worlds may orbit the Sun far beyond Neptune, sculpting the geometry of the outer Solar System. So far, astronomers have spotted only a handful of these objects — but if more are out there, they could explain why other distant bodies move in the way they do.

Proof of a large number of these worlds could weaken the case for Planet Nine — an unseen planet bigger than Earth that is hypothesized to orbit beyond Neptune. Their collective gravitational influence would tug some planetary objects into orbits that move away from the inner Solar System.

“Smaller bodies cause this detachment, not an unseen ninth planet,” says Jacob Fleisig, an undergraduate student at the University of Colorado Boulder. He presented the work on 4 June at a meeting of the American Astronomical Society in Denver.

Fleisig and Ann-Marie Madigan, an astrophysicist at the university, were inspired by recent discoveries of extreme Solar System objects. These include the world known as Sedna, which never gets closer to the Sun than about 76 times the Earth–Sun distance. (Neptune, the outermost of the planets, lies at roughly 30 times the Earth–Sun distance.)

Planetary outcasts

Sedna and other extreme objects are puzzling. They are thought to have formed closer to the Sun, along with planets, asteroids and other objects that coalesced from a primordial spinning disk of gas and dust. Some unknown factor later pushed a few of these objects out to extreme orbits.

In 2016, Michael Brown and Konstantin Batygin of the California Institute of Technology in Pasadena proposed the existence of Planet Nine, which would never get closer to the Sun than about 200 times the Earth–Sun distance. Planet Nine’s gravity could have pushed out the orbits of objects such as Sedna, they said.

But Fleisig and Madigan say they don’t need to invoke the existence of a Planet Nine. They ran supercomputer simulations of how bodies might interact in the outer Solar System far beyond Pluto, in the icy region known as the Kuiper belt. They found that a flock of Moon-sized worlds could do many of the same things as Planet Nine.

Over millions of years, the collective gravity of these smaller worlds would nudge the orbits of distant objects. The worlds would jostle one another like bumper cars and, occasionally, cause an object to move into a very distant orbit. Their simulations suggest that more-massive objects would be flung into the most distant orbits — as some observations have suggested1.

Farthest fringes

The work looks “very cool” and “a really promising way” to explain how distant worlds got into their orbits, says Samantha Lawler, an astronomer at the NRC Herzberg Institute of Astrophysics in Victoria, Canada. It could also explain several other mysteries of the outer Solar System, Madigan says — such as an object reported last month, called 2015 BP519, which zips around the Sun at a steep angle relative to the plane of the planets2. Earlier simulations suggested that the gravitational interactions can also make things unstable, kicking objects into these severe angles3.

Planet Nine supporters are not giving up easily. Batygin says that his calculations do not show that the gravitational pull of distant worlds would affect the orbits of other bodies4. “If another model could explain the anomalous structure of the distant Kuiper belt without invoking Planet Nine, that would be of tremendous interest,” he says. “Unfortunately, I don’t think this is that model.”

Scientists continue to unearth new objects that could help to answer questions about the outer Solar System's evolution. The Outer Solar System Origins Survey, an international collaboration that ended last December, found more than 840 icy worlds orbiting far beyond Neptune. Thirty of those are in detached orbits.