A possible new dwarf planet skirts the solar system’s edge

A possible cousin of Pluto seems to be circling the far reaches of the solar system.

The dwarf planet candidate 2017 OF201 travels in a superwide orbit, with the sun relatively near one end of its huge elliptical path, researchers report in a paper submitted May 21 to arXiv.org. Its route differs from those of other distant large objects in a way that may challenge the idea that the solar system houses an undiscovered planet.

For around a decade, researchers have been hunting for a planet beyond Pluto, demoted to a dwarf planet in 2006. The gravitational pull of an unknown giant planet — dubbed Planet Nine or Planet X — could explain the clustered orbits of several faraway objects, whose routes are all roughly oriented the same way.

“You would expect them to all have random orientations. Why are they all off to one side?” says astronomer Chad Trujillo of Northern Arizona University in Flagstaff, who was not involved in the new work.

In hopes of finding Planet Nine, astrophysicist Sihao Cheng of the Institute for Advanced Study in Princeton, N.J., and colleagues searched through public archival data for objects orbiting the sun. But instead of identifying a full-blown planet, the team spotted 2017 OF201 in 19 telescope images captured from 2011 to 2018.

Positions and travel speed determined from the multiyear observations revealed the moving object’s extremely wide orbit. Its farthest point from the sun is about 245 billion kilometers, or more than 1,600 times Earth’s distance from our star. The dwarf planet candidate’s closest point to the sun is about 7 billion kilometers, roughly 45 times that of Earth’s distance. One trip around the sun takes more than 24,000 years.

Furthermore, the researchers estimate that 2017 OF201 has a diameter of 700 kilometers and a mass comparable to that of Earth’s moon. That’s large enough for the newly discovered object’s gravitational pull to create a round surface, making it count as a dwarf planet, Cheng says. He and his colleagues are seeking additional data to confirm these estimates, which would help solidify its status.

But 2017 OF201’s path contrasts with the clustered orbits used to support the existence of Planet Nine. “The orientation seems to be … 90 degrees, probably, of the cluster,” Cheng says.

The team’s simulations with a recently proposed Planet Nine orbit suggest 2017 OF201’s route doesn’t work with the hypothesis. But the finding doesn’t rule out the possibility just yet.

“People have considered a whole host of different orbits for these hypothetical planets,” Trujillo says. “This object being incompatible with one is not [detrimental] to Planet X.”

Cheng agrees and is running more simulations to determine whether 2017 OF201’s orbit is stable and how it might be influenced by other possible routes of an undiscovered planet.