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

Known as 2017 OF201, it’s a suspected dwarf planet — a round rock too big to be an asteroid but too small to be a full-fledged planet. 

Finding this object doesn’t just add a new name to the solar system census. It also challenges the idea that the solar system houses an unseen giant planet on its outer edges. That proposed yet undiscovered world is called Planet Nine, or Planet X. 

Researchers at Princeton University in New Jersey shared their findings May 21 on arXiv.org. Studies posted to that site have not yet passed peer review by other scientists. 

Planetary hide and seek 

For around a decade, scientists have been hunting a Planet Nine beyond the dwarf planet Pluto. They thought it might exist because the gravity of a giant planet in this area would explain the orbits of several other objects. 

The routes those distant rocks took around the sun are all roughly oriented the same way. “You would expect them to all have random orientations. Why are they all off to one side?” asks Chad Trujillo. This astronomer was not involved in the new research. He works at Northern Arizona University in Flagstaff. 

Sihao Cheng’s team at Princeton hoped to find Planet Nine. They searched through old data for objects orbiting the sun. But they didn’t find a true planet. Instead, they spotted 2017 OF201. The surprise object emerged in 19 telescope images captured between 2011 and 2018. 

Its speed and position over those years traced out its orbit. This object’s path around the sun is a huge, stretched-out oval with our sun near one end. Its farthest point from the sun is more than 1,600 times Earth’s distance from our star. The closest it gets to the sun is roughly 45 times Earth’s distance.  

Each trip 2017 OF201 takes around the sun lasts more than 24,000 years. 

The Princeton group estimates that 2017 OF201 is about 700 kilometers (435 miles) across. Its mass is about 2 percent of Pluto’s. That’s large enough for its gravity to pull it into a round shape. If so, that would make it a dwarf planet, says Cheng. He’s an astrophysicist at Princeton’s Institute for Advanced Study. 

His team is now looking for more data to confirm that 2017 OF201 really is a dwarf planet. 

What it means for Planet Nine  

This object’s orbit may throw a wrench into theories about Planet Nine.  

The reason: Its path around the sun differs from the clustered orbits used to support the existence of Planet Nine. “The orientation seems to be … 90 degrees, probably, of the cluster,” Cheng says. If a giant Planet Nine exists, one would expect it to have shepherded 2017 OF201 in line with other neighboring objects.  

Computer models helped the team investigate further. Those models included a recently proposed orbit for Planet Nine. And in those simulations, 2017 OF201 could not take its observed route around the sun.  

Still, the possibility of Planet Nine isn’t ruled out just yet. 

“People have considered a whole host of different orbits” for the potential giant planet, Trujillo says. If 2017 OF201 doesn’t fit with one of them, then perhaps Planet Nine simply takes a different path around the sun. 

Cheng agrees. He’s now running more models to find out how 2017 OF201’s orbit might be influenced by other possible orbits for a Planet Nine.