Planet Nine? Not quite, but some astronomers think they've spotted a new dwarf planet

NBC News

04-06-2025

A possible new dwarf planet has been discovered at the edge of our solar system, so far-flung that it takes around 25,000 years to complete one orbit around the sun.
The object, known as 2017 OF201, was found by researchers at the Institute for Advanced Study and Princeton University who were searching for 'Planet Nine,' a hypothetical planet larger than Earth that is thought to orbit beyond Neptune. Some astronomers theorize that a mysterious ninth planet, which so far remains undetected, could explain an unusual clustering of objects and other anomalies observed in the outer solar system.
In searching for the elusive Planet Nine, researchers instead turned up a different resident in our cosmic backyard.
'It's not very different from how Pluto was discovered,' said Sihao Cheng, a member at the Institute for Advanced Study who led the research team. 'This project was really an adventure.'
If confirmed, the newfound dwarf planet would be what Cheng calls an 'extreme cousin' of Pluto. The findings were published on the preprint website arXiv and have not yet been peer-reviewed.
Cheng and his colleagues estimate that 2017 OF201 measures about 435 miles across — significantly smaller than Pluto, which measures nearly 1,500 miles across.
A dwarf planet is classified as a celestial body that orbits the sun that has enough mass and gravity to be mostly round, but unlike other planets, has not cleared its orbital path of asteroids and other objects.
Eritas Yang, one of the study's co-authors and a graduate student at Princeton University, said that one of 2017 OF201's most interesting features is its extremely elongated orbit. At its farthest point from the sun, the object is more than 1,600 times more distant than the Earth is to the sun.
The researchers found the dwarf planet candidate by meticulously sifting through a huge data set from a telescope in Chile that was scanning the universe for evidence of dark energy. By cobbling together observations over time, the researchers identified a moving object with migrations that followed a clear pattern.
2017 OF201 is likely one of the most distant visible objects in the solar system, but its discovery suggests there could be other dwarf planets populating that region of space.
'We were using public data that has been there for a long time,' said Jiaxuan Li, a study co-author and a graduate student at Princeton University. 'It was just hidden there.'
Li said the object is close to the sun at the moment, which means the researchers need to wait about a month before they can conduct follow-up observations using ground-based telescopes. The scientists are also hopeful that they can eventually secure some time to study the object with the Hubble Space Telescope or the James Webb Space Telescope.
In the meantime, Cheng said he hasn't given up searching for Planet Nine. The new discovery, however, may throw a wrench into some long-standing theories of the planet's existence.
The hypothesis behind Planet Nine is that a planet several times the size of Earth in the outer solar system could explain why a group of icy objects seem to have unusually clustered orbits.
'Under the influence of Planet Nine, all objects that do not have this specific orbital geometry will eventually become unstable and get kicked out of the solar system,' Yang said.
2017 OF201's elongated orbit makes it an outlier from the clustered objects, but Yang's calculations suggest that the orbit of 2017 OF201 should remain stable over roughly the next billion years.
In other words, 2017 OF201 likely would not be able to remain if Planet Nine does exist. But Yang said more research is needed, and the discovery of the new dwarf planet candidate is not necessarily a death knell for Planet Nine.
For one, the simulations only used one specific location for Planet Nine, but scientists don't all agree on where the hypothetical planet lurks — if it's there at all.
Konstantin Batygin, a professor of planetary science at the California Institute of Technology, proposed the existence of Planet Nine in a study published with his Caltech colleague Mike Brown in 2016.
He said the discovery of 2017 OF201 doesn't prove or disprove the theory. The objects in the outer solar system that are likely to show a footprint of Planet Nine's gravity, Batygin said, are the ones where the closest points on their orbits around the sun are still distant enough that they don't strongly interact with Neptune.
'This one, unfortunately, does not fall into that category,' Batygin told NBC News. 'This object is on a chaotic orbit, and so when it comes to the question of 'What does it really mean for Planet Nine?' The answer is not very much, because it's chaotic.'
Batygin said he was excited to see the new study because it adds more context to how objects came to be in the outer solar system, and he called the researchers' efforts mining public data sets 'heroic.'
Cheng, for his part, said he hasn't abandoned hope of finding Planet Nine.
'This whole project started as a search for Planet Nine, and I'm still in that mode,' he said. 'But this is an interesting story for scientific discovery. Who knows if Planet Nine exists, but it can be interesting if you're willing to take some risks.'