At the very edge of our solar system, beyond the grasp of Neptune and even past the icy sprawl of Pluto’s domain, a new frozen world has emerged from the shadows. It’s not a new Earth or the mythical Planet Nine, but its discovery is a stark reminder: the solar system is far from fully mapped.
The object, named 2017 OF201, is a minor planet — one of the largest found in nearly a decade. At an estimated 700 kilometers wide, it’s likely large enough to qualify as a dwarf planet. Its orbit, however, is what truly sets it apart.
A Discovery from the Archives
The object was first spotted not in real time, but in the digital remains of past sky surveys. Sihao Cheng, a researcher at the Institute for Advanced Study in Princeton, led the team that unearthed 2017 OF201. By digging through images from telescopes like the Dark Energy Camera and the Canada-France-Hawaii Telescope, Cheng and colleagues Jiaxuan Li and Eritas Yang traced a slow, deliberate movement — too sluggish for a nearby asteroid, too consistent to be a fluke.
“It became rather clear that they correspond to a single moving object that has an extremely wide and eccentric orbit,” the authors wrote in their newly posted paper on arXiv.
They found 19 separate images of 2017 OF201 spanning seven years, from 2011 to 2018. With those, they plotted its orbit in stunning detail.
They mapped a nearly 25,000-year journey around the sun. Its closest approach, or perihelion, is about 45 times Earth’s distance from the sun — roughly the same as Pluto’s. But at its farthest point, it travels over 1,600 times farther than Earth. So, it has a very elliptical orbit.
A Challenge to Planet Nine
This orbit places 2017 OF201 among a class of objects known as extreme trans-Neptunian objects (ETNOs). These are icy bodies so far away and slow-moving that they barely seem tethered to the sun at all.
But unlike other ETNOs, 2017 OF201 doesn’t follow the usual pattern. Many of its distant cousins appear to “cluster” together in space, their orbits mysteriously aligned as if shepherded by an invisible force. This curious arrangement has led some astronomers to suggest the presence of an undiscovered planet — often dubbed Planet Nine.
But 2017 OF201 doesn’t fit.
“Notably, the longitude of perihelion of 2017 OF201 lies well outside the clustering observed in extreme trans-Neptunian objects,” Cheng and his team write. “This could challenge the Planet Nine hypothesis.”
In simulations, they went further. When they inserted a theoretical Planet Nine into their models, 2017 OF201 got kicked out of the solar system altogether — an outcome that casts doubt on whether such a planet could exist in the form currently proposed.
Bigger Than It Looks
Despite being so distant, 2017 OF201 is unusually bright. That, combined with its estimated size, suggests it may be round — shaped by its own gravity into a sphere. If so, it qualifies as a dwarf planet, joining a class that includes Pluto, Eris, and Haumea.
Based on its brightness and an assumed reflectivity, the team estimates its diameter at 700 kilometers. That’s about one-third the size of Pluto, but nearly as large as Ceres, the biggest object in the asteroid belt.
And it’s not alone, Cheng says. Because 2017 OF201 is only visible during about 0.5% of its orbit, its discovery suggests a vast hidden population of similar objects. If that’s true, the total mass of this distant group could be 1% that of Earth — comparable to the entire known Kuiper Belt.
Where Did It Come From?
The scientists think this object’s journey began with a nudge from Neptune. It may have once orbited closer to the sun, only to be flung outward by gravitational kicks. As it drifted farther, the subtle pull of the Milky Way’s tidal forces — yes, the galaxy itself — gradually lifted its path, detaching it from the planetary region and steering it into its current, lonely orbit.
“The last time 2017 OF201 passed close to us was in November of 1930,” the researchers wrote, “and it will come back again in about 25,000 years.”
That date is rich with coincidence. Pluto was discovered that very same year. And the Institute for Advanced Study, where Cheng now works, was founded that year. Some celestial rhythms, it seems, play out over human generations.
The Fringes Still Hold Secrets
The discovery of 2017 OF201 is a triumph of data sleuthing. By repurposing telescope data meant for distant galaxies, Cheng’s team has revealed a new chapter in the story of our solar system.
For now, 2017 OF201 drifts slowly through the void, an ancient iceberg orbiting in a cosmic backwater. But its discovery brings us a little closer to understanding the outer edge of our planetary family — and reminds us that even in the 21st century, our solar system still holds surprises.
