Deep in the frozen outer reaches of our solar system, roughly 6 billion kilometers from the Sun, a small icy rock has done something it was never supposed to do: it has grown an atmosphere.
The object, known as (612533) 2002 XV93, is a trans-Neptunian body (TNO), one of thousands of frozen worlds drifting beyond Neptune’s orbit in the region known as the Kuiper Belt. Roughly 500 kilometers in diameter, and located about 40 times farther from the Sun than Earth, it takes 247 years to complete a single orbit. By any standard measure, it was considered unremarkable, just another icy fragment left over from the early solar system. Until now.
An Unexpected Discovery
The finding was made by a team of Japanese professional and amateur astronomers led by Ko Arimatsu, associate professor and senior lecturer at the Ishigakijima Astronomical Observatory of the National Astronomical Observatory of Japan (NAOJ). The researchers observed a rare stellar occultation, a moment when the object passed directly in front of a background star, in January 2024.
Under normal circumstances, if a body in space has no atmosphere, the starlight behind it would vanish instantly as it passes. But that’s not what the team observed. Instead, the star’s light dimmed gradually over about 1.5 seconds, indicating refraction by an atmosphere surrounding the object. The gradual fade was the telltale signature of something enveloping the body, bending light as it passed through.
The observations were conducted from four locations in Japan, using both professional and amateur telescopes. The primary instrument was the 1.05-meter telescope at the University of Tokyo’s Kiso Observatory, supplemented by smaller telescopes equipped with CMOS cameras that independently captured the gradual dimming.
The Only One of Its Kind
Until this discovery, Pluto held a unique distinction in the outer solar system: it was the only trans-Neptunian object known to host an atmosphere, a thin exosphere of nitrogen, methane, and carbon monoxide that forms when Pluto’s surface ices vaporize near its closest approach to the Sun. Dwarf planets Eris, Haumea, Makemake, and dwarf planet candidate Quaoar, the largest known TNOs after Pluto, do not appear to have atmospheres.
This would make 2002 XV93 only the second known object beyond Neptune with a detectable atmosphere, after Pluto. The difference in scale is striking: while Pluto has a diameter of about 2,377 kilometers, 2002 XV93 spans only around 500 kilometers — less than a quarter the size.
More specifically, 2002 XV93 is classified as a “plutino”, meaning that, like Pluto, it orbits in a 2:3 mean-motion resonance with Neptune, completing two trips around the Sun for every three completed by Neptune.
An Atmosphere Against the Odds
The researchers calculated that 2002 XV93 has an atmosphere about 5 million to 10 million times thinner than Earth’s. Its surface pressure falls between 100 and 200 nanobars, for comparison, Pluto’s atmosphere averages around 10 millibars, itself already extraordinarily thin. The likely composition includes methane, nitrogen, and carbon monoxide, though the precise chemical makeup remains unknown.
What makes this discovery so puzzling is the size of the object. In the deep cold of the Kuiper Belt, scientists expect only the most volatile ices, such as methane, nitrogen, and carbon monoxide, to feed an atmosphere. On a body the size of 2002 XV93, those gases should escape too quickly for any long-term atmosphere to survive. Yet it has one. The James Webb Space Telescope had detected frozen carbon dioxide on the surface, but no hypervolatile compounds like methane or nitrogen, suggesting, as researchers note, that most of them had already vaporized and escaped into space long ago.
Two Theories for How It Happened
Arimatsu’s team is currently investigating two main hypotheses. The first is cryovolcanism, essentially frozen volcanoes, in which internal gases such as methane, nitrogen, or carbon monoxide seep upward from beneath the object’s icy surface and into space. The study notes that cryovolcanic-like seepage might still occur under special conditions, such as unusual concentrations of ammonia or methanol, or tidal forcing from an undiscovered satellite, though current data can neither confirm nor rule out such a companion.
The second theory is a recent impact. A comet or another Kuiper Belt object might have slammed into 2002 XV93, releasing subsurface gases in a sudden burst. If this is the case, the atmosphere would be temporary, as at these temperatures and with such weak surface gravity, a thin gas envelope would dissipate into space within roughly a thousand years.
Wider Implications
The discovery is already prompting a rethink of what astronomers thought they knew about small bodies in the outer solar system. As the researchers put it, “the traditional idea that dense atmospheres form only around large bodies must be revised.”
The finding also highlights recent activity on 2002 XV93, whether it be the eruption of frozen gases or the aftermath of material slowly falling back onto the object’s surface. “This shows the Kuiper Belt is not a cold dead place,” wrote Scott Sheppard of the Carnegie Institution for Science in a comment, “but is teeming with activity and has many of the building blocks for life.”
The next step for researchers is to determine the precise chemical composition of the exosphere, something the James Webb Space Telescope is uniquely positioned to deliver. Monitoring how the atmosphere’s density changes over the coming years will also help resolve the central mystery: is this a fleeting aftermath of a cosmic collision, or an ongoing geological process on a world far stranger than anyone expected?
The findings were published on May 4, 2026, in the journal Nature Astronomy. In an interview with Reuters, Arimatsu noted that he hopes the object will eventually receive a formal name, perhaps connected to Okinawan mythology, such as Amamikyu, the creator deity in Okinawan tradition, though formal naming must follow the procedures of the International Astronomical Union.
