Astronomers find atmosphere around a solar system object that shouldn’t have one
A rare observation opportunity revealed the presence of a thin atmosphere around a small object beyond Pluto.
A view shows the Milky Way galaxy shining brightly above a lone tree in a field, captured with long-exposure photography in Ankara, Turkiye, on August 23, 2025. (Photo Credit: Ercin Erturk/Anadolu via Getty Images)
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(CNN) — Astronomers have detected a thin atmosphere around a tiny celestial body in the outer solar system for the first time — an object previously thought to be too small to support the presence of an atmosphere.
Thousands of frozen, rocky bodies called trans-Neptunian objects, or TNOs, exist in the Kuiper Belt on the edge of our solar system, remnants from its formation 4.5 billion years ago.
The dwarf planet Pluto is the largest of these TNOs, so named because they’re found beyond the orbit of Neptune.
The frigid temperatures and weak surface gravity of the small bodies has long caused astronomers to believe they aren’t capable of retaining atmospheres — with the exception of Pluto, which has a thin one. Atmospheres, especially dense ones, typically form around large planets or moons, including Saturn’s biggest satellite, Titan.
Meanwhile, dwarf planets Eris, Haumea, Makemake and dwarf planet candidate Quaoar, the largest TNOs after Pluto, don’t appear to have atmospheres.
During a rare observation opportunity, astronomers in Japan spotted the thin shell of an atmosphere around a TNO known as (612533) 2002 XV93, according to a study published Monday in the journal Nature Astronomy.
While Pluto has a diameter of 1,477 miles (2,377 kilometers), 2002 XV93 only spans about 311 miles (500 kilometers) across.
The unexpected discovery — made by Dr. Ko Arimatsu, associate professor and senior lecturer at the National Astronomical Observatory of Japan, and his colleagues — could offer an unprecedented glimpse into how an atmosphere forms and remains around a small object, and change how astronomers think about objects in the Kuiper Belt.
Seizing the observation opportunity
As January 2024 neared, Arimatsu and his colleagues prepared for the unique chance to observe a TNO as it passed in front of a bright star, as seen from Japan.
2002 XV93 has a standard orbit for a Kuiper Belt object and is smaller than a dwarf planet, so it wasn’t considered to be different from other TNOs.
But such moments when a TNO is illuminated by a star in the cosmic background, called stellar occultations, are rare opportunities to study the size, shape and features of a small, distant object, Arimatsu said. The researchers set up at three different locations across Japan, using observatories in Kyoto and the Nagano Prefecture, as well as a citizen scientist-run telescope in Fukushima.
The star’s light gradually faded as the TNO moved in front of it, suggesting the presence of of an atmosphere. If an object has no atmosphere, a star disappears and reappears much more sharply.
“The observation data showed a smooth change of the star’s brightness near the edge of the shadow, lasting about 1.5 seconds,” Arimatsu wrote in an email. “This kind of smooth brightness change is naturally explained if the starlight was bent by a very thin atmosphere around the object.”
The researchers calculated that 2002 XV93 has an atmosphere about 5 million to 10 million times thinner than Earth’s — and suspect two possibilities as to what created it.
The atmosphere could be the product of cryovolcanoes on the small, icy body, which release internal gas such as methane, nitrogen or carbon monoxide from beneath its surface. Or, another Kuiper Belt object such as a comet might have struck 2002 XV93, also releasing gases from the subsurface.
If the atmosphere was created due to impact, it may only last for several hundred years, Arimatsu said. But if regular cryovolcanic activity routinely replenishes the atmosphere with the release of gas, it could last much longer, he added.
Getting to know 2002 XV93
Future observations of 2002 XV93, either through more stellar occultation opportunities or by using the powerful James Webb Space Telescope, will help astronomers to better characterize the nature of the atmosphere and determine its origin, as well as how the atmosphere evolves over time.
“If the future occultation observations reveal a steady decrease in the pressure, it would suggest a short-timescale impact origin,” Arimatsu said.
The Webb telescope also could detect methane or carbon monoxide emissions coming from the object and identify the atmosphere’s composition.
Arimatsu’s team is continuing the search for atmospheres around other TNOs by relying on stellar occultation observations. Their findings could help determine if 2002 XV93 is a rare exception to the rule, or if other similar small objects also possess atmospheres.
“This was an exciting discovery to read about,” said Dr. Scott S. Sheppard, staff scientist at the Carnegie Institution for Science in Washington, DC. “It was thought that objects like 2002 XV93 would be too small to have an atmosphere, but this result shows that is not true.”
Sheppard did not participate in the research, but he has studied and discovered TNOs.
The finding also highlights the discovery of recent activity on 2002 XV93, Sheppard noted, 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,” Sheppard wrote in an email, “but is teeming with activity and has many of the building blocks for life.”
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