A small, icy world lurking in the outer reaches of our solar system may be wrapped in a thin atmosphere—a finding that, if confirmed, would mark the first time such an envelope has been detected on an object of its size.
The discovery suggests that small bodies beyond Neptune are not the static, frozen relics scientists once assumed. Instead, they may host dynamic, changing environments.
Ko Arimatsu, an observational astronomer at Japan’s National Astronomical Observatory, and his colleagues published their findings on May 4 in Nature Astronomy. Until now, Pluto remained the only trans-Neptunian object with a confirmed atmosphere. This new observation expands the known diversity of atmospheric bodies in the Kuiper Belt and beyond.
How Telescopes Caught a Faint Signal
The team focused on 2002 XV93, a small solar system body located farther from the Sun than Pluto. On January 10, 2024, the object passed directly in front of a distant star—a phenomenon known as an occultation.
Using a network of telescopes across three locations in Japan, astronomers monitored the event with precision. If 2002 XV93 were a bare rock or ice ball, the star’s light would have vanished and reappeared instantly, like a switch being flipped.
However, the data told a different story. The star’s light faded and recovered gradually over approximately 1.5 seconds. This smooth dimming indicates that starlight passed through a medium that refracted the light before it reached Earth.
“This smooth dimming is best explained by starlight passing through and being refracted by a tenuous atmosphere,” Arimatsu explains.
The inferred atmosphere is incredibly faint, with a pressure roughly one ten-millionth that of Earth’s. Yet, its presence is significant given the object’s physical constraints.
Why This Discovery Defies Expectations
2002 XV93 is approximately 470 kilometers wide —roughly the length of the Grand Canyon. At this size, an object’s gravity is typically too weak to retain gases for long periods.
“I was genuinely surprised,” Arimatsu admits.
Theoretical models suggest that any atmosphere on such a small body should dissipate into space within thousands of years unless it is constantly replenished. This raises critical questions about the object’s activity and history. The presence of an atmosphere implies one of two main scenarios:
- A Recent Impact: A collision with a comet or other icy body may have recently kicked up dust and gas, creating a temporary atmosphere that the astronomers happened to observe at the perfect moment.
- Ongoing Geological Activity: The object may possess “icy volcanoes” (cryovolcanoes) that regularly release gas from its interior, sustaining the atmosphere over longer periods.
Unanswered Questions and Future Observations
While the occultation data strongly supports the existence of an atmosphere, it does not provide a complete picture. A single observation cannot entirely rule out alternative explanations, such as a cloud of dust rather than gas. Furthermore, the current data does not reveal the chemical composition of the atmosphere or its vertical extent above the surface.
Future observations will be crucial in distinguishing between the impact and volcanic hypotheses.
- If the atmosphere fades over the next few years, it likely resulted from a transient impact event.
- If it persists or varies with the object’s seasons, it points toward internal geological processes like cryovolcanism.
Conclusion
The potential atmosphere of 2002 XV93 challenges the long-held view that small icy worlds in the deep solar system are inert. Whether driven by recent collisions or internal heat, this finding highlights that even the smallest and most distant objects in our neighborhood can harbor complex, evolving environments.
