A group of professional and amateur astronomers in Japan has uncovered evidence that a small, distant object in the outer Solar System is surrounded by a thin atmosphere. The finding is surprising because the object is far too small to hold onto gas for long, raising new questions about how and when this atmosphere formed. Future observations will be needed to better understand its composition and origin.
Far beyond Neptune's orbit, thousands of icy bodies known as trans-Neptunian objects (TNOs) circle the Sun. Pluto is the most well-known example and is one of the few with a confirmed thin atmosphere. For most TNOs, however, the combination of extremely low temperatures and weak gravity makes it unlikely for them to retain any gases. As a result, scientists generally expect these distant objects to be airless.
A Rare Stellar Alignment Reveals Clues
Astronomers took advantage of a rare opportunity to test that assumption using a TNO called (612533) 2002 XV93. This object, commonly shortened to 2002 XV93, measures about 500 km across, much smaller than Pluto, which spans 2,377 km.
On January 10, 2024, 2002 XV93 passed directly in front of a background star as viewed from Japan. Events like this, known as stellar occultations, allow scientists to study distant objects in detail. If the object has no atmosphere, the star's light should disappear abruptly as it is blocked. If an atmosphere is present, the starlight fades more gradually as it passes through surrounding gas.
Observations Suggest a Thin Atmosphere
A research team led by Ko Arimatsu at NAOJ Ishigakijima Astronomical Observatory observed this event from multiple locations across Japan. Their measurements showed a pattern consistent with the gradual dimming expected when light passes through a thin atmosphere. These results strongly suggest that 2002 XV93 is surrounded by a layer of gas.
A Short-Lived and Mysterious Origin
Further analysis indicates that this atmosphere should not last long. Calculations suggest it would dissipate in less than 1000 years unless it is continuously replenished. This means the atmosphere must have formed or been renewed relatively recently.
Data from the James Webb Space Telescope provide another piece of the puzzle. Observations show no evidence of frozen gases on the surface that could slowly turn into vapor and sustain an atmosphere. This leaves scientists considering other explanations. One possibility is that material from inside the object was brought to the surface, releasing gas. Another is that a comet impact delivered or released enough material to create a temporary atmosphere.
More observations will be needed to determine which scenario is correct and to better understand how such a small, distant object can briefly hold onto an atmosphere at all.
