Study: Possible Thin Atmosphere Detected Around Small Trans‑Neptunian Object 2002 XV93

Astronomers say they have detected the first atmosphere around a trans-Neptunian object other than Pluto, reporting a thin gaseous envelope around the distant icy body (612533) 2002 XV93 in a peer-reviewed paper published May 4 in Nature Astronomy.

If confirmed, the finding would challenge long-standing ideas about which worlds in the outer solar system can hold onto atmospheres. But the claim also comes with an important caveat: It is based on a single stellar occultation, a one-time alignment in which the object passed in front of a background star, so independent follow-up will be crucial.

The study, led by Ko Arimatsu of the National Astronomical Observatory of Japan and colleagues at other Japanese institutions, focused on 2002 XV93, a plutino — a class of objects locked in a 2:3 orbital resonance with Neptune, like Pluto. During an occultation on Jan. 10, 2024, observers at multiple sites in Japan watched the star’s light dim as the object crossed in front of it. Positive detections came from Kyoto and Kiso, and a third site in Fukushima recorded a possible gradual drop.

What stood out was not simply that the star disappeared, but how. Instead of the abrupt blink expected if an airless solid body blocked the light, the star faded and recovered more gradually. The researchers interpret that signature as atmospheric refraction, with a tenuous atmosphere bending the starlight. From that signal, they estimated a surface pressure of about 100 to 200 nanobars, or 0.1 to 0.2 microbar.

That is an extraordinarily thin atmosphere by Earth standards, and about 100 times thinner than Pluto’s, according to the paper. Even so, the result is striking because 2002 XV93 is relatively small, about 250 kilometers in radius, or roughly 500 kilometers across.

Stellar occultations are one of the best ways to probe distant small bodies that are too far away and faint to study in detail directly. When an object with no atmosphere passes in front of a star, the light curve typically drops and returns sharply. An atmosphere can smooth out that change because gas around the object refracts the starlight before the solid body fully blocks it.

Until now, Pluto had been the only trans-Neptunian object known to have a substantial atmosphere. Previous occultation studies of larger outer solar system bodies including Eris, Haumea, Makemake and Quaoar found no detectable atmospheres, instead setting upper limits typically in the 1 to 100 nanobar range. That makes the new claim especially provocative: a smaller body appears to show what bigger ones did not.

The authors say that outcome strains existing volatile-retention models, which generally suggested that only the largest and, in some cases, warmest trans-Neptunian objects could preserve atmospheres over time. To explain the apparent gas around 2002 XV93, they suggest it may be transient, perhaps released by ongoing cryovolcanic activity or by a recent impact from a small icy object. Japanese press material accompanying the work said such an atmosphere might survive less than 1,000 years unless it is replenished.

The team also said the raw observations, reduced light curves and model profiles have been deposited on Zenodo, making the result publicly documented and reproducible in principle.

Arimatsu told The Associated Press that “it changes our view of small worlds in the solar system, not only beyond Neptune.” But even supporters stress caution. Alan Stern, the planetary scientist who led NASA’s New Horizons mission to Pluto, told the AP: “This is an amazing development, but it sorely needs independent verification. The implications are profound if verified.”