Second Trans-Neptunian Object with Atmosphere Discovered Beyond Pluto by Japanese Astronomers

Astronomers have long studied the Kuiper Belt, a distant region of minor planets beyond Neptune, where objects exhibit unusual gravitational interactions. Some researchers speculate the belt may harbor a hidden ninth planet. However, the extreme distance from the Sun makes these trans-Neptunian objects (TNOs) notoriously difficult to observe, leaving their properties shrouded in mystery.

Now, a team led by Ko Arimatsu, associate professor at the National Astronomical Observatory of Japan, has identified only the second TNO with an atmosphere after Pluto. Published in Nature Astronomy, their research describes a minor planet—2002 XV93—measuring just 311 miles across, wrapped in an extremely thin gas envelope. If confirmed, this discovery could reshape our understanding of how atmospheres form around small, distant objects and what other hidden neighbors may exist in the Kuiper Belt.

Discovery Through Stellar Occultation

The team observed 2002 XV93 as it passed in front of a bright star in January 2024, a rare event known as a stellar occultation. First spotted in 2002, the object lies billions of miles from Earth. Given its small size—typical for the Kuiper Belt—researchers expected it to resemble other similar TNOs. However, the starlight did not vanish abruptly but faded gradually over about 1.5 seconds, indicating the presence of an atmosphere.

“The observation data showed a smooth change of the star’s brightness near the edge of the shadow, lasting about 1.5 seconds. This kind of smooth brightness change is naturally explained if the starlight was bent by a very thin atmosphere around the object.” — Ko Arimatsu, National Astronomical Observatory of Japan

The atmosphere around 2002 XV93 is estimated to be five to ten million times thinner than Earth’s. Researchers propose two possible explanations for its existence: cryovolcanic activity, where eruptions of water, ammonia, or methane release gases, or a recent impact by a small icy object.

If the atmosphere resulted from an impact, it could dissipate within hundreds of years. If caused by cryovolcanism, it may persist much longer. The team plans to further study 2002 XV93 using NASA’s James Webb Space Telescope, which could detect additional gases like methane or carbon monoxide.

Implications for Solar System Science

This discovery challenges long-held assumptions about atmosphere formation on small, icy bodies. Previously, only Pluto was known to have a significant atmosphere among TNOs. The findings suggest that even tiny objects in the outer solar system may retain thin atmospheres, raising new questions about the prevalence of such features in the Kuiper Belt.

The research underscores the importance of advanced observational techniques, such as stellar occultations, in uncovering the secrets of distant celestial bodies. As astronomers continue to explore the outer solar system, discoveries like this one could redefine our understanding of planetary formation and the potential for habitable environments beyond Earth.