Astronomers have long been fascinated by the unusual behavior of objects orbiting the Sun in the Kuiper Belt, a scattered disc of minor planets beyond the orbit of Neptune.

Many of these frozen objects appear to be affected by the gravitational influence of much more massive ones, with some astronomers controversially arguing that the region could be home to a hidden ninth major planet in the solar system.

However, the region’s extreme distance from the Sun makes it incredibly difficult to study, shrouding the nature of “trans-Neptunian objects” (TNOs) in enduring mystery.

Now, a team of astronomers in Japan says it’s spotted only the second TNO to have an atmosphere after the dwarf planet Pluto, the largest known TNO by volume. As detailed in a paper published in the journal Nature Astronomy this week, the researchers say they’ve identified a minor planet only about 311 miles across — roughly a fifth of Pluto’s diameter — that’s wrapped in an extremely thin envelope of gases.

If confirmed, the research could force us to reconsider how atmospheres can form around small objects, and what other mysterious neighbors we might expect to find in the Kuiper Belt.

The team of astronomers, led by National Astronomical Observatory of Japan associate professor and senior lecturer Ko Arimatsu, observed the TNO, dubbed 2002 XV93, as it passed in front of a bright star — rare events known as stellar occultations — in January 2024.

As its name suggests, the object was first discovered in 2002, and is located several billion miles away from Earth. But thanks to its diminutive size, typical for the Kuiper Belt, researchers expected it to closely resemble other similarly-sized TNOs.

However, as it passed in front of the distant background star, Arimatsu and his colleagues saw the starlight gradually fade away, suggesting the presence of an atmosphere since if it didn’t have one, the light would be cut off far more abruptly.

“The observation data showed a smooth change of the star’s brightness near the edge of the shadow, lasting about 1.5 seconds,” he told CNN. “This kind of smooth brightness change is naturally explained if the starlight was bent by a very thin atmosphere around the object.”

It’s certainly not nearly as thick as the Earth’s atmosphere, and is anywhere from five million to ten million times thinner.

In their paper, the researchers suggest that “cryovolcanic activity” — the eruption of water, ammonia or methane instead of molten rock on the surface of icy moons or dwarf planets — or the “recent impact of a small icy object” could help explain the thin atmosphere around 2002 XV93.

In case of the latter, Arimatsu said the atmosphere could dissipate within hundreds of years. In case of the former, such a thin atmosphere could stick around for much longer.

The researchers are now excited to have another chance to observe the intriguing TNO using NASA’s James Webb Space Telescope, which could reveal the presence of other gases, including methane or carbon monoxide.

“That is why future monitoring is so important,” Arimatsu told NBC News. “If the atmosphere fades over the next several years, that would support an impact origin. If it persists, or varies seasonally, that would point more toward ongoing internal gas supply” from cryovolcanic activity.

More on TNOs: Scientists Detect Mysterious Object in Deep Solar System