PARIS: A tiny, little-known world beyond Pluto appears to have an atmosphere, Japanese astronomers said Monday, defying what had been thought possible for icy objects in our cosmic backyard.

If confirmed, the roughly 500-kilometre-wide (310-mile) rock would become just the second world past Neptune in our Solar System to host an atmosphere -- after only Pluto itself.

Formerly classified as a planet, Pluto was demoted to dwarf planet status in 2006, in part because astronomers were discovering other similar objects in a distant region called the Kuiper Belt.

While NASA under US President Donald Trump has floated the idea of restoring Pluto's planet status, the discovery of another atmosphere nearby could undermine the argument for its reinstatement.

For the new discovery, Japanese researchers and an amateur astronomer pointed their telescopes at an object with the unwieldy name of (612533) 2002 XV93.

The icy world is nearly 40 times further from the Sun than Earth -- or roughly six billion kilometers away.

These dark objects can only be seen when they pass in front of a distant star.

When this happened in January 2024, the astronomers observed that the starlight did not immediately reappear, suggesting a thin atmosphere was filtering some of the light.

They estimate that the world has an atmosphere five to 10 million times thinner than Earth's, according to a new study in Nature Astronomy.

"This is important because, until now, Pluto was the only trans-Neptunian object with a confirmed atmosphere," lead study author Ko Arimatsu of the National Astronomical Observatory of Japan told AFP.

Such small worlds had not been thought capable of hosting an atmosphere.

"This discovery therefore challenges the conventional view that small icy worlds in the outer Solar System are mostly inactive and unchanging," he added.

Ice volcanoes or comet smash?

The researchers could not say for sure what created the atmosphere, which is nowhere near thick enough to support life.

But they suggested it could have been made by gas being shot out from the world's interior by erupting ice volcanoes.

Or it could have been kicked up by a comet smashing into the world, which would mean it will gradually disappear.

Jose-Luis Ortiz, a Spanish astronomer not involved in the research who studies dwarf planets beyond Neptune, said the results were interesting, but urged caution.

"I still doubt that it is an atmosphere. We need more data," he told AFP.

An alternative explanation for the observations could be that the object has a ring close to its body, Ortiz said.

Ko Arimatsu acknowledged that he could not rule out "exotic alternatives" to an atmosphere.

However, "a nearly edge-on ring does not seem consistent with the main features of our observations," he added.

Both astronomers called for further observations to reveal more about this strange world -- particularly with the James Webb space telescope.

There have also been suggestions that a dwarf planet called Makemake, which is slightly smaller than Pluto, could have a very thin atmosphere, though some scientists are sceptical.

Last week, NASA administrator Jared Isaacman suggested the US space agency was considering reinstating Pluto as a fully-fledged planet.

Isaacman made the comments while endorsing a proposal to halve NASA's science budget, riling some astronomers, Nature reported.

"It's wild to 'make Pluto a planet again' while decimating the careers of those of us that study it!" planetary scientist Adeene Denton wrote on Bluesky.

Pluto

Mysterious world beyond Pluto may have an atmosphere: astronomers