By BlueShift | 2026-05-08

A tiny, little-known "plutino" beyond Pluto appears to have an atmosphere, Japanese astronomers revealed in a study published May 4 in the Nature Astronomy journal.

If confirmed, the plutino, which is about one-seventh the diameter of Earth's moon and about one-fifth the diameter of Pluto, would become just the second world past Neptune in our solar system to host an atmosphere, after Pluto.

Discovered in December 2002 by the Palomar Observatory's Samuel Oschin telescope, in the US state of California, the trans-Neptunian object (TNO) has been designated (612533) 2002 XV93.

TNOs are minor bodies in the solar system that orbit the Sun at a greater average distance than Neptune; while a plutino is a type of TNO in the Kuiper Belt in a 2:3 orbital resonance with Neptune.

These bodies, which complete two orbits for every three of Neptune's, have a similar orbital rhythm to Pluto and account for about a fourth of known objects in the Kuiper Belt, a region in the outer solar system extending beyond Neptune.

The icy plutino is less than 500km in diameter, and nearly 40 times further from the Sun than Earth, a distance of roughly six billion kilometers.

Its surface contains high amounts of water ice and some frozen carbon dioxide, but no detectable signs of frozen methane, nitrogen, or carbon monoxide, according to the researchers.

Its mass, density, shape and rotation is unknown.

For the new discovery, the researchers and an amateur astronomer pointed their telescopes at (612533) 2002 XV93 from several locations in Japan. They also drew on data from the James Webb Space Telescope.

Dark objects like this can only be seen when they pass in front of a distant star.

When this occultation 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 plutino has an atmosphere five to 10 million times thinner than Earth's.

"Our findings suggest that a fraction of distant icy minor planets can exhibit atmospheres, potentially sustained by ongoing cryovolcanic activity or produced by a recent impact of a small icy object," according to the study's abstract.

"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.

Spanish astronomer Jose-Luis Ortiz, who studies dwarf planets beyond Neptune and is not involved in the research, 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 he could not rule out "exotic alternatives" to an atmosphere, but added that "a nearly edge-on ring does not seem consistent with the main features of our observations."

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

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