Astronomers say a distant world beyond Pluto may have an atmosphere

TOKYO: A small and little-studied object beyond Pluto appears to possess an atmosphere, Japanese astronomers said on Monday, in a finding that challenges long-held assumptions about icy bodies in the outer reaches of the Solar System.

If the result is confirmed, the object — estimated to be about 500 kilometres wide — would become only the second known world beyond Neptune in the Solar System with an atmosphere, after Pluto.

Pluto, once classified as a planet, was reclassified as a dwarf planet in 2006, partly because astronomers were identifying other comparable objects in the distant Kuiper Belt. Nasa under US President Donald Trump has floated the possibility of restoring Pluto’s planetary status, but the identification of another nearby atmosphere could weaken that case.

The latest finding centres on an object known as (612533) 2002 XV93. Japanese researchers, working with an amateur astronomer, observed the icy body as it passed in front of a distant star. The object lies nearly 40 times farther from the Sun than Earth, or around six billion kilometres away.

During the January 2024 event, the astronomers found that the starlight did not return immediately after the object moved past, which they said suggested that a thin atmosphere had filtered part of the light. In a study published in Nature Astronomy, they estimated that the atmosphere is around five to 10 million times thinner than Earth’s.

Lead study author Ko Arimatsu of the National Astronomical Observatory of Japan said the finding was significant because Pluto had until now been the only trans-Neptunian object with a confirmed atmosphere.

“This is important because, until now, Pluto was the only trans-Neptunian object with a confirmed atmosphere,” Arimatsu told AFP that such small worlds had not been expected to retain 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.

Possible origins and calls for caution

The researchers said they could not determine with certainty how the atmosphere formed. They suggested one possibility was gas being expelled from the object’s interior through erupting ice volcanoes. Another was that material had been thrown up by a comet impact, in which case the atmosphere could gradually fade away.

Jose-Luis Ortiz, a Spanish astronomer who was not involved in the research and studies dwarf planets beyond Neptune, described the results as interesting but said more evidence was needed.

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

Ortiz said another possible explanation was that the object might have a ring close to its body. Arimatsu said he could not exclude what he called “exotic alternatives” to an atmosphere, but added that a nearly edge-on ring did not appear to match the main features of the observations.

Both astronomers said further observations would be needed to better understand the object, particularly using the James Webb space telescope.

The report also noted that there have been suggestions that Makemake, a dwarf planet slightly smaller than Pluto, may have a very thin atmosphere, although some scientists remain doubtful.

Last week, Nasa administrator Jared Isaacman said the US space agency was considering restoring Pluto as a full planet. Nature reported that Isaacman made the remarks while backing a proposal to cut Nasa’s science budget by half, drawing criticism from some astronomers.

“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.