Japan-led team reports possible ultrathin atmosphere on distant Kuiper Belt plutino

Fresh analysis indicates a compact, ice-rich body orbiting beyond Pluto could be shrouded in an extremely tenuous gas layer, possibly vented by ice volcanism or unleashed during a comet collision.

Ko Arimatsu of the National Astronomical Observatory of Japan, who leads the work, said the target spans roughly 300 miles (500 kilometres). A firm detection would mark the least massive solar-system object yet shown to keep a planet-wide atmosphere gripped by its own gravity.

Alan Stern of the Southwest Research Institute, chief scientist on NASA’s New Horizons flight past Pluto, had no role in the paper. Stern said: "This is an amazing development, but it sorely needs independent verification. The implications are profound if verified,"

The measurement illuminates the remote, frigid Kuiper Belt. In 2024 researchers in Japan aimed three telescopes at the body as it crossed in front of a distant star, catching a short-lived dip in the star’s brightness.

In an email, Arimatsu said: "It changes our view of small worlds in the solar system, not only beyond Neptune," He said spotting air around so slight a world was "genuinely surprising", and runs counter to "the conventional view that atmospheres are limited to large planets, dwarf planets and some large moons".

Officially listed as minor planet (612533) 2002 XV93, the object is a plutino, circling the sun twice while Neptune completes three laps. During the campaign it lay more than 3.4 billion miles (5.5 billion kilometres) sunward—beyond Pluto’s range. Until now Pluto has been the sole Kuiper Belt resident whose atmosphere has been observed.

Appearing Monday in Nature Astronomy, the study models the envelope as five million to ten million times rarer than Earth’s air—about fifty to one hundred times more diffuse than Pluto’s.

Arimatsu said methane, nitrogen or carbon monoxide are the top chemical suspects; each could reproduce the dimming recorded when the body occulted the star.

He expects NASA’s Webb Space Telescope and additional monitoring to nail down the composition. Arimatsu said: "That is why future monitoring is so important," He went on: "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 towards ongoing internal gas supply" from ice volcanoes.