Scientists have identified what may be the smallest world in the solar system with its own atmosphere, a mere 300-mile-wide body at the frozen edge of the Kuiper Belt. The finding, reported Monday in Nature Astronomy, could upend decades of planetary science and expand the known diversity of worlds beyond Neptune, researchers said.
The object, formally known as (612533) 2002 XV93, is a plutino — a class of minor planets that orbit the sun twice for every three orbits of Neptune. At the time of the study, it lay more than 3.4 billion miles from Earth, farther even than Pluto, the only other Kuiper Belt body known to possess an atmosphere.
The atmosphere is incredibly tenuous: it is 5 million to 10 million times thinner than Earth’s, and 50 to 100 times thinner than Pluto’s. Yet its presence is “genuinely surprising,” said lead author Ko Arimatsu, an astronomer at the National Astronomical Observatory of Japan.
“It changes our view of small worlds in the solar system, not just beyond Neptune,” Arimatsu wrote in an email. “Finding an atmosphere around such a small object challenges the conventional view that atmospheres are limited to large planets, dwarf planets and some large moons.”
The detection was made in 2024 using three telescopes in Japan. The team observed the tiny world as it transited, or passed in front of, a distant star, causing a brief dimming of starlight. The nature of that dimming — its duration and the way the starlight faded and recovered — suggested the presence of an atmosphere.
Arimatsu said the most likely chemical components are methane, nitrogen or carbon monoxide, any of which could produce the observed occultation signal. But determining the exact composition will require further observations, ideally with NASA’s James Webb Space Telescope.
Alan Stern, principal investigator of the New Horizons mission that flew by Pluto in 2015 and has since explored farther into the Kuiper Belt, called the finding “an amazing advance” but emphasized caution.
“This urgently needs independent verification,” Stern said. “The implications are profound if confirmed.”
Stern was not involved in the study.
Arimatsu plans to monitor the object over the coming years. If the atmosphere fades, it would suggest it was produced by a comet impact — a transient event. If it persists or varies seasonally, it would point to an ongoing internal supply, perhaps from icy volcanoes.
The discovery opens a new window into the outer solar system’s smallest residents, underscoring how much remains unknown about the cold, dark frontier beyond Neptune.