On the night of January 10, 2024, a faint star above Japan flickered for between 15 and 20 seconds. Astronomers across the country, both professionals and hobbyists, were watching carefully. They expected the star to disappear suddenly as a frozen rock four billion kilometers away passed in front of it.
Instead, the starlight dimmed gently and then brightened again. This soft, gradual fade lasting about 1.5 seconds on each side of the blackout showed that light was bending through a gas. The frozen rock had an atmosphere.
The rock is called 2002 XV93. It is roughly 500 kilometers across, smaller than a quarter of Pluto's diameter, and it should not have any atmosphere at all. This discovery, published in Nature Astronomy by a team led by Ko Arimatsu of the National Astronomical Observatory of Japan, is the first detection of an atmosphere on any object beyond Neptune except Pluto.
2002 XV93 sits about 40 astronomical units from the Sun, in extremely cold conditions where almost everything has frozen solid. Yet the atmosphere exerts a surface pressure of just 100 to 200 nanobars, between five and ten million times thinner than Earth's air. For a world this small, this atmosphere should disappear into space in less than a thousand years without constant supply.
Scientists suggest two possibilities: cryovolcanism, where icy worlds erupt frozen gases, or a recent collision with a comet that created temporary gas. However, the James Webb Space Telescope found no obvious frozen ice on the surface to explain this mystery. The outer solar system may be more active than previously thought.