Surprising Signs of an Atmosphere Around a Tiny World, Billions of Miles Away

Astronomers have discovered evidence of an atmosphere surrounding 2002 XV93, a small icy world roughly 300 miles wide located 3.5 billion miles from the sun—nearly as distant as Pluto. The findings, published in *Nature Astronomy* by Ko Arimatsu of Japan’s National Astronomical Observatory and his team, challenge expectations, as such a tiny object’s weak gravity should have long since lost any atmospheric gases to space. The discovery stems from observations made on January 10, 2024, when 2002 XV93 passed in front of a distant star, causing a gradual dimming and brightening rather than an abrupt cutoff. Telescopes in Kyoto and Kiso, Japan, recorded the event, noting a 1.5-second delay in the star’s disappearance and reappearance—a pattern consistent with light scattering through an atmosphere. A third telescope in Fukushima detected partial dimming, indicating the star passed behind the atmosphere but not the solid body of the object. The atmosphere is extremely thin, with pressure estimated at just one to two 10-millionths of Earth’s. Given the frigid temperatures in the outer solar system, most gases would freeze solid, yet the presence of this tenuous layer remains unexplained. Michael Brown, a planetary astronomer at Caltech who was not involved in the study, called the finding ‘super weird,’ though he acknowledged the data’s convincing nature. 2002 XV93 is one of thousands of trans-Neptunian objects (TNOs) orbiting beyond Neptune, many discovered since the 1990s. Like Pluto, it is locked in a 3:2 orbital resonance with Neptune, meaning it completes three orbits for every two of the planet’s. The object’s existence and now its atmosphere raise new questions about the dynamics of distant solar system bodies and the conditions that might allow such features to persist in extreme environments.