An icy world, perhaps just 300 miles wide and far beyond Pluto, may have air, challenging the assumption that small space objects can't hold onto an atmosphere.

This world, known as 2002 XV93, orbits in a distant region sprinkled with leftover rubble from our solar system's early planet-building days. Astronomers often study this ancient detritus, referred to as Trans-Neptunian Objects, by waiting for remnants to pass in front of stars. During rare alignments, researchers can measure how the starlight changes to learn about an object's size and features, such as rings or debris.

What scientists saw instead for this object was startling. On Jan. 10, 2024, the little object eclipsed a faint star — a star so anonymous it doesn't even have a common name. But that event, seen from East Asia, revealed thin air on 2002 XV93.

Typically, if an object has no atmosphere, the background starlight disappears abruptly as the object blocks it. Instead, telescopes recorded a gradual fading out and in of the light at the beginning and end of the event — a sign that an atmosphere had bent or scattered it.

"Our findings suggest that a fraction of distant icy minor planets can exhibit atmospheres," the researchers wrote in a new paper published in Nature Astronomy. 

The researchers estimated that an extremely thin atmosphere, with a surface pressure between about 100 and 200 nanobars, surrounds the object. That level is far thinner than Pluto's atmosphere but stronger than anything previously measured around similar small bodies in the outer solar system.

In a post on X, lead author Ko Arimatsu said this is the first detection of an atmosphere for a Trans-Neptunian Object beyond Pluto. It's not known yet what gases are in the world's air, but methane, nitrogen, or carbon monoxide are the team's best guesses, based on their observations.

The discovery shocked scientists because astronomical models indicate small, distant objects should not be able to maintain an atmosphere — at least not for very long. Under weak gravity, gases tend to escape into space, meaning any atmosphere would likely dissipate within a few hundred to a few thousand years.

That may suggest the researchers caught a temporary atmosphere for the object at just the right time. 

The authors proffer a few scenarios for how 2002 XV93's atmosphere could have formed. One possibility is that a comet or asteroid could have crashed into the world recently, releasing trapped gases. Another possibility is that cold volcanoes could have vented gases instead of lava, or internal heat or chemistry could have pushed gases to the surface. 

If the air fades over time, it likely came from a collision. If it continues or changes in cycles, that may mean internal mechanisms are responsible. The team favors the former explanation over the latter.

Scientists say they'll need to study the object more to solve the mystery. They already have James Webb Space Telescope observations of the target that show no indication of frozen material that could have turned into gas.