Astronomers have confirmed, for the first time, a trans-Neptunian object with a thin atmosphere—something previously thought exclusive to Pluto among similar objects.
Trans-Neptunian objects (TNOs) are small icy bodies orbiting the Sun beyond Neptune. In a Nature Astronomy paper published today, researchers led by Ko Arimatsu of the National Astronomical Observatory of Japan describe (612533) 2002 XV93, a TNO with a very thin atmosphere. Arimatsu explained that trans-Neptunian objects are so cold and distant that scientists have long assumed they’re largely unchanging and inactive, which made Pluto stand out among them.
“Our result shows that even a much smaller object, only about 500 kilometers (311 miles) across, can have an atmosphere, at least temporarily,” Arimatsu told Gizmodo. “Discoveries like this show that the outer Solar System may be more dynamic than we expected.” 2002 XV93, known as a Plutino-class object, orbits around 38 astronomical units from Earth—roughly 3.5 billion miles (5.6 billion kilometers)—putting it at a distance comparable to Pluto’s.
A star-crossed discovery
According to Arimatsu, the team did not directly image the atmosphere surrounding 2002 XV93. The discovery came from ground-based campaigns with contributions from both professional and amateur astronomers across four different observatories in Japan. Specifically, the team caught a stellar occultation, which is when a cosmic object in the solar system passes in front of a background star.
“If the object has no atmosphere, the starlight should disappear and reappear sharply,” Arimatsu explained. “If it has an atmosphere, the starlight can be bent by refraction and changes more gradually.”
When the team observed hints of the latter phenomenon, it turned to analyzing the resulting light curves, testing three simplified models of atmospheric composition against previous investigations, according to the paper. As a result, the researchers concluded that atmospheric refraction best reflected the data.
Unlikely worlds of ice and rocks
The findings challenge the conventional account of TNOs, which were generally thought to be too small to “retain an observable atmosphere” for extended periods of time, Arimatsu said. Indeed, according to the paper, “despite targeted searches using stellar occultations,” no sizeable TNO other than Pluto appeared to have a measurable atmosphere.
Planetary scientists are interested in finding these sorts of atmospheres because they “control how heat is transported, how surface ices evaporate or freeze, how material escapes to space, and how the surface evolves over time,” Arimatsu explained. Even if a TNO’s atmosphere is nowhere close to Earth’s, understanding its presence allows scientists to get a better grasp of how volatile materials such as methane, nitrogen, or carbon monoxide move in and out of the cosmic object, he added.
A shifting solar system
It’s still not clear if the object’s atmosphere is a temporary or permanent feature. That could be the subject of follow-up investigations on the origins of the atmosphere, he told Gizmodo. Is it coming from cryovolcanic activity from inside the TNO? Or did another small, icy body slam into the object?
“If the atmosphere were produced by an impact, it may fade over the next several years,” he speculated. “If it persists or varies seasonally, that would favor ongoing internal gas supply.”
It’s fascinating how small changes in starlight can reveal such significant features of tiny, faraway worlds, he added. Moving forward, the team plans to conduct similar multi-site occultation campaigns to see whether Pluto could have even more relatives.
“The outer Solar System is not simply a collection of frozen, unchanging worlds,” he concluded. “There is still a frontier in astronomy where creativity, timing, and collaboration can make a major discovery.”
Related article: NASA Chief Reignites Debate Over Pluto’s Planetary Status and Yes We’re Doing This Again
