TOKYO - In a discovery that challenges long-held assumptions about the outer solar system, astronomers have detected a thin, delicate atmosphere surrounding a tiny icy world beyond Pluto - marking only the second time such a phenomenon has been observed on a trans-Neptunian object .
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The object, formally designated (612533) 2002 XV93, is a plutino approximately 500 kilometers (310 miles) in diameter - less than a quarter the size of Pluto. It resides in the Kuiper Belt, a vast region of icy bodies beyond Neptune, at a distance of roughly 3.4 billion miles (5.5 billion kilometers) from the Sun .
How Scientists Made the Discovery
The breakthrough came in January 2024, when the object passed in front of a distant star - an event known as a stellar occultation. Using three telescopes across Japan, including one operated by an amateur astronomer, researchers observed that the star's light did not disappear abruptly as expected when the solid surface of the object intervened .
Instead, the starlight faded and reappeared gradually over approximately 1.5 seconds - a smooth brightness change consistent with starlight being bent (refracted) by a thin atmospheric layer .
"These observations are consistent with the presence of a thin global atmosphere," said lead researcher Ko Arimatsu of the National Astronomical Observatory of Japan, whose findings appear in the journal Nature Astronomy .
The Atmosphere: Extremely Thin but Significant
The detected atmosphere is extraordinarily tenuous:
Surface pressure: 100-200 nanobars - approximately 5 to 10 million times thinner than Earth's atmosphere
Comparison to Pluto: Roughly 50 to 100 times thinner than Pluto's already wispy atmosphere
Likely composition: Methane, nitrogen, or carbon monoxide - any of which could reproduce the observed dimming
So small an object was not previously believed capable of holding onto an atmosphere, given its weak gravity and the extreme cold of the outer solar system, where most gases freeze solid .
Two Possible Origins: Ice Volcanoes or a Recent Collision
The atmosphere's existence presents a mystery: without ongoing replenishment, 2002 XV93 would lose its atmospheric gases within roughly 1,000 years . Researchers have proposed two competing explanations :
Cryovolcanism (Ongoing Activity): The atmosphere may be sustained by ice volcanoes - eruptions of volatile gases such as methane or nitrogen seeping from the object's interior through cracks in its icy crust .
Impact Origin (Temporary): Alternatively, a relatively recent collision with another Kuiper Belt object or comet could have released subsurface gases, creating a temporary atmosphere that will gradually dissipate .
The two scenarios could be distinguished through continued observation: "If the atmosphere fades over the next several years, that would support an impact origin. If it persists or varies seasonally, that would point more toward ongoing internal gas supply," Arimatsu explained .
Expert Reactions and Skepticism
The scientific community has responded with both excitement and caution.
Alan Stern, lead scientist of NASA's New Horizons mission to Pluto (who was not involved in the study), called it "an amazing development, but it sorely needs independent verification. The implications are profound if verified" .
David Jewitt of UCLA found the team's methodology convincing: "The occultation feature is symmetric, like an atmosphere, too well determined to be due to measurement noise and too close to the parent body to be due to a ring. Refraction by a near-surface gas envelope is pretty much the remaining explanation" .
However, Jose-Luis Ortiz, a Spanish astronomer studying dwarf planets beyond Neptune, urged caution: "I still doubt that it is an atmosphere. We need more data" . He suggested the observations could potentially be explained by a ring close to the object's body, though Arimatsu noted that "a nearly edge-on ring does not seem consistent with the main features of our observations" .
Implications and What's Next
The discovery fundamentally alters our understanding of the outer solar system. Until now, Pluto was the only known trans-Neptunian object with a confirmed atmosphere -larger bodies such as Eris, Haumea, Makemake, and Quaoar have shown no detectable atmospheric signatures .
"It changes our view of small worlds in the solar system, not only beyond Neptune," Arimatsu said. Finding an atmosphere around such a small object was "genuinely surprising" and challenges "the conventional view that atmospheres are limited to large planets, dwarf planets and some large moons" .
The team hopes to conduct follow-up observations, particularly using the James Webb Space Telescope, to verify the atmosphere's composition and determine its origin . Further stellar occultation campaigns could also reveal whether other trans-Neptunian objects harbor similar atmospheric surprises.
"This suggests that even in a distant, cold world, there are dynamisms we haven't imagined," said study co-author Junichi Watanabe .
