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Ko Arimatsu from the National Astronomical Observatory of Japan (NAOJ) detected signs of an atmosphere around the trans-Neptunian object (612533) 2002 XV93, a 500 km body orbiting the Sun at 6 billion km, with a pressure of 100 to 200 nanobars, in a study published in Nature Astronomy on May 4, 2026.
Japanese astronomers have for the first time identified signs of an atmosphere around a small icy body orbiting the Sun far beyond Neptune, a discovery that challenges what science considered possible for objects of this scale. The body, designated (612533) 2002 XV93 and only 500 km in diameter, passed in front of a distant star on January 10, 2024, in an event known as a stellar occultation, and analysis of how the light “faded” gradually instead of disappearing abruptly supports the hypothesis that a tenuous atmosphere surrounds the object, according to a study led by Ko Arimatsu of the National Astronomical Observatory of Japan (NAOJ) and published on May 4, 2026, in the journal Nature Astronomy. “This is a surprising development, but one that urgently needs independent verification,” Arimatsu declared in an interview with the Associated Press agency at the time of publication, a caveat that differentiates rigorous scientific discovery from a sensationalist headline.
The pressure of the atmosphere detected around the object is estimated between 100 and 200 nanobars, a value that seems insignificant but is surprising for a body with such weak gravity. Until now, Pluto was the only trans-Neptunian object with a detected global atmosphere, and its atmospheric pressure of approximately 10 microbars (10,000 nanobars) is 50 to 100 times greater than that measured on 2002 XV93, while Earth’s atmosphere at sea level has 1 billion nanobars, meaning it is 5 to 10 million times denser than the atmosphere Japanese astronomers identified 6 billion kilometers from the Sun. The discovery implies that other small objects in the Kuiper Belt may have atmospheres that science did not look for because they were believed to be impossible.
How astronomers detected the atmosphere of an object invisible to telescopes
The technique that allowed the detection of the atmosphere bypasses the impossibility of directly observing such dark and distant objects. A stellar occultation occurs when a Solar System body passes exactly in front of a distant star from the perspective of an observer on Earth, and if the object has no atmosphere, the star’s light disappears abruptly when it passes and reappears abruptly when it exits, whereas the presence of an atmosphere produces gradual dimming and reappearance as the light passes through layers of gas around the body. It was precisely this gradual pattern that telescopes recorded during the 1.5 seconds that 2002 XV93 blocked the star’s light on January 10, 2024.
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The observation was coordinated from multiple points in Japan, including the Kiso Schmidt telescope in Nagano prefecture, observatories in Kyoto, and a telescope operated by citizen scientists in Fukushima. The participation of amateur astronomers in the discovery published in Nature Astronomy is a detail that humanizes the research and demonstrates that dedicated observers with modest equipment can contribute to cutting-edge science when coordinated with professional institutions like NAOJ. In at least two observation sites, the records showed a progressive decrease in light consistent with the presence of an atmosphere around the object, a pattern that would be statistically improbable as an instrumental artifact.
What is 2002 XV93 and why did no one expect to find an atmosphere on it
The object that surprised the scientific community with signs of an atmosphere belongs to a specific class of icy bodies on the fringes of the Solar System. Classified as a plutino, a category of trans-Neptunian objects whose orbit is resonant with Neptune (completing 2 orbits around the Sun for every 3 orbits of Neptune), 2002 XV93 orbits approximately 38 to 40 astronomical units from the Sun, takes 247 years to complete one orbit, and formed over 4.5 billion years ago, at the beginning of the Solar System. Its diameter of 500 km is about one-fifth that of Pluto (2,377 km) and significantly smaller than that of Eris (2,326 km), the two largest known trans-Neptunian objects.
Science considered it unlikely that such a small body would retain an atmosphere because the gravity on its surface is too weak to prevent gases from escaping into space. The general rule in planetology is that the smaller the body, the less its ability to retain an atmosphere, and objects below one thousand kilometers in diameter in the outer Solar System were treated as inert icy rocks with no possibility of maintaining a stable gaseous envelope. Previous studies on other large trans-Neptunian objects had established upper limits of only 1 to 100 nanobars for a possible atmosphere, which makes the 100 to 200 nanobars detected on 2002 XV93 a surprisingly high value for a body of this category.
What are the two hypotheses that explain the detected atmosphere
Researchers propose two explanations for the presence of an atmosphere around such a small and distant object. The first hypothesis suggests internal release of gases by processes similar to cryovolcanism, a type of icy volcanism in which internal processes of the object release volatiles that form the atmosphere, a scenario in which atmospheric pressure should remain relatively stable or vary seasonally according to the object’s position in its 247-year orbit. The second hypothesis proposes that a comet or another small body recently collided with 2002 XV93, releasing surface gases that formed the temporary atmosphere.
Data from the James Webb Space Telescope helps differentiate the two hypotheses. Previous observations with James Webb did not detect frozen gases on the surface of 2002 XV93, a result that weakens the continuous evaporation hypothesis (if there were a permanent source, the ice would be visible) and strengthens the possibility of a recent impact as the origin of the atmosphere. If the impact hypothesis is correct, models suggest that the atmosphere may dissipate within 100 to 1,000 years without replenishment, and future observations in the next 5 to 10 years showing a constant decrease in pressure would confirm this interpretation. If the pressure remains stable, 2002 XV93 would be a geologically active body hidden on the farthest fringes of the Solar System.
Why the discovery of the atmosphere matters for understanding the Solar System
The detection of an atmosphere on 2002 XV93 forces science to reconsider what it thought about the Kuiper Belt and its inhabitants. If a 500 km body can maintain an atmosphere, even if tenuous and possibly temporary, other similarly sized objects may have atmospheres that telescopes never looked for because current hypotheses dismissed them as impossible. The study published in Nature Astronomy itself highlights that “the traditional idea that dense global atmospheres form only on larger planets needs to be revised,” a conclusion that expands the universe of potentially interesting worlds for future exploration.
Next steps include independent verification and continued observation. Arimatsu and his team plan to monitor 2002 XV93 in the coming years through new stellar occultations and direct observation with James Webb, work that will determine whether the atmosphere is a permanent phenomenon indicating internal geological activity or a transient trace of a recent impact that will disappear within a few human generations. In either scenario, the discovery has already altered the map of what science considers possible in the coldest and most remote regions of our planetary system, where the temperature is close to absolute zero and sunlight arrives a thousand times weaker than on Earth.
And you, do you think it’s possible that other small objects beyond Neptune also have an atmosphere? Does this discovery change what we think about the Solar System? Leave your opinion in the comments.
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