The discovery of an atmosphere on a tiny Kuiper...

In science, no matter how confident we are in our theories, there’s no substitute for interrogating the natural world by asking it questions about itself directly: through observation and experiment. Sometimes, that requires setting up conditions in a laboratory to create certain events whose outcomes we can measure to whatever precision we desire. At other times, however, it requires looking out into space — at the natural laboratory of the Universe — to observe how nature behaves. No matter what our expectations were beforehand, there’s no substitute for actual data in figuring out how things actually are. Although Pluto was the first object ever discovered out beyond Neptune, spotted way back in 1930 for the very first time, its atmosphere was only directly discovered and measured in 1988: when an observatory in Earth’s southern hemisphere observed it occulting, or passing in front of, a background star. While many other Kuiper belt occultations have occurred since, only Pluto had ever been shown to have an atmosphere, rendering it unique among the known trans-Neptunian objects. Until now. In an all new 2026 study led by Ko Arimatsu, one of the smaller Kuiper belt objects known, 2002 XV93, was precisely observed from three separate locations during a 2024 stellar occultation, and was determined to have an atmosphere, after all. This makes it the second known Kuiper belt object, after Pluto, to have an atmosphere. Here’s how we figured it out, and what it just might mean for the fields of astronomy and planetary science. When we think about the bodies in our Solar System, what we view today is the end result of a 4.5+ billion year tale of survival. Early on, when our planets, moons, asteroids, and Kuiper belt objects were forming out of the protoplanetary material that surrounded our young Sun, there were many volatiles present: molecules that would eventually be vaporized, sublimated, or otherwise evaporated away by too much energetic radiation, such as direct sunlight. Around the planets and moons of the inner Solar System, volatiles are rare, with materials such as: - hydrogen ice, - helium gas, - nitrogen ice, - methane ice, - carbon monoxide ice, - carbon dioxide ice, - and even water ice, only rarely found. They’re most abundant in permanently shadowed craters, deep beneath a planetary surface, or — in the case of water ice — as part of a temporary configuration on a world with liquid and gas-phase water as well. The farther out in the Solar System you go, farther away from the Sun, the longer these volatiles can persist. In the case of Pluto, the largest body presently known in the Kuiper belt, the New Horizons flyby in 2015 showed a surface rich in many of these different ice species, an atmosphere rich in clouds and hazes containing these volatile molecules, and even snows where these ices precipitate. The atmosphere was thin but substantial, with a pressure of around 10 microbars (about 1 Pa), and was thought to be generated by the routine sublimation of those surface ices as Pluto regularly reaches perihelion, bringing it closer to the Sun than Neptune. In the many years since the first discovery of Pluto’s atmosphere, an enormous number of objects from out beyond Neptune — primarily, but not exclusively, populating the Kuiper belt — have been discovered. Starting in the 1990s and continuing into the 21st century, these worlds are actually quite abundant, with many of them ranging from hundreds of kilometers up to a thousand or more kilometers in size. Many of them are now classified as dwarf planets, and it’s theoretically possible that many of the smaller objects just a few hundred kilometers in size, dominated by ices, have already pulled themselves into hydrostatic equilibrium as well. One such object is was discovered in 2002: 2002 XV93, which makes two revolutions around the Sun for every three revolutions that Neptune makes. After its 2002 discovery, astronomers searched for what we call “precovery” observations: where historical data is mined to look for any other images of this object, now that we know it exists and where it should have been. Observations dating back as far as October of 1990 were found to contain this object, allowing astronomers to reconstruct its orbit and determine a great number of its orbital properties quite precisely: its perihelion, its semimajor axis, and its eccentricity among them. There are many objects known that are extremely similar to 2002 XV93: they form a class known as plutinos. Pluto, Charon, Orcus, Achlys, and Ixion are the largest known plutinos, and hence they all have names related to mythological creatures associated with the underworld. 2002 XV93, being smaller and having no other known remarkable properties, hasn’t received any such designation: at least, not yet. In fact, prior to this latest study, the most remarkable thing that happened to 2002 XV93 was that it was observed spectroscopically by JWST back in 2022, allowing us to acquire a series of spectra of ice-rich plutinos, including this one. Those JWST observations, which also looked several other Kuiper belt objects, including numerous plutinos (at right, below), revealed crystalline water ice at 3.1 microns, which provides strong indirect present for amorphous water ice on those bodies (with dips at 1.5 and 2.0 microns), as well as carbon dioxide ices (with a dip at 4.27 microns) on 2002 XV93. However, there were no signs of what one might call “hyper-volatile” compounds that can sublimate into the gas phase at temperatures typically achieved at these roughly 40 AU distances: methane, nitrogen, or carbon monoxide. These JWST observations painted a very similar picture for 2002 XV93 as for other similar plutinos found in the Kuiper belt: suggesting that if it ever had a substantial atmosphere, it sublimated away and escaped into space long ago. Since the temperature of such a world should be determined by its distance from the Sun, and its ability to “hold onto” volatile compounds should be determined by its surface gravity, 2002 XV93 was expected to be an airless world, like all the other known Kuiper belt objects, including Pluto’s giant (and much larger than this object) moon Charon, had previously been determined to be. But in science, we don’t simply take the observations we have and our best theories for how we predict things should be and call it a day; we seek to probe, measure, and test the Universe directly, wherever possible. Only by looking directly, and by making direct observations and measurements of the quantities we’re seeking to understand, can we truly uncover how the Universe works. That’s part of the key process of what science is at its core: putting questions about “what’s going on in the Universe” to the Universe itself, and interrogating it in such a way that it reveals the answers to our questions directly, through experimentation, observation, and measurement. That’s what led to the key 2024 occultation measurements, which revealed the existence of 2002 XV93‘s atmosphere. The only way to have an occultation is where, from the perspective of any given observer, a foreground object lines up with and, from that perspective, passes in front of a luminous, background object, obscuring its light. We normally approximate: - Earth as a point, - distant objects in our Solar System as a point, - and the even-more distant stars as a point, but when it comes to occultations, that’s not a good enough approximation. These distant objects don’t behave as truly point-like objects, but rather as small disks in relative motion to the background of the fixed, more point-like stars, where there’s a time that the occultation begins, a duration for the span of the occultation, and a time that the occultation ends. Moreover, Earth can’t be treated like point-like either. Just as your left eye and right eye see different perspectives when you switch between them, particularly when you’re viewing nearby objects relative t…

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The discovery of an atmosphere on a tiny Kuiper belt world