A potential dwarf planet and Plutino — only about 250 km in radius — at the extreme edges of our solar system was observed to have an atmosphere. The discovery comes as a first for Trans-Neptunian objects outside of Pluto and presents new considerations for objects smaller than 1000km to maintain an atmosphere.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're talking about the amazing object 2002 XV93. Not a very exciting name, but with a pretty exciting discovery — it has an atmosphere.
Let's begin with what is 2002 XV93, which I'll shorten to just XV93. It was discovered in 2002 during a time when Caltech's Dr. Michael Brown was surveying the outer solar system for distant objects — and XV93 is distant. At its closest, it is nearly 34 astronomical units or AU away and it gets as far as 44 AU away from the sun. As a reference, Neptune's orbit is at 30 AU. XV93's orbital distance changes by 25% over the course of its 247-year orbit around the sun, and is inclined by 13 degrees compared to the rest of our eight planets in a plane called the ecliptic. XV93 is roughly 300 miles across — or about the distance across Missouri north to south — so it's quite small, yet it is considered a dwarf planet like Pluto — though Pluto is five times larger. But to be in the dwarf planet category, the object must be round — though to be fair, we do not have any resolved images of XV93's surface or shape. It's so far away and so small that it's just a dot, even in the best telescopes. So XV93 is a dwarf planet, but it's also called a trans-Neptunian object, a Kuiper Belt object, and a Plutino.
Wow, that is a lot.
Both trans-Neptunian and Kuiper Belt objects are bodies beyond Neptune's orbit, traditionally with a range between 30 and 50 AU, which XV93 fits into nicely. However, there are now at least a dozen trans-Neptunian objects whose orbits extend out past 150 AU, and the object named Sedna's orbit goes out to nearly 940 AU, so even the trans-Neptunian definition is a bit fuzzy. And lastly, a Plutino is an object that has a 3-2 resonance with Neptune. Pluto has this property, which means it makes two orbits for every three orbits of Neptune. While none of the major planets in our solar system have resonances with each other's orbits, it is not an uncommon feature among exoplanets. And we do see it in our solar system with small objects in resonances with large ones like asteroids with Jupiter or XV93 in Pluto with Neptune.
Phew, that's a lot, so let's summarize.
XV93 is a small, probably round object past Neptune with an orbit that has been shaped by Neptune, is somewhat elongated and inclined, and has an atmosphere.
Wait... If XV93 is only a dot in the best telescopes, how do we know it has an atmosphere?
We know that and its shape and the shape of most asteroids from a technique called occultation. That's when a foreground object goes in front of a background one. In this case, XV93 and a distant star some 2,000 light-years away. Basically when this occurs, the shape and size of the foreground object is projected onto the Earth's surface. So if an occultation occurs at night — which is the only time we can see the background star — a set of telescopes can be used to very carefully time the dimming and re-brightening of the background object.
During the night of January 10, 2024, XV93 occulted the distant star for less than 20 seconds, and it was observed by a set of telescopes in Japan. Dr. Ko Arimatsu of the National Astronomical Observatory of Japan and collaborators reported their findings in the May 2026 issue of Nature Astronomy. The occultation lasted 16.5 seconds in Kiso and 18 seconds in Kyoto, and in both cases the background star didn't just disappear — like for an airless object — but dimmed for one and a half seconds before and after the occultation. And a third telescope in Fukushima only saw the dimming caused by the atmosphere with no occultation.
And that was a surprise.
Up to this publication, Pluto is the only trans-Neptunian object known to have an atmosphere, and even that is 100 times thinner than ours. But XV93's atmosphere is estimated to be about 10 million times thinner than ours, hardly an atmosphere at all, yet enough to cause the dimming observed. Dr. Arimatsu and collaborators speculate that it could be a transitory atmosphere from the eruption of a cryovolcano or a recent meteor impact. At XV93's distance, the temperature is nearly 400 degrees below zero Fahrenheit and even gases like nitrogen and methane can freeze into solids. The chemistry at such temperatures is very interesting and objects like XV93 could teach us a lot about conditions at the extreme boundary of our solar system.