What a frozen world that is no wider than 500 kilometers made planetary scientists realize? The answer is that some very small worlds, like 2002 XV93, may retain or repeatedly reconstruct their atmospheres, despite what experts believed. 2002 XV93 is the plutino-type Kuiper Belt Object, located further from the Sun than Neptune. For decades, any small body of similar size was considered to be unable to host a significant gaseous shell due to lack of mass and low temperature. Only the famous Pluto managed to sustain a thin atmosphere of frozen volatiles sublimating at its surface. However, observations of 2002 XV93 indicate that even an extremely tiny world can be surrounded with an exosphere, although quite a flimsy and temporary one.
The atmosphere of this minor planet was detected not with a spacecraft probe or a direct photo, but thanks to an occultation of a distant star. On January 10, 2024, Japanese scientists registered gradual darkening of a star while it was eclipsed by a passing 2002 XV93. Unlike an instant extinction typical of an airless object, it dimmed for a second or so close to the leading edge of the object’s shadow, meaning that its faint light was refracted and absorbed by an atmosphere.
The exosphere found is incredibly thin. Its surface pressure is estimated at 100-200 nanobars, about five to ten million times less than that of Earth and substantially less than the thin atmosphere of Pluto. Still, its presence alone is a scientific breakthrough because even such a thin shell proves that there are actually some gases around the celestial body, or, to say more accurately, these gases are produced by the world.
The mystery of its formation lies in the fact that the only reasonable assumption concerning the composition of an atmosphere of a Kuiper Belt Object is the presence of such gases as nitrogen, methane, and carbon monoxide. They can sublimate at the low temperatures of the outer Solar System. However, none of the previous observations revealed the presence of these gases on its surface. Neither water ice nor carbon dioxide are possible since it is extremely cold for sublimation. Hence, two options remain: the atmosphere formed as a result of a recent impact or some cryovolcanic eruptions releasing the gas.
In both cases, the results obtained prove that some small Kuiper Belt Objects may have been overlooked. First, if an atmosphere formed after an impact, it means that 2002 XV93 was observed shortly after its creation. In other words, a brief observation was made in a very tight timeframe of tens of centuries. Secondly, if the world produces an atmosphere, then some other small Kuiper Belt Objects may also have one, proving their geological activity.
The discovery is important not just for understanding the nature of Kuiper Belt Objects in general but also for revealing new details concerning the processes taking place within the outer Solar System. For instance, Pluto proved that even at such distances, gas atmospheres can evolve quite complexly due to mutual interaction between sunlight and the planet’s surface and internal activity. Thus, if a small Kuiper Belt Object can mimic the same process, the list of targets to be observed will considerably extend.
Further research will probably include occultations or observations from the James Webb Space Telescope. Thus, scientists will check if the atmosphere of 2002 XV93 continues to dissipate or stays steady. The latter case implies the existence of ongoing volcanic eruptions feeding fresh material to the atmosphere of the dwarf planet. For what used to be another minor celestial body, this small Kuiper Belt Object turned out to be quite an interesting test case for studying the outer solar system.