In the unknown territory beyond Neptune's orbit, astronomers have discovered a tiny celestial body that defies our understanding of the sky.
This mysterious object is only about 500 kilometers (310 miles) in diameter, and while its gravity is so weak that it seems unlikely it could sustain an atmosphere for long, it actually does. Of course, it is a thin and unstable atmosphere, but the general view is that a small object like (612533) 2002 XV93 should not have an atmosphere at all.
It may sound insignificant, but this extremely small chunk of ice and rock in the cold, dark corners of the solar system could change our understanding of atmospheric retention.
In the process of studying this icy object, astronomers showcased cutting-edge technologies for detecting distant, faint phenomena.
2002 XV93, as it is known for short, is a type of object known as a plutino. It's a small body that shares an orbit similar to Pluto's orbital rhythm, at around 40 times Earth's distance from the Sun, in resonance with Neptune's orbit.
These small icy worlds are thought to represent a fossil record of the early Solar System, recording information about what it was made of and how things moved around in it. The resonance with Neptune, for example, shows that Neptune moved outwards, sweeping things up as it went.
But the space out beyond Neptune – the Kuiper belt – is a strange sort of no-man's-land for astronomy. It's filled with small, icy objects that are extremely difficult to find, let alone study in detail, because they're too far from the Sun to reflect much light we can detect.
This object, known as 2002 XV93, is an asteroid called Plutino. Plutino is a small body located about 40 times the distance from the Sun as Earth, exhibiting an orbital period similar to that of Pluto, and it resonates with Neptune's orbit.
These small icy bodies are considered the fossil record of the early solar system, containing information about the components of the solar system and the movements of celestial bodies. For example, the resonance with Neptune demonstrates that as Neptune moved outward, it attracted surrounding celestial bodies.
However, the space beyond Neptune—the Kuiper Belt—is a vastly unknown region astronomically. This area is filled with small, icy bodies, which are so far from the Sun that they reflect very little light that we can observe, making them extremely difficult to find and study in detail.
Scientists often have to rely on indirect observation methods. In the case of 2002 XV93, the observation method relied heavily on chance. In 2024, a research team led by Ko Arimatsu at the National Astronomical Observatory of Japan happened to be in the optimal position to observe a stellar occultation—the phenomenon where this star passes in front of a distant star.
The research team captured the occultation at three locations in Japan, recording in great detail the moment when the much closer Pluto temporarily blocked the starlight.
In the case of Pluto when its starlight is obscured, the change in light during the occultation process is very distinct and clear. The light suddenly disappears as 2002 XV93 passes in front of the star, and brightens again once the occultation ends.
However, that was not what astronomers observed. The entire phenomenon lasted only about 15 to 20 seconds, depending on the observation location, and the luminosity curves showed gradual dimming and brightening for approximately 1.5 seconds just before and after the total solar eclipse, respectively.
This gradual dimming can only be explained by starlight being refracted as it passes through the atmosphere.
Based on these dimming and brightening, the research team constructed a refraction model to understand what kind of atmosphere generated the signal. Based on Pluto's atmosphere, they assumed a specific temperature structure and a main component consisting of methane, nitrogen, or carbon monoxide.
Then, the researchers simulated how atmospheric density would vary at different altitudes and how light would be refracted as it passed through the atmosphere.
The closest results suggested that the atmospheric density is merely 100 to 200 nanobars, which is about 5 to 10 million times thinner than the atmospheric density at Earth's sea level.
This is a surprising result for several reasons. First, we currently possess observational instruments sensitive enough to detect the refraction of light passing through the extremely thin atmosphere coming from the outer reaches of the solar system.
Second, according to the research team's model, this extremely thin atmosphere will disappear within just a few hundred to a thousand years. The most likely scenario for the existence of an atmosphere on 2002 XV93 currently is that it is being replenished in some way.
Because there are many celestial bodies in the Kuiper Belt, the researchers propose a scenario in which a comet collided with 2002 XV93, releasing gas that formed a temporary atmosphere but soon dissipated.
Another possibility is that, like Pluto, 2002 XV93 possesses active cryogenic volcanoes that are ejecting ice chunks and volatile substances from within the asteroid to replenish the constantly leaking atmosphere.
Related Article: 'Ridiculous Hypothesis' About Pluto Confirmed for the First Time in Science
In any case, this is the first instance, excluding Pluto, where an atmosphere has been discovered on a small trans-Neptunian object (TNO). This discovery suggests that small bodies can possess atmospheres, and, even more interestingly, that if we are lucky, they can be detected even when there is almost no atmosphere present.
In their paper, the research team stated, "This discovery suggests that the existing idea that dense atmospheres can form across an entire planet must be revised."
"Even Neptune-orbiting objects (TNOs) hundreds of kilometers in size can possess atmospheres, at least temporarily, which raises questions about the typical volatile matter preservation scenario. The results of this study suggest that some distant icy asteroids may possess atmospheres, possibly formed by sustained cryogenic volcanic activity or recent collisions with small icy bodies."
This study was published in Nature Astronomy.
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