A tiny frozen world in the outer Solar System has been found to have an atmosphere, making it only the second object beyond Neptune, after Pluto, known to have one.
The finding, published in Nature Astronomy, could mean that such mini worlds are much more active than planetary scientists had believed.
The world, known as 2002 XV93, is a minor planet and a Trans Neptunian Object (TNO), meaning it orbits the Sun somewhere beyond the orbit of Neptune in a region known as the Kuiper Belt.
A miniature Pluto
The atmosphere of 2002 XV93 is around 5 to 10 million times thinner than Earth’s – so thin you wouldn’t be able to feel a breeze if you were standing on the surface.
So far, the only other TNO found with even a hint of an atmosphere is Pluto, which at 2,377km (1,480 miles) in diameter is over five times larger than 2002 XV93.
The small size of the minor planet means it has a very weak gravitational pull, so any atmosphere around it can easily escape into space. In fact, the study predicts that the atmosphere should completely disappear within 100 to 1,000 years.
That means something must be replenishing the atmosphere – and there are two leading theories as to what that might be.
The first is cryovolcanism, which occurs when it’s so cold that ‘volatiles’ – things that are liquids or gas at room temperature, such as water, methane and ammonia – begin to act like rock and magma.
“Volatile material could escape from beneath the icy surface, perhaps through cryovolcanic-like activity,” said Ko Arimatsu from the National Observatory of Japan, who led the study. These gases might then form the atmosphere.
However, cryovolcanic activity has only ever been seen on larger worlds before now.
The second possibility is a recent impact. “A small icy body may have hit 2002 XV93 recently and released gas or exposed volatile-rich material,” said Arimatsu.
“Such impacts are probably rare, so observing a transient atmosphere at the right time may have a low probability, but the impact scenario cannot be ruled out.”
In other words, it’s unlikely the researchers happened to see this specific phenomenon when they were observing 2002 XV93, but it’s worth considering.
Observing by eclipse
It takes 2002 XV93 around 247 years to orbit the Sun. This puts it in a similar position to Pluto – but the distance means it’s very difficult to observe such objects directly.
Instead, Arimatsu’s team made the discovery using a technique known as ‘occultation’. This is where the planetary body passes in front of a distant background star, briefly eclipsing it.
“If the object has no atmosphere, the star’s light should disappear and reappear sharply,” said Arimatsu. If there is an atmosphere, however, then its gases slightly bend the light as it comes through, softening the change.
On 10 January 2024, three observatories in Japan were able to watch an occultation by 2002 XV93.
“In our observations, especially from Kiso Observatory in Japan, the starlight faded and recovered gradually near the edge of the shadow. This gradual change is best explained by the star’s light being bent by a very thin atmosphere around 2002 XV93,” said Arimatsu.
The team have requested time on the James Webb Space Telescope, which should be able to detect what the atmosphere is made of, but the best way to pick apart what’s happening is more occultation observations.
“These can show whether the atmosphere is fading, staying present, or changing with time,” said Arimatsu.
A steadily fading atmosphere suggests a temporary atmosphere is slowly escaping without being replenished, pointing towards the impact theory. A more changeable or steady atmosphere suggests it is being topped up by gases coming from inside the planet.
The problem is that occultations require very precise alignment. According to Arimatsu, “only a small number (about 10) [of observatories] have had the right geometry and data quality to search for very thin atmospheres around objects of this size.”
That could mean there’s a long wait to find out exactly what’s going on with 2002 XV93’s atmosphere.
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