An object in the outer solar system appears to have had a huge chunk taken out of it in a violent collision, despite such impacts being rare at this distance from the sun.
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| An artist’s impression of 2002 MS4, made before the new measurements of its shape Courtesy Superpartes/Space Engine planetary Database Wiki, CC-BY-SA. |
Flavia Rommel at the Federal University of Technology in Brazil and her colleagues enlisted the help of hundreds of amateur astronomers to study the distant world 2002 MS4, found beyond Neptune’s orbit in the Kuiper belt. The so-called trans-Neptunian object (TNO), discovered in 2002, is so remote that it can barely be seen by telescopes, despite being some 800-kilometres wide.
Rommel and her team used a technique called occultation to study the object in more detail, recruiting telescopes from around the world to watch as it passed in front of a background star in 2020, blocking the star’s light. That allowed them to refine the size and shape of 2002 MS4, but also threw up a surprise.
“It was not round in the north-east region,” says Rommel. The researchers concluded that they were seeing a huge topographical feature: a massive depression in the object, probably a crater, around 45 kilometres deep and 320 kilometres across. “This is the first example of a large crater beyond Neptune,” says Rommel.
The rim of the crater appears to rise about 25 kilometres above the surface of 2002 MS4, which would make it taller than Olympus Mons on Mars, the biggest mountain in the solar system. Only once before have we seen anything like this vast feature on a world so far from the sun, a deep crack twice the size of the Grand Canyon on a TNO called 2003 AZ84.
Bruno Sicardy at the Paris Observatory in France, who also worked on the study, says the depression was “quite unexpected”, adding that there is “no other body which is 800 kilometres with such a large topographic feature”.
However, it is possible that the feature seen by the team isn’t a crater at all, but actually a moon orbiting the TNO, which would distort its apparent shape. That explanation is unlikely, says Sicardy, requiring “a lot of luck”, but can’t be ruled out yet.
No other telescope – not even NASA’s James Webb Space Telescope (JWST) – can observe 2002 MS4 in as much detail as the occultation technique because the object is so far away, around 40 times as distant from the sun as Earth is. However, JWST could help confirm or refute the existence of an orbiting moon.
If the feature is a giant crater, it would point to a violent collision involving this object at some point. Such collisions are common closer to the sun, but rare at this distance, where objects are more spread out. “This was not expected,” says Rommel.
The discovery could give us a unique window into our solar system’s evolution, says Alex Dias de Oliveira at Avenues: The World School in São Paulo, Brazil. “These objects work as fossils for the solar system because they are so far from the sun,” he says. “Knowing their shape will give us more information about their chemical composition. If you understand their composition, you understand the past of the solar system.”
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