Simulations shed light on how snowman-shaped body in Kuiper belt may have formed | Space
It is the most distant and primitive object ever visited by a spacecraft from Earth: Researchers now say they have new information about how the 4 billion-year-old ultra-red body, known as Arrokoth, acquired its distinctive snowman-like shape.
Arrokoth lies in the Kuiper Belt, a large, thick ring of icy objects located beyond the orbit of Neptune. This region of space is home to most of the known dwarf planets as well as comets and small piles of solid rubble called planetesimals – the building blocks of planets.
Not all of these planetesimals are rounded: in fact, astronomers estimate that 10 to 25 percent of those found in the Kuiper Belt, including Arrokoth, have two lobes, meaning they look a bit like a peanut or a snowman.
Experts have previously said that Arrokoth’s shape, composition and small number of craters suggest that the two lobes formed at the same time and non-violently, suggesting that this could have happened through a process known as gravitational collapse. However, the details of how this could have happened have been debated.
Now, researchers have used computer simulations to show that gravitational collapse can indeed produce such double-lobed objects and to shed light on the mechanism.
“It’s so exciting because we can actually see this for the first time,” said Jackson Barnes, the first author of the research, based at Michigan State University. “This is something we never got to see from start to finish, confirming this whole process.”
As Barnes notes, the Kuiper Belt is a remnant of the solar system’s primordial protoplanetary disk, within which vast clouds of rotating pebbles are thought to have formed. In the gravitational collapse scenario, gravitational forces within these clouds caused pebbles to form in clusters, or planetesimals, of different sizes.
In the Monthly Notices of the Royal Astronomical Society, Barnes and colleagues report how they performed 54 simulations involving an initial pebble cloud containing 105 particles, each with a radius of about 2 km (1.25 miles). This is a low-resolution model of the real situation, as real pebble clouds are thought to have contained around 1024 millimeter-sized particles.
The team found that in some cases, two small planetesimals end up rotating around each other, eventually spiraling inward until, at speeds of about 5 meters per second or less, they touch and join together, forming a double-lobed planetesimal, or “contact binary.”
“Some of the contact binaries in our model look strikingly similar to Arrokoth,” Barnes said.
He noted that researchers had previously simulated gravitational collapse but, unlike the new approach, had not taken into account the physics of how particles rest on top of each other when they come into contact. As a result, these simulations suggest that any collision between smaller planetesimals would simply result in the formation of a larger spherical object.
Barnes said the new simulations were also important because they confirmed the long-standing idea that planetesimals in general formed through gravitational collapse.
Alan Stern, a planetary scientist at the Southwest Research Institute and principal investigator of NASA’s New Horizons mission to the Kuiper Belt, welcomed the study.
“This is consistent with previous work and support[s] “The hypothesis that the Kuiper Belt object Arrokoth, which New Horizons explored during a close flyby, is the result of gentle formation processes,” he said.
Alan Fitzsimmons, emeritus professor of astronomy at Queen’s University Belfast, noted that simulations suggested only 4% of objects “out there” formed as contact binaries.
“Telescopic surveys involve much higher fractions,” he said. “It may be that Mother Nature prefers other ways of doing them, or that future even more complex simulations may narrow the gap between what is calculated and what we see.”
