It is the most distant and primitive object ever visited by a spacecraft from Earth: now researchers say they have fresh insights into how the ultra-red, 4bn-year-old body known as Arrokoth came to have its distinctive snowman-like shape.
Arrokoth sits in the Kuiper belt, a vast, thick ring of icy objects that lies beyond the orbit of Neptune. This region of space is home to most of the known dwarf planets as well as comets and small, solid rubble heaps called planetesimals – the building blocks of planets.
Not all of these planetesimals are rounded: indeed, astronomers estimate 10-25% 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 Arrokoth’s shape, composition and small number of craters suggests both lobes formed at the same time and in a non-violent way, proposing that this could have occurred through a process known as gravitational collapse. However, the details of just how this would 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 that we’ve never been able to see from beginning to end, confirming this entire process.”
As Barnes notes, the Kuiper belt is a remnant of the solar system’s primordial protoplanetary disk, within which vast rotating clouds of pebbles are thought to have formed. In the gravitational collapse scenario, gravitation forces within these clouds caused the pebbles to form into clumps, or planetesimals, of different sizes.
Writing in the Monthly Notices of the Royal Astronomical Society, Barnes and colleagues report how they ran 54 simulations involving an initial pebble cloud containing 105 particles, each with a radius of about 2km (1.25 miles). This is a low-resolution model of the true situation as it is thought real pebble clouds would have contained about 1024 millimetre-sized particles.
The team found that in some cases two small planetesimals ended up orbiting each other, eventually spiralling inwards until, at velocities of about 5 metres a second or less, they touched and joined, forming a doubled-lobed planetesimal, or “contact binary”.
“Some of the contact binaries in our model look strikingly like Arrokoth,” Barnes said.
He noted that researchers had simulated gravitational collapse before but, unlike the new approach, they did not take into account the physics of how particles rest upon each other when they make contact. As a result, these simulations suggested any collision between smaller planetesimals would simply result in one larger, spherical object.
Barnes said the new simulations were also important because they supported the long-held view that planetesimals in general were 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.
“It’s in agreement with previous work and support[s] the hypothesis that Kuiper belt object Arrokoth, which New Horizons explored in a close flyby, is the result of gentle formation processes,” he said.
Alan Fitzsimmons, an emeritus professor of astronomy at Queen’s University Belfast, noted that the simulations only suggested 4% of objects “out there” formed as contact binaries.
“Telescopic surveys imply much higher fractions,” he said. “It may be that Mother Nature prefers other ways of making them, or that future even more complex simulations can close the gap between what is calculated and what we see.”