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Close-Up of Arrokoth Reveals How Planetary Building Blocks Were Constructed

Peanut-shaped Arrokoth is the farthest and most primitive object in the Solar System ever to be visited by a spacecraft. | NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

New observations on the farthest, most primitive object in the Solar System ever to be visited by a spacecraft – a tiny, frozen, bi-lobed world known as Arrokoth – offer a unique glimpse into the early formation of our Solar System and perhaps the planet Earth.

The new findings expand upon the first published observations of the Kuiper Belt object, announced in a May 2019 issue of Science, which were based on a small amount of data sent from the New Horizons spacecraft shortly after its New Year’s Day 2019 encounter.

Using more than ten times as much data – including the highest-resolution images from the flyby – the New Horizons team describes Arrokoth in unprecedented detail across three reports in the February 14 issue of Science.

Also known as 2014 MU69 and previously nicknamed Ultima Thule, the Kuiper Belt planetesimal was recently officially named Arrokoth, a Native American term meaning “sky” in the Powhattan/Algonquin language.

These studies provide a far more complete picture of the composition and origin of Arrokoth and help to resolve a longstanding scientific controversy about how planetesimals – the primordial building blocks of our Solar System’s planets – were formed.

After passing Pluto in 2015, NASA’s New Horizons spacecraft continued its mission deeper into the Kuiper Belt – a vast ring of icy bodies spread beyond the orbit of Neptune – to investigate the objects observed slowly orbiting in the outer reaches of the Solar System. Its target: Arrokoth, a peanut-shaped Kuiper Belt object discovered by the Hubble Telescope the previous year. New Horizons flew past the distant bi-lobed world after a journey spanning more than three years and one billion miles.

Like the asteroids of the well-known asteroid belt between Mars and Jupiter, many of the objects drifting in the Kuiper Belt are remnants left over from the formation of the Solar System. One of the class of bodies called Cold Classical Kuiper Belt Objects, Arrokoth is an ancient relic preserved from the time when the planets first began to form nearly 4.5 billion years ago. Scientists still debate the process by which the dust and gases surrounding the new Sun coalesced to become the eight planets we recognize today.

“To understand planet formation, we have to understand planetesimal formation, since this is a key first step,” said Lowell Observatory astronomer William Grundy, lead author of one of the studies. “Arrokoth is a planetesimal – an essential stepping-stone on the way to building planets, starting from dust in a protoplanetary nebula.”

While there are planetesimals closer to Earth in the asteroid belt, according to Grundy, those visited via space probes have all been badly battered by colliding with each other or cooked by the sun, looking  far different than when they first formed. However, in the cold, vast and remote Kuiper Belt, primordial planetesimals like Arrokoth remain largely untouched – even by the heat of the Sun – thus avoiding many of the processes that have obscured or erased the earliest histories of other worlds. As a result, Kuiper Belt planetesimals are the best-preserved time capsules of the early stages of planet formation known to exist.

“For decades, we’ve been thinking about how planetesimals are made and debating the pros and cons of the various hypothesized mechanisms,” said Grundy. “So, it’s thrilling to finally be able to see one still pretty much just as it was after its formation.”

New Horizon’s visit to Arrokoth was brief – the spacecraft rocketed past the roughly 22-mile-long object at almost 32,000 miles per hour before careening even deeper into the Kuiper Belt. However, the encounter was close and long enough to characterize, map and image the distant object’s geological and geophysical features, including its most striking aspect – its unique shape.

Images from New Horizons show that Arrokoth is what’s known as a contact binary. Its shape – something akin to a slightly flattened snowman – is the product of two separate, mutually orbiting planetesimals, which gently merged with one another. According to John Spencer, a planetary scientist at the Southwest Research Institute and lead author of another study in the issue, this is something that could only happen during the early stages of Solar System formation.

“The gentle merger provides strong support for the idea that planetesimals formed from local clouds of material that collapsed under their own gravity,” rather than the longstanding theory of hierarchical accretion, said Spencer.

The hierarchical accretion hypothesis – whereby colliding dust grains become larger, and so on up through pebbles, cobbles and boulders, colliding ever more forcefully into one another as they gradually grow to become the size of planets – is the currently accepted method of planetesimal formation.

However, the formation and merging of Arrokoth’s lobes bear no signs of such violent processes. Instead, the new evidence from Arrokoth suggest a rapid but gentle process of planetesimal formation – a formally underdog hypothesis known as local cloud collapse – where local concentrations of protoplanetary dust and debris come together under the influence of their own collective gravity.

“It’s hard to communicate about something to the public when we don’t even have a good idea of what it looks like,” said Grundy. “Just knowing that it plays an important role as a building block of planets isn’t enough.”

“Now we finally have that picture and a much clearer idea of how it forms,” he said.

New Horizons is speeding onwards in the outer fringes of the Solar System, already 300 million miles beyond Arrokoth, and its exploration of the Kuiper Belt is ongoing. According to the researchers, distant observations of other Kuiper Belt Objects will help to place Arrokoth’s observations in context. And, however unlikely, the team hopes to find another potential target for a close flyby.

“I entered this field because I was inspired by Carl Sagan’s point that only a couple of generations of scientists in all of human history would be the first to send craft to explore the planets of our Solar System,” said NASA’s Alan Stern, New Horizons’ principal investigator. “I have been fortunate to be a part of that scientific revolution.”

“We successfully accomplished the most distant flyby in the history of planetary exploration of a world we didn’t know existed five years ago,” said Spencer. “Seeing a new world for the first time – there’s nothing else like that experience.”