Close encounter

Deformed object in the Kuiper Belt defies planetary formation theory

Arrokoth is weird — it is also proving revelatory.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

As it flew by Pluto in 2015, NASA’s New Horizons mission spotted something weird: An ancient, deformed planetary body the size of Seattle swam into view, perplexing scientists here on Earth.

This strange encounter turned out to be a scientific boon. Data from the object reveals new details about how planetary bodies, including our own Earth, formed in the early Universe, offering rare insight into the ancient cosmos.

Dubbed Arrokoth, this weird rock is the furthest, most primitive object ever visited by a spacecraft.

Located some billion miles from Pluto, the object lies in the Kuiper Belt, a region beyond the Solar System. The Belt is believed to contain the leftover remains of the Solar System's early formation.

On New Year’s Day, 2019, New Horizons approached Arrokoth at a distance of 2,200 miles, flying directly past it. For reference, New Horizons was four billion miles away from Earth at this point.

NASA's data from the close encounter were published Thursday in three studies in the journal Science. Together, the research supports an unlikely theory of how planetary bodies formed in the early universe.

Close encounter

Arrokoth is around 22 miles wide, and is a binary object made up of two lobes that sort of looks like a misshaped, red snowman. It is classified as a "planetesimal" — an object believed to have formed around 3.8 billion years ago, during the Solar System's infancy.

Arrokoth is not the only object that dates to the time. Ancient comets and asteroids which also date from this febrile time have come closer to Earth, but because their orbits bring them near the Sun, they have also been affected by the star's radiation. As a result, they have been severely altered, obscuring some of the data scientists might gain on how they formed. But the Kuiper Belt is cold, and far from the Sun's harmful rays. It also contains some of the first planetary bodies ever formed in the Solar System, making it a potential astrophysics goldmine.

This brief animation moves between two New Horizons spacecraft views of Arrokoth, the spacecraft's New Year's 2019 flyby target in the Kuiper Belt.NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

“These Kuiper Belt objects are a much better preserved sample or window back into the formation stage of planets, or planetesimals,” Alan Stern, principal investigator of New Horizons, said during a press conference to announce the findings on Thursday.

Arrokoth not only reveals some of the secrets of this process, but the data also defy the conventional theory of how planets form.

Slow and steady

The argument over how planets form has various competing theories, but the two at the center of this new research are the theory of hierarchical accretion and the cloud collapse hypothesis. Hierarchical accretion is a process whereby planetary bodies formed through high speed collisions of small rocks and grains. Cloud collapse, by contrast, is an altogether more gentle process.

Cloud collapse suggests that particles were gradually drawn to each other, until they suddenly collapsed under their gravitational pull, forming massive bodies over time.

The uniform color and composition of Arrokoth’s surface shows the Kuiper Belt object formed from a small, uniform, cloud of material in the solar nebula, rather than a mishmash of matter from more separated parts of the nebula. The former supports the idea that Arrokoth formed in a local collapse of a cloud in the solar nebula.NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

According to the new data dump, the science supports the second theory. Arrokoth's two lobes were formed from the same cloud of material — as separate entities, they orbited around each other before they fused together to form the binary shape, the scientists reveal.

To make the conclusion, the researchers looked at different numerical simulations of interacting bodies at different speeds and angles. The models suggest that an object of this size most likely came together at a low speed — consistent with cloud collapse theory, and not hierarchical accretion.

Other data collected during the flyby jibes with this result: Arrokoth has no signs of a violent origin, and no large craters or fractures — suggesting that it had a rather gentle birth.

“The only way to account for this body is the merger speed was very low,” William McKinnon, NASA's deputy leader of the Geology and Geophysics Investigation team for New Horizons, said Thursday.

“The shape is simply inconsistent with higher velocity accretion, but it does match what we expect to see in the low velocity merger.”

New Horizons launched in January, 2006. The spacecraft was designed to peer into the outskirts of the Solar System by flying by Pluto and venturing on into the Kuiper Belt. Arrokoth was not on the original agenda for the mission, but in this one flyby, the misshapen object ca now be added to its magnificent scientific legacy.

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