New Horizons flyby reveals critical clues about planetary formation

Data from New Horizons' encounter with Arrokoth in the Kuiper Belt is unlocking the secrets of the Solar System.

Kuiper Belt Object Arrokoth. Credit: ASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Published: February 14, 2020 at 8:45 am
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Data from the New Horizons spacecraft's Kuiper Belt flyby on 1 January 2019 is providing scientists with new insight into how planets and their building blocks - called planetesimals - were formed in the early Solar System.


New Horizons made its close approach at Arrokoth (2014 MU69) - formerly called Ultima Thule - on New Year's Day 2019, over 4 billion miles from Earth.

The object is part of the Kuiper Belt: a ring of debris on the edge of the Solar System left over from the formation of the planets.

Learning more about the Kuiper Belt could reveal more about what the early Solar System was like and how it formed.

Read more about the New Horizons mission:

Data on Arrokoth's shape, geology, colour and composition has enabled New Horizons scientists to make a leap in their understanding of the origins of the planets.

Images from the flyby revealed Arrokoth is formed of 2 connected lobes.

Computer simulations enabled the team to decipher the object's history, revealing that Arrokoth consisted of two separate bodies that formed nearby each other, orbited each other, and eventually merged at a slow speed to form the 22-mile long Kuiper Belt Object.

This tells the team that Arrokoth formed during the collapse of a cloud of solid particles under gravity in the primordial solar nebula: not by another method theorised called 'hierarchical accretion'.

According to the latter theory, planetismals slammed into each other at breakneck speeds to form larger bodies, whereas the New Horizons team have concluded Arrokoth formed under a much gentler process.

An illustration showing the merger of the two lobes of Arrokoth. Credit: Credit: ASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
An illustration showing the merger of the two lobes of Arrokoth. Credit: Credit: ASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

“Just as fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space,” says William McKinnon, a New Horizons co-investigator from Washington University in St. Louis.

“Arrokoth looks the way it does not because it formed through violent collisions, but in more of an intricate dance, in which its component objects slowly orbited each other before coming together.”

Also supporting this theory are Arrokoth's uniform colour and surface composition, showing it formed from nearby material, just like under the the local cloud collapse model, rather than a collection of matter from distant parts of the nebula, as the hierarchical model suggests.

Arrokoth's flattened lobes, as well as the close alignment of their poles and equators, also point to a more gentle merger.

“Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell,” says New Horizons principal investigator Alan Stern

"It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation.

“All of the evidence we’ve found points to particle-cloud collapse models, and all but rule out hierarchical accretion for the formation mode of Arrokoth, and by inference, other planetesimals.”


New Horizons spacecraft is now 4.4 billion miles from Earth, travelling deeper into the Kuiper Belt at nearly 31,300 miles per hour.


Iain Todd, BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Staff Writer. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.


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