A weird shaped rock provides insights into how our planets formed

In 2019, the New Horizons Space Probe discovered this rock, called Arrokoth, in the outer solar system beyond Neptune.

Arrokoth (space rock)
NASA/Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute, Roman Tkachenko
The formation of MU69
New Horizons, NASA, JHUAPL, SwRI, James Tuttle Keane

Our solar system consists of the Sun and eight planets orbiting it (sorry, Pluto is only a dwarf planet). The formation of these planets is thought to be from gradual accumulation (accretion) of dust and gas. During accretion, small pieces of matter called planetesimals form first. Continued accretion of planetesimals eventually leads to the formation of the first planets. However, the mechanism of how these planetesimals led to the formation of the planets in the solar system is still elusive.

Now, scientists are looking at tiny pieces of rocks (less than 100 km wide) beyond Neptune, the last planet of our solar system. In a region called Kuiper Belt, these rocks (one of the larger one being Pluto) have been relatively untouched since the formation of our planets. The New Horizons space probe, after watching the dwarf planet Pluto, recently discovered another of these rocks, a smaller one called Arrakoth​​.

Arrokoth has a weird shape. It is a “contact-binary”, meaning that it is basically two separate objects that touch each other. Researchers believed that understanding how Arrokoth formed would give clues to how planetesimals accreted into planets.

In three papers (paper 1, paper 2 and paper 3) published in Science, researchers, including many from the New Horizons Space Probe team, reported results from extensive analysis of Arrakoth.

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. It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation. - Alan Stern, Principal Investigator and author in all Arrokoth papers cited in this article

The researchers used “sophisticated” computer simulations and analysis to analyse how Arrokoth formed​. Their analysis indicates that the lobes of this “contact binary” object were once separate bodies that formed close together. They also think that these two bodies orbited each other at low velocity and then gently merged to create the 22-mile long rock​​.

Researchers also pointed out that the colour and composition of the two lobes of Arrokoth are very similar indicating that they had a common origin. The shape of the lobes suggest that the objects were spiralling close to each other and eventually came into contact.

The flattened shapes of each of Arrokoth’s lobes, as well as the remarkably close alignment of their poles and equators, also point to a more orderly merger from a collapse cloud. Further still, Arrokoth’s smooth, lightly cratered surface indicates its face has remained well preserved since the end of the planet formation era.

This indicates Arrokoth formed during the gravity-driven collapse of a cloud of solid particles in the primordial solar nebula, rather than by the competing theory of planetesimal formation called hierarchical accretion. Unlike the high-speed collisions between planetesimals in hierarchical accretion, in particle-cloud collapse, particles merge gently, slowly growing larger.

Results from the work presented in papers discussed in this article​ give us more insights into how planets, including our earth, may have formed. This understanding is important to debunk theories of “creation” which have no evidence whatsoever. Today, we have some clues as to how rocks, untouched since the formation of the planets, were formed. This brings us a step closer in our never-ending endeavour to understand the origin of our cosmos. 

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