Discovered in 2014, it’s a weird one, shaped like a giant flat peanut. Two planetesimals joined at their axes, with unusual and possibly revealing geology:
Arrokoth’s two lobes appear to have once been separate objects, orbiting each other in synchronicity until merging together at a very slow speed – no more than a couple meters per second, perhaps even slower.
Those objects were likely formed in the very early years of our solar system, over four billion years ago. Then, the solar system was a nebulous swirl of gas and dust. Microscopic dust particles began to stick together into larger and larger chunks. Soon, those chunks become pebble-sized clouds that continue to collapse together, creating larger objects. In some cases, such as Arrokoth’s, those objects formed in pairs, leading to a binary system.
This could also explain certain aspects of Arrokoth’s chemical compositions, with possible methanol ices and complex organics that were also present in the early solar system nebula seen on the object as well. However, those organics could have emerged after Arrokoth had formed through chemical processes spurred on by cosmic radiation affecting methane on its surface. Unlike many similar objects, there is an absence of water ice, which appears to have been depleted or is obscured from our view.
Because Arrokoth’s formation was so benign, with no major collisions or aberrations apparent on its surface, the object can give us insight into this early phase of the solar system when planets and other objects were still forming. Its interior is likely preserved as well, leaving the same mixture of ices, organics and dust from the nebula of materials that created our solar system.“Arrokoth Revealed: A First In-Depth Look at a Pristine World” at NASA Ames (February 14, 2020)
It’s definitely slow. The Max Planck Society tells us, “It took several million years for Arrokothn to acquire its bizarre, pancake-flat shape.” No ka-POW! But mysteries remain:
“We like to think of the Kuiper Belt as a region where time has more or less stood still since the birth of the Solar System,” explains Ladislav Rezac from MPS, one of the two first authors of the current publication. More than four billion kilometers away from the Sun, the bodies of the Kuiper Belt have remained frozen and unchanged, so is the common belief. New Horizon’s images of Arrokoth challenge this idea by its apparently smooth surface without signs of frequent cratering events and by its peculiar, flattened shape. Scientists assume that the Solar System was formed 4.6 billion years ago from a disk of dust: the particles from this nebula agglomerated into ever larger clumps; these clumps collided and merged into even larger bodies. “There is as yet no explanation as to how a body as flat as Arrokoth could emerge from this process,” says Rezac.Max Planck Society, “It took several million years for Arrokothn to acquire its bizarre, pancake-flat shape” at Phys.org (October 5, 2020)
The name means “sky” in a native American language. Earlier, the object had been called Ultima Thule, as in the vid below.