Distant Kuiper Belt object reveals deep secrets of planetary formation
Bit by bit, NASA’s New Horizons probe is bringing into sharper focus the most distant space object ever explored by a spacecraft. Known as Arrokoth, this ancient, icy body sits far beyond Pluto in a ring of frigid objects that form the Kuiper Belt. Scientists have now got their hands on the latest observations of this faraway world, which significantly advances our understanding of how planetary bodies were formed.
New Horizons whizzed past Arrokoth in January last year, coming as close as 3,500 km (2,200 mi) from the surface and gathering detailed images and data on its composition. This painted a picture of a smooth, flat world consisting of two connected lobes that appeared to be in pristine condition, boding well for the study of how the building blocks of of planets, known as planetesimals, came together billions of years ago.
“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, of the Southwest Research Institute in Boulder, Colorado. “It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation.”
Scientists have been using the New Horizons flyby data to form an idea of how Arrokoth formed, and in three newly published papers this week present what they believe to be a relatively complete picture. The team believes that its two lobes were once separate bodies that long ago, began to orbit each other very slowly and then gently merged together over a long period of time.
The scientists suspect this was driven by a gravity-driven collapse of solid particles in the primordial solar nebula, the collection of dust and gas believed to have given rise to the solar system. Adding extra weight to this theory is the uniform color and flat composition of Arrokoth’s surface, indicating it built up gently and slowly from "local" materials in the solar nebula coming together.
This runs counter to a competing theory on planetesimal formation known as hierarchical accretion, in which a variety of distant particles from different sections of the nebula are smashed together at high speed.
“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.”
The scientists say the latest papers on Arrokoth, which are based on 10 times as much data as those published last year, “all but rule out” the theory of hierarchical accretion for how the body was formed, and by extension, other planetesimals like it.
In the meantime, New Horizons continues to observe the Kuiper Belt as it flies through the disk of icy objects at almost 31,300 mph (50,400 km/h). In the next few months, the team will use telescopes here on Earth to start searching for other Kuiper Belt objects that New Horizons may be able to perform close flybys of, fuel permitting.