Gravitational Collapse Explains Why So Many Kuiper Belt Objects Are Shaped Like Snowmen

Far past Neptune, in the cold outer reaches of the solar system, roughly 1 in 10 of the small icy bodies drifting through the Kuiper Belt has an unmistakable silhouette: two spheres pressed together, like a snowman. These contact binaries have puzzled astronomers since NASA's New Horizons spacecraft returned up-close images of the Kuiper Belt object Arrokoth in January 2019, prompting a rethink of how these structures came to exist.

A simulation developed by Jackson Barnes, a graduate student at Michigan State University, now provides the most physically realistic explanation to date. His model, published in the Monthly Notices of the Royal Astronomical Society, reproduces the characteristic two-lobed shape through gravitational collapse - without exotic collisions, special events, or conditions that would be too rare to explain objects that account for 10% of all planetesimals.

What Previous Models Got Wrong

Earlier computational models of planetesimal formation treated the objects as fluid blobs. In fluid-blob simulations, when two pieces of material come together, they merge into a single sphere. That behavior makes physical sense for liquids, but it cannot produce the distinct double-lobed structures observed in the Kuiper Belt, where each lobe retains its rounded shape and the two spheres rest against each other at a narrow contact point.

The problem was computational: modeling material with structural strength - the kind that allows an object to hold its shape rather than merging into a blob - requires much more computing power. Barnes had access to high-performance computing resources through MSU's Institute for Cyber-Enabled Research (ICER), which made it possible to run simulations with sufficient physical realism to allow objects to maintain their integrity.

How Contact Binaries Form in the Simulation

In Barnes's model, the sequence begins with the formation of the earliest planetesimals from the disk of dust and pebbles that surrounded the young Sun. As a rotating cloud of pebbles collapses under its own gravity, it occasionally fragments into two separate objects rather than collapsing entirely into one. These two objects orbit each other as a binary pair.

Over time, gravitational and tidal interactions cause the two objects' orbits to decay. They spiral slowly inward toward each other. When they finally make contact, they do so gently - gentle enough that each body retains its spherical shape rather than shattering. The result is the characteristic two-lobe structure. Because the Kuiper Belt is sparsely populated and distant from the chaotic inner solar system, most contact binaries formed this way remain undisturbed for billions of years, accumulating few craters and preserving their original shape.

"If we think 10 percent of planetesimal objects are contact binaries, the process that forms them can't be rare," said Seth Jacobson, Earth and Environmental Science Professor at MSU and senior author on the paper. "Gravitational collapse fits nicely with what we've observed."

Why It Matters for Solar System Science

The Kuiper Belt is a fossil of the early solar system. The objects there have changed little in the 4.5 billion years since they formed, making them time capsules that preserve information about the conditions under which the planets assembled. Understanding how contact binaries form is part of reading that record accurately.

The double-lobe structure itself is informative. If contact binaries form from pairs that spiral together rather than from collisions, that tells scientists something about how early gravitational collapse proceeded - specifically, that it frequently produced binary systems rather than single objects. That in turn constrains models of how the early solar disk behaved.

Barnes expects the model will help interpret binary systems involving three or more objects, which are also observed in the Kuiper Belt. The team is also developing a new simulation that models the initial gravitational collapse process with higher fidelity. As NASA missions continue to explore the outer solar system, more contact binaries with detailed shape measurements will provide additional tests of the model.