Simulations reveal how Kuiper Belt snowman worlds form

by Riko Seibo
Singapore (SDX) Feb 21, 2026

Astronomers have wondered for years why many icy objects in the distant Kuiper Belt resemble stacked snowmen, and new work from Michigan State University points to gravitational collapse as a straightforward explanation. The Kuiper Belt, a cold, sparsely populated region beyond Neptune, contains ancient planetesimals that preserve conditions from the early solar system, and roughly one in ten of these bodies appears as a contact binary shaped like two joined spheres.

Graduate researcher Jackson Barnes has developed what the team describes as the first simulation that naturally produces these two-lobed shapes through gravitational collapse. Earlier numerical models treated colliding bodies like fluid droplets that easily merged into more spherical forms, which made it difficult to generate the distinctive double-lobed structures seen in spacecraft imagery.

Using the computing resources of MSU's Institute for Cyber-Enabled Research high performance cluster, Barnes implemented a more realistic approach in which planetesimals retain material strength and can rest against one another without deforming into a single sphere. In this framework, the forming objects behave more like rigid aggregates of pebbles rather than idealized fluids, allowing stable, two-sphere contact binaries to emerge.

Alternative explanations for contact binaries have invoked rare or exotic processes, such as finely tuned collision histories or special environmental conditions, but these mechanisms struggle to account for the relatively high fraction of such objects in the Kuiper Belt. As Earth and Environmental Science professor Seth Jacobson notes, if about ten percent of planetesimals are contact binaries, then their formation pathway must be common, and gravitational collapse provides a natural, frequently occurring route.

NASA's New Horizons spacecraft offered the first close-up look at a contact binary in January 2019 when it flew past Kuiper Belt object 2014 MU69, informally called Ultima Thule, revealing a clear two-lobed shape. Those observations prompted scientists to reexamine other Kuiper Belt objects, leading to the realization that contact binaries account for roughly a tenth of the local planetesimal population and often show little evidence of heavy cratering.

In standard models of early solar system evolution, a rotating cloud of dust and pebbles in the outer protoplanetary disc collapses under its own gravity to form the first kilometer-scale planetesimals. Barnes' simulations begin from such a cloud of tiny materials, which clump together into larger aggregates much like individual snowflakes compacted into a snowball, building up the seeds of future dwarf planets, comets, and other icy bodies.

As the cloud spins, gravitational forces can cause it to fall inward and fragment into two main clumps that become separate planetesimals orbiting a common center. Many binary planetesimals are observed today in the Kuiper Belt, and in the new work the orbits of such partners gradually spiral inward until the bodies gently touch and fuse, preserving their roughly spherical individual forms while creating a bilobed overall shape.

Once formed, these two-lobed planetesimals can survive for billions of years because the Kuiper Belt is so sparsely populated that further disruptive collisions are unlikely. Barnes points out that many known binaries in this region are not heavily cratered, consistent with a long, largely undisturbed residence in the distant outer solar system where impacts are rare.

Researchers have suspected for some time that gravitational collapse might underlie the origin of contact binaries, but comprehensive tests of this idea were limited by earlier modeling approaches. Barnes' work is the first to incorporate the necessary physics to reproduce the observed shapes directly in numerical experiments, providing a concrete test of the long-standing hypothesis.

The team expects that the same modeling framework can extend to more complex systems containing three or more interacting bodies, potentially clarifying how higher-order binary and multiple systems form in the Kuiper Belt and beyond. Barnes and colleagues are now developing an improved simulation that will more accurately represent the full collapse process, including additional physical effects that could influence the final architectures of these distant objects.

With future NASA missions poised to explore still-uncharted regions of the solar system, the researchers anticipate that more contact binaries will be discovered, offering new laboratories for testing theories of planetesimal formation. Each newly imaged snowman-shaped world will provide further constraints on how small bodies assembled in the early solar system and how gravitational collapse sculpted the icy frontier over cosmic time.

Research Report:Direct contact binary planetesimal formation from gravitational collapse