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New Simulations Show Gravitational Collapse Can Build Kuiper Belt 'Snowman' Worlds

Using solid-strength physics with low-speed contacts, a peer-reviewed MSU study reproduces the bilobed forms seen in the outer solar system.

In 2019, NASA's New Horizons mission arrived at 486958 Arrokoth--formerly nicknamed Ultima Thule--for a flyby. While imaging the Kuiper Belt object, scientists discovered evidence of sublimation--the transition of an object from solid to gas--that had occurred 4.5 billion years prior. Image via NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute, National Optical Astronomy Observatory
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Overview

  • Michigan State University researchers Jackson Barnes and Seth Jacobson report the first simulations that naturally form contact-binary planetesimals, published February 19 in Monthly Notices of the Royal Astronomical Society.
  • Across 54 runs the team resolved 834 bodies, with 29 evolving into two-lobed contact binaries, providing quantitative support that such shapes can arise without rare events.
  • The model preserves internal strength of solids so components can rest against each other instead of flowing together, addressing a key shortcoming of earlier fluid-like approaches.
  • Low relative speeds of only a few meters per second allow gently merging lobes after inward-spiraling binary orbits, producing stable, Arrokoth-like forms.
  • The results align with estimates that roughly 10% of Kuiper Belt planetesimals are bilobed, while the authors develop next-generation simulations to refine rates and more complex outcomes.