Space snowmen in the solar system may form when binary planetesimals gently join.

A new simulation study models planetesimals as clouds of pebble-sized particles and finds that spinning clouds can produce binary planetesimals that spiral together and gently fuse into contact binaries, or 'space snowmen.' The simulations produced contact binaries in about 4% of runs, and the results were published Feb. 19 in Monthly Notices of the Royal Astronomical Society.

In the outer solar system many icy bodies appear as pairs of joined spheres often called "space snowmen" or contact binaries. A new simulation study models planetesimals as clouds of pebble-sized particles to explore how these shapes form. Earlier models treated planetesimals as single colliding spheres and typically produced only spherical outcomes. The new approach lets spinning clouds split into multiple planetesimals that can then orbit and interact. Key points: - The study modeled planetesimals as collections of individual particles rather than single perfect spheres. - Spinning clouds sometimes formed two planetesimals that orbited one another instead of a single body. - Those binaries could spiral inward under mutual gravity and make gentle contact, producing fused shapes including spherical, flat, cigar-shaped, and snowman-like bodies. - The final shape depended on the relative speeds of the bodies and the strength with which their constituent particles interlocked. - Contact binaries appeared in about 4% of the simulated planetesimals, lower than earlier estimates of 10–25% reported by other research. - The authors note simulation limits tied to the number and size range of particles used and say more detailed particle models might change the formation rates. Summary: The study offers a straightforward formation pathway for contact binaries and other shapes through low-speed orbital decay and pebble cohesion, providing a possible explanation for many Kuiper Belt snowmen. The simulated contact-binary fraction was about 4%, and the team reported that expanding particle numbers and sizes in future simulations could affect that fraction. The researchers are also investigating how spinning clouds might form triple systems and how those outcomes relate to observed relict triples in the Kuiper Belt.