In the distant reaches of our solar system, beyond the orbit of Neptune, lies the mysterious and icy expanse known as the Kuiper Belt. This vast region is home to countless ancient remnants from the solar system’s formation—small bodies called planetesimals, composed primarily of ice and rock. Among these objects, a curious subset captures the imagination of astronomers and the public alike: contact binary planetesimals. These bodies resemble cosmic snowmen, consisting of two lobes gently fused together, yet the origins of their unique shapes have long been shrouded in mystery.
Recent groundbreaking research from Michigan State University has shed light on the processes that craft these two-lobed formations. Utilizing a state-of-the-art high-performance computing system, graduate student Jackson Barnes has developed the first computational simulation that naturally forms contact binaries through gravitational collapse, without relying on improbable or exotic events. Published in the Monthly Notices of the Royal Astronomical Society, this work opens new avenues for understanding the early evolutionary pathways of small bodies in the outer solar system.
Traditional models faced significant limitations, often approximating these small icy objects as fluid blobs that, upon collision, merged into singular spheres. Such simplifications failed to reproduce the characteristic dual-lobed structure observed in about 10% of Kuiper Belt planetesimals. Barnes’ simulations mark a breakthrough by incorporating the mechanical strength and granular nature of these bodies. His approach allows the simulated planetesimals to rest against each other, maintain their distinct shapes, and ultimately fuse gently rather than violently.
The insights from Barnes’ research are critical because they align with the observed abundance of contact binaries. If 10% of planetesimals exhibit this fused shape, the formation mechanism must be a relatively common event in the early solar system, rather than a product of rare or catastrophic phenomena. Earth and Environmental Science Professor Seth Jacobson, a senior author on the paper, emphasizes that gravitational collapse is a compelling and elegant explanation consistent with empirical data acquired through decades of observation.
NASA’s New Horizons mission provided the first close-up images of a contact binary in January 2019 when it flew past the Kuiper Belt object known as 2014 MU69, nicknamed Ultima Thule. These crisp images revealed a distinctly two-lobed shape with smooth lobes fused at a narrow neck, challenging prior assumptions about planetesimal formation. Following this discovery, astronomers revisited other Kuiper Belt objects and identified that approximately one in ten follows this binary configuration, with little evidence of disruptive collisions owing to the sparse population density in that cosmic neighborhood.
The Kuiper Belt, formed remnant from the protoplanetary disk that once encircled the Sun, is an archive of primordial matter dating back over four billion years. Planetesimals are among the first large solid bodies to arise from this disk, developing through the slow agglomeration of pebble-sized fragments pulled together by mutual gravitational attraction. This formative stage is analogous to compaction of snowflakes into a snowball, except occurring over cosmic time scales and within a rotating circumstellar environment.
Barnes’ simulations highlight a fascinating dynamical process: as a rotating cloud of pebbles collapses under gravity, irregularities often lead to the initial formation of binary systems—two planetesimals orbiting each other. Over time, their orbits decay, spiraling closer until they make contact gently. The simulated binaries retain their smooth, rounded shapes without blending into a single sphere, thus reproducing the iconic snowman-like morphology observed in actual Kuiper Belt objects.
A key question that arises is how these delicate binary structures persist over billions of years without disruption. Barnes explains that the Kuiper Belt’s low-density environment minimizes chances of catastrophic collisions that could separate or shatter these contact binaries. This tranquil setting preserves the integrity of their shapes, consistent with the lack of significant cratering seen on many observed binaries.
While the gravitational collapse hypothesis had been proposed before, quantitative and realistic modeling was lacking due to computational constraints and oversimplifications. Barnes’ work pioneers a physics-rich simulation capable of resolving the mechanical and dynamical subtleties necessary to form and sustain contact binaries. This represents a major advancement in small-body astrophysics.
Looking forward, Barnes anticipates that his model will inspire further studies examining more complex multi-lobed systems, where three or more bodies coalesce through related mechanisms. The research team also aims to refine their simulations by incorporating more detailed physics to replicate the collapse and accretion processes with even greater fidelity.
Moreover, ongoing and future space missions venturing into the outer solar system may uncover additional contact binaries, revealing whether these “cosmic snowmen” have distant, untapped cousins. Such discoveries will further deepen our understanding of the delicate balance between gravitational forces and collisional histories that shape the architecture of our solar system’s frontier.
This new insight into the origin of contact binary planetesimals marks a significant milestone in planetary science. It not only clarifies how these peculiar objects form but also enhances our comprehension of the early conditions and evolutionary processes that govern the distant Kuiper Belt. As computational capabilities continue to expand, such interdisciplinary efforts bridging observation, theory, and simulation promise to unravel even more cosmic mysteries.
Subject of Research: Formation of contact binary planetesimals in the Kuiper Belt through gravitational collapse
Article Title: Direct contact binary planetesimal formation from gravitational collapse
News Publication Date: 19-Feb-2026
Web References: http://dx.doi.org/10.1093/mnras/stag002
Image Credits: NASA
Keywords
Kuiper Belt, contact binaries, planetesimals, gravitational collapse, New Horizons, solar system formation, computational simulation, binary planetesimals, outer solar system, planetary science
