Have you ever wondered how these strange floating “snowmen” manage to survive in the deep freeze of the Kuiper Belt outside the orbit of Neptune? These oddly shaped, double-lobed rocks called bilaterians, like the famous Arrokoth rocks, appear fragile, yet they have survived for billions of years without collapsing.
Astronomers have been searching for an answer throughout the ages.Enter Jackson Barnes, a sharp graduate student at Michigan State University. He built the first computer simulation proving that these exotic planetesimals form naturally from swirling pebble clouds that collapse under their own gravity. There’s no magic needed, just physics doing its work in the cosmic dust. Solve the puzzle.
How gravity created “iceman” worlds in space
Beyond the asteroid belt lies a frozen marvel: the Kuiper Belt, a large ring of icy remains from the birth of our solar system.
Among many of these are the “iceman” planetesimals, which are fragile, double-lobed contact binaries such as Arrokoth. These strange shapes, stuck together like cosmic snowballs, have puzzled astronomers. How could they survive billions of years without decomposing? For years, the mystery persisted. Then Jackson Barnes, a graduate student at Michigan State University, solved this problem. His pioneering computer simulations revealed that these objects form naturally from cobblestone clouds in the early solar system.
Gravity causes the clouds to collapse, naturally giving birth to these clumpy binary structures without the need for collisions. This breakthrough rewrites our understanding of planetary formation. It shows that gentle gravitational processes can sculpt resilient survivors in the frozen void, suggesting similar worlds orbiting other stars. The secrets of the Kuiper Belt continue to be revealed, one simulation at a time.
Michigan State University hack. Jackson Barnes is leading the charge
Researchers at Michigan State University (MSU) have uncovered a simple but elegant phenomenon behind it: gravitational collapse.
Graduate student Jackson Barnes has developed the first computer simulation showing how two-lobed “contact binaries” arise naturally from cobblestone clouds.Older models treated the colliding planetesimals as fluid-like blobs that coalesced into smooth spheres, and failed to recreate contact binaries. Taking advantage of high-performance computing, Barnes simulated objects that retain their structural integrity and gently settle upon contact.
Expert insights from Professor Seth Jacobson
“If we think that 10% of planetary objects are contact binaries, then the process that forms them cannot be rare,” said Seth Jacobson, assistant professor of Earth and Environmental Sciences at Michigan State University, and lead author of the paper. “Gravity collapse fits well with what we observed.”
The science behind floating “snowmen”: Understanding the gravitational collapse that defines the process
As the Dictionary of Astrobiology describes, gravitational collapse is “the collapse of a region of matter under the influence of its own gravity, for example, the dense core of an interstellar cloud on its way to becoming a star.”
This occurs when local self-gravity overwhelms restoring forces such as thermal gas pressure or turbulence.In protoplanetary disks, millimeter-sized pebbles are concentrated in a pebble cloud via flowable instabilities. Self-gravity then causes collapse, giving birth to planetesimals. Barnes’ model captures this precisely.
Real World Notes: Arrokoth and New Horizons
Contact binaries gained fame in January 2019 when NASA’s New Horizons spacecraft flew by one in the Kuiper Belt.
Dubbed Ultima Thule (which later officially became Arrokoth), the lobed “iceman” shape fascinated scientists. These globules are spread throughout the Kuiper Belt, neither shatter on impact nor collapse on their own, indicating gentle formation.
Details of the pilot simulations published in monthly notices
In the Monthly Notices of the Royal Astronomical Society, Barnes and colleagues detail 54 simulations of a primordial pebble cloud containing 105,105 particles, each with a radius of about 2 km (1.25 mi).
This low-resolution setting reflects reality, where real pebble clouds likely contain 10,241,024 millimeter-sized particles.
Key Findings: Orbital dance of the ascendant
The team found that in some cases, two small planets from the pebble cloud entered a common orbit. They gradually spiraled inward, reaching speeds of 5 meters per second or less before they made contact. Forming the double-lobed shape, upon contact, the particles realistically settle down, merging into a double-lobed planetesimal or “contact double”.
“Some of the contact binaries in our model bear a striking resemblance to Arrokoth,” Barnes noted.Previous gravitational collapse simulations had ignored the physics of particle contact, and were predicting collisions between smaller planets would result in a single spherical object. Barnes’ innovative model of how pebbles stabilize and stick explains the intact “snowmen” shapes.
Implications for the origins of the solar system
This work comes as a transformative insight into planetesimal formation.
Contact binaries, which make up 10% of Kuiper Belt objects, indicate that gravitational collapse in cobblestone clouds is common, producing “debris piles” that survive the ages. It is consistent with Arrokoth’s low-density, compact structure observed by New Horizons.Similar shapes appear among near-Earth asteroids, meaning this process works across the entire solar system. Future missions could test these predictions.
Simulations and future observations
High-resolution cobblestone cloud models, powered by advanced computing, promise deeper insights. Telescopes like the James Webb Space Telescope may discover more contact binaries in distant disks in the coming days.Jackson Barnes’ simulation not only solves the “snowmen” mystery, it redefines how planetesimals, and eventually planets, emerge from cosmic dust.