Far beyond Neptune, at the cold edge of our solar system, millions of icy objects drift quietly in a region called the Kuiper Belt.
These frozen leftovers from the birth of the solar system are known as planetesimals—small building blocks that never grew into full planets.
For years, astronomers have puzzled over a strange feature of many of these distant objects: a surprising number look like snowmen, made of two round lobes stuck together.
Scientists call these objects “contact binaries,” meaning two separate bodies that gently merged and remained attached.
About one in ten Kuiper Belt planetesimals appears to have this shape.
The mystery has been how they formed without smashing into each other and shattering apart.
Now, researchers at Michigan State University believe they have found a simple explanation. Graduate student Jackson Barnes created the first computer simulation showing how these snowman-like objects could form naturally through a process called gravitational collapse.
His findings were published in the journal Monthly Notices of the Royal Astronomical Society.
Earlier computer models treated colliding space rocks like blobs of liquid that would merge into a single sphere. That made it nearly impossible to explain how two distinct lobes could remain.
Barnes used powerful supercomputers to build a more realistic simulation in which the objects behave like solid bodies that can keep their shape and settle against each other.
The process begins in a cloud of dust and pebbles left over from the early solar system. Gravity pulls this material together, forming a rotating clump.
As the clump collapses inward, it can split into two separate bodies that end up orbiting one another as a pair. Over time, their orbits slowly shrink until the two objects touch and fuse gently, preserving their rounded shapes instead of flattening into one.
Images from NASA’s New Horizons spacecraft helped spark renewed interest in these unusual shapes. In 2019, the spacecraft flew past a Kuiper Belt object named Arrokoth, which looked strikingly like a flattened snowman.
After that discovery, astronomers reexamined other distant objects and realized contact binaries are surprisingly common.
Because the Kuiper Belt is so sparsely populated, these merged pairs can survive for billions of years without major collisions that would break them apart. Many of them even show few impact craters, suggesting they have remained largely undisturbed since the solar system formed.
Scientists had long suspected gravitational collapse might be responsible, but until now they lacked the tools to test the idea properly. Barnes’s simulation is the first to include the detailed physics needed to recreate the process realistically.
Researchers hope this new model will also help explain more complex systems involving three or more connected objects.
As future space missions explore the outer solar system, astronomers expect to discover many more of these cosmic snowmen.
What once seemed like a strange coincidence may turn out to be a natural outcome of how planets—and the smaller bodies around them—first came into existence.
