For decades, astronomers have tried to understand why so many icy bodies in the outer solar system resemble snowmen, with two rounded sections joined together. Researchers at Michigan State University now report evidence pointing to a surprisingly straightforward process that can explain how these unusual shapes form.
Beyond the turbulent asteroid belt between Mars and Jupiter lies the Kuiper Belt, a distant region past Neptune filled with frozen remnants from the solar system’s earliest days. These primitive objects, known as planetesimals, are leftover building blocks from planet formation. About 10 percent of them are classified as contact binaries, meaning they consist of two connected lobes that give them a snowman-like appearance. Until recently, scientists did not know how such forms could develop naturally.
New Simulation Supports Gravitational Collapse
Jackson Barnes, a graduate student at MSU, developed the first computer simulation capable of naturally producing these double-lobed structures through gravitational collapse. His findings were published in the Monthly Notices of the Royal Astronomical Society.
Previous computer models simplified impacts by treating colliding bodies as if they were fluid masses that blended into smooth spheres. That assumption prevented researchers from recreating the distinctive two-part shape seen in contact binaries. Using the high performance computing cluster at MSU’s Institute for Cyber-Enabled Research, or ICER, Barnes created a more realistic digital environment. In his model, forming objects retain their structural strength, allowing them to settle against each other rather than merge into a single sphere.
Some earlier explanations relied on rare cosmic events or unusual conditions. While those scenarios are possible, they would not easily explain why such objects are relatively common.
“If we think 10 percent of planetesimal objects are contact binaries, the process that forms them can’t be rare,” said Earth and Environmental Science Professor Seth Jacobson, senior author on the paper. “Gravitational collapse fits nicely with what we’ve observed.”
NASA New Horizons and the Kuiper Belt
Contact binaries gained widespread attention when NASA’s New Horizons spacecraft captured close-up images of one in January 2019. The images led scientists to examine additional Kuiper Belt objects more closely, revealing that about one in 10 planetesimals share this shape. In the sparsely populated Kuiper Belt, these distant bodies drift with relatively few collisions, allowing fragile structures to survive.
The Kuiper Belt itself is a relic of the early Milky Way, when the galaxy existed as a rotating disc of gas and dust. That ancient material still lingers in this region, including dwarf planets like Pluto, comets, and countless planetesimals.
How Planetesimals Form and Merge
Planetesimals were among the first sizable objects to form from the swirling disc of dust and pebbles that surrounded the young Sun. Similar to how snowflakes stick together to build a snowball, tiny particles were drawn together by gravity into larger clusters.
As these rotating clouds collapsed, they sometimes split into two separate bodies that began orbiting one another. Astronomers frequently observe such binary planetesimals in the Kuiper Belt. In Barnes’ simulation, the pair gradually spirals inward. Instead of colliding violently, the two bodies gently come into contact and fuse, preserving their rounded shapes and creating the familiar snowman form.
Why Contact Binaries Survive
Once joined, these objects can remain intact for billions of years. According to Barnes, their long-term stability comes from the low chance of further impacts. In the remote Kuiper Belt, collisions are rare. Without a disruptive crash, there is nothing to separate the two lobes. Many binary objects even show few craters.
Although scientists had suspected gravitational collapse played a role in forming contact binaries, previous models lacked the detailed physics needed to test the idea thoroughly. Barnes’ work is the first to include the necessary processes to successfully recreate them.
“We’re able to test this hypothesis for the first time in a legitimate way,” Barnes said. “That’s what’s so exciting about this paper.”
Barnes believes the model could also help researchers study more complex systems involving three or more connected objects. The team is currently developing an improved simulation to better represent how collapsing clouds behave.
As future NASA missions continue to explore distant regions of the solar system, Jacobson and Barnes expect that even more snowman-shaped worlds may be discovered.
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