An American student has resolved the longstanding mystery of one our solar system’s strangest objects – cosmic ‘snowmen’ that populate its outer reaches.
Astronomers have long debated why the globular icy objects look the way that they do.
But researchers at Michigan State University now say they have evidence that a surprisingly simple process could explain how these shapes form.
Beyond the turbulent asteroid belt between Mars and Jupiter lies the Kuiper Belt, a vast region past Neptune filled with icy remnants from the birth of the solar system.
These ancient building blocks, known as planetesimals, have remained largely untouched for billions of years.
Around one in ten are “contact binaries” – objects made up of two connected spheres, reminiscent of a snowman.
How they formed without being smashed together by violent collisions has remained an open question.
Now Jackson Barnes, a graduate student at the university, has developed the first computer simulation to show how such two-lobed shapes can arise naturally through gravitational collapse.
This is the process by which matter contracts under its own gravity, overpowering forces that would otherwise pull it apart. The research has been published in Monthly Notices of the Royal Astronomical Society.
Earlier computer models treated colliding objects as fluid-like blobs that quickly merged into single spheres, making it impossible to recreate contact binaries.
But using high-performance computing facilities, Barnes’ simulations instead allow objects to retain their strength and settle gently against one another.
Other theories have suggested that rare events or exotic conditions might be required to produce these shapes, but researchers say such explanations are unlikely to account for their apparent abundance.
‘If we think 10% of planetesimal objects are contact binaries, the process that forms them can’t be rare, said earth and environmental science assistant professor Seth Jacobson, the study’s senior author.
‘Gravitational collapse fits nicely with what we’ve observed.’
Contact binaries were first seen in close detail in January 2019, when NASA’s New Horizons spacecraft flew past a Kuiper Belt object later nicknamed Ultima Thule.
The images prompted scientists to re-examine other distant bodies, revealing that about 10% of planetesimals share the same distinctive shape.
In the sparsely populated Kuiper Belt, these objects drift largely undisturbed and are rarely hit by other debris.
In the early history of the Milky Way, the galaxy consisted of a disc of gas and dust. Remnants of that era persist in the Kuiper Belt today, including dwarf planets such as Pluto, along with comets and planetesimals.
Planetesimals are among the first solid bodies to form as dust and pebble-sized material clumps together under gravity. Much like snowflakes compressed into a snowball, they are loose aggregates pulled from clouds of tiny particles.
Barnes’ simulation shows that as one of these clouds rotates, it can collapse inward and split into two separate bodies that begin orbiting each other.
Such binary planetesimals are commonly observed in the Kuiper Belt. Over time, their orbits spiral closer until the pair gently touch and fuse, preserving their rounded shapes.
The reason these fragile-looking structures survive for billions of years, Barnes explains, is simple chance.
In such a remote region, collisions are rare. Without a major impact, there is little to pull the two bodies apart, and many contact binaries show few, if any, impact craters.
Scientists have long suspected gravitational collapse was responsible, but until now they lacked models capable of testing the idea properly.
‘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.’
He believes the model could also help researchers understand more complex systems involving three or more bodies. The team is already working on simulations that better capture the details of the collapse process.
As future space missions venture deeper into the outer solar system, the researchers say the familiar snowman shape may turn out to be far more common than once thought.
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