Scientists have long debated how the Kuiper Belt’s “snowball” world formed. Did two separate bodies collide at just the right speed and stick together, or did they originate as a pair from the beginning?
A new computer simulation shifts the balance toward a simpler answer. This suggests that two-lobed Kuiper Belt objects can form when a collapsing debris cloud splits into two and then slowly snaps back together due to gravity alone.
If that’s true, the most famous example of the contact binary may not be a lucky accident at all, but a direct fossil of its early collapse.
Arrokoth sets the standard.
Arrokoth, a small two-lobed object explored by New Horizons in 2019, clearly preserves that process.
Works at Michigan State University (MSU), Jackson Barnes demonstrated that a rotating cloud can collapse and separate into two joined lobes, which can then be brought back into contact without shattering either.
Rather than melting into a single sphere, the paired objects settle together at low speed, holding round halves connected by a narrow neck.
If that path is maintained, many similar objects kuiper belt They may have already been born combined, making their shape a direct record of planet formation rather than the product of a later chance encounter.
A place where the world of snowmen continues
Beyond Neptune lies the Kuiper Belt, a wide region of icy debris where weak sunlight keeps the surface cool. With fewer approaches and collisions, many objects were able to avoid the shape-changing shattering of asteroids closer to the Sun.
Scientists call many of these clumps planetesimalthe early building blocks that later solidified into planets and moons. This area remains so calm that the delicate two-lobed form can persist long enough for telescopes to spot it.
In 2004, careful analysis suggested that many Kuiper Belt objects are actually stuck pairs. Astronomers call these contact binaries “contact binaries.” The two rounded protrusions meet at a narrow neck that preserves a record of how they were formed.
Approximately 10% of known planetesimals appear to fall into this category. That is, the processes behind them should be routine.
Its everyday origins can probably be traced back to the earliest collapse events, when surrounding clouds were still shaping the evolution of the pair.
Gravity binds the lobes together
Dust and ice swirled long before the planet cleared its orbit. protoplanetary disk Around the young sun. As the clumps became denser, gravity pulled the pebbles inward, while repeated collisions lost energy and slowed them down.
During compression, the rotation became faster and some of the masses split into pairs that circled around each other instead of forming one body. But to become contacts binarythose pairs still had to lose orbital energy.
In Burns’ model, nearby debris pulled on each pair, exchanging energy until the orbits stiffened. Each nudge steals a small motion from the binary and passes it to the passing material in the same cloud.
Eventually, the lobes made contact at low speed and fused at the neck without breaking into a single mass. Because the contact was gentle, heating was minimized and volatile ice was allowed to remain on the surface.
Why did older simulations miss it?
For years, planet formation models may have been too tidy. Most previous simulations treated colliding particles like soft clay, merging them into a single smooth mass.
This shortcut erased the very feature scientists were trying to explain: the narrow waist between the two lobes.
Burns took a different approach. Instead of blending particles, we used a discrete element method that allows particles to push, slide, and bounce realistically.
Small frictional forces and small bounces create a natural pileup, keeping the seams visible instead of rounded.
“That’s what’s so interesting about this paper,” Burns said after the updated code finally allowed him to distinguish between the lobes.
Mathematically, a gentle merge is advantageous
When the team ran the simulation dozens of times, a clear pattern emerged. Contact binaries formed only when two lobes approached each other at less than about 13 miles per hour.
Even at such low speeds, the bodies did not shatter or melt into a single sphere. They gently settled together, keeping their rounded halves intact and connected by a thin neck.
After the merger, many of the simulated objects rotated once every 8 to 12 hours. This was strikingly similar to the slow rotation rates astronomers observed in the Kuiper belt.
This match allows the decay path to be tested. If the future telescope If the study finds clusters with similar rotational speeds, it would strengthen the theory that gravity, rather than rare collisions, built these Kuiper belt worlds.
Gravity builds the Kuiper belt family
The simulation also produced some surprises. In some runs, a third object remained in orbit around the newly formed pair. Instead of creating just a binary, small systems could also be built through gravitational collapse.
During their chaotic inward fall, gravity can trap extra objects into stable orbits, suggesting that the same process could explain the triplex structures and even more complex groups already seen in the Kuiper belt.
If this idea is correct, a single collapse event could form not just one object, but an entire series of icy objects.
Test the role of gravity
Even with better impact models, the simulations could not reproduce all the details seen in Arrokoth’s large lobes. Computers still had to bundle countless pebbles together to form large stand-ins, which meant that the code was smooth across most of the microscopic surface textures.
Later changes may also have had an impact. Billions of years of slow collisions may have subtly tweaked the object’s rotation and reshaped parts of its surface without disturbing its basic two-lobed structure.
However, once gravitational collapse is already capable of producing contact binaries on its own, those additional processes move from necessary materials to optional purification.
This change points to a simpler origin story. One new simulation set ties this commonality together outer solar system In the Kuiper belt, normal gravity changes shape rather than rare, finely tuned collisions.
As telescope fields get sharper and simulations become more sophisticated, scientists will be able to determine how often gravitational collapse creates these pairs of worlds, and how many have endured largely unchanged since the early days of our solar system.
The research will be published in a journal Royal Astronomical Society Monthly Notices.
Image credit: NASA
—–
Like what you read? Subscribe to newsletter We bring you fascinating articles, exclusive content, and the latest updates.
Please check it out earth snapThis is a free app provided by. Eric Ralls and Earth.com.
—–