Bold claim: the tiny worlds beyond Neptune aren’t just cold rocks — they’re shaped by the quiet, stubborn force of gravitational collapse. And this is where the story gets surprising: the familiar “snowman” silhouettes you see in the Kuiper Belt may form right at the very start, not from later drama after two bodies drift together. Here’s how new simulations shed light on a long-standing puzzle, with clear ideas and concrete numbers to help you follow along.
A common shape needs a common origin
Scientists have proposed many ways contact binaries — two bodies touching and sticking together — could form. Some ideas involve later events that push two separate objects into contact, while others point to gas drag, orbital resonances, or combinations that gradually tighten their orbits. But in a population as sizable as the Kuiper Belt, the mechanism behind such frequent bilobed shapes must not be a rarity.
“If we think 10 percent of planetesimal objects are contact binaries, the formation process can’t be a one-off fluke,” explains Seth Jacobson, Earth and Environmental Science professor and senior author of the study. “Gravitational collapse fits nicely with what we’ve observed.”
The study, led by Michigan State University graduate student Jackson Barnes and published in Monthly Notices of the Royal Astronomical Society, asks a simple but powerful question: can contact binaries arise directly during the initial collapse of a pebble cloud, before any later interactions take place?
Not a liquid blob collision
Earlier simulations treated colliding bodies as fluid blobs that merge into a sphere, a simplification that helps some problems but erases a key detail: solid objects can preserve their shape, lean against one another, and stay distinct after a gentle contact.
Barnes used a modeling approach that allows fine-grained contacts instead of forcing perfect mergers. He employed PKDGRAV, an N-body code, together with a soft-sphere discrete element method (SSDEM). SSDEM uses spring-and-dashpot forces to model how particles touch and slide against each other. In practical terms: rocks can bump, rub, and settle rather than instantly fuse.
Why this matters: the early “pebble cloud” story is central. Gravitational collapse gathers tiny solids into self-gravitating planetesimals, bypassing intermediate sizes that face growth barriers. As the cloud contracts, it spins faster. It can’t simply shrink into a single fast-rotating object. Instead, it can break into two near-equal partners, or even into multiple pieces.
The big question becomes: could this initial binary stage naturally evolve into contact — without an extra, later trigger?
A gentle inward spiral
To test this, Barnes and colleagues ran 54 simulations of collapsing clouds. Each cloud had the mass roughly corresponding to a 100-kilometer-scale planetesimal system. Rather than tracking countless tiny pebbles, the team represented the cloud with 10,000 “superparticles,” each about 2 kilometers in radius.
Across all the runs, roughly 3 percent of resolved planetesimals formed as contact binaries. The researchers identified 29 contact-binary planetesimals among a sample of 834, using a by-eye criterion: the object remained distinctly bilobate after contact. Of those, 24 showed a clear two-lobed form, while five were borderline with a less pronounced neck. Objects too small to resolve clearly were not counted.
Every single one of these contact binaries began as a gravitationally bound binary pair. During collapse, interactions with other bodies drained orbital energy from the system. The mutual orbits tightened until the pair collided.
Most collisions were mild. The vast majority occurred at speeds between 0.4 and 5.8 meters per second, with one outlier at about 16.9 meters per second. A cluster of impacts fell in the 2.9–5.0 m/s range — a band that geophysical and geomorphological studies have tied to Arrokoth’s lobes’ collision.
Barnes sums up the payoff clearly: “We’re able to test this hypothesis for the first time in a legitimate way. That’s what’s so exciting about this paper.”
Arrokoth, what the simulations do and do not match
Arrokoth (also known as 486958 Arrokoth) resides in the cold classical Kuiper Belt, far enough from giant planets to experience minimal dynamical change. Its surface shows a relatively modest crater record with ages similar on both lobes. Wenu and Weeyo lack strong albedo or color differences and share similar volatile inventories, which supports a shared origin and a gentle, post-contact history.
The simulations produced contact binaries with rounded, elongated lobes that resemble a subset of suspected primordial bilobed bodies found in the solar system. They also yielded post-contact spin rates typically between about 2.1 and 3.0 revolutions per day — below a commonly cited spin-breaking threshold of roughly 3.6 revolutions per day for equal-lobe-mass binaries with densities around 1 g/cm³. Arrokoth spins at about 1.51 revolutions per day (a 15.93-hour period).
The paper offers a reason for the discrepancy in spin: long-term cratering could slow Arrokoth through largely inelastic impacts that compact the surface rather than deeply excavating it. Collisions with hundreds of kilometer-scale Kuiper Belt objects could contribute to this slow-down.
Shape is another area where match quality depends on the model. No simulated binary perfectly matches a distinctly flattened shape that some Arrokoth shape models proposed. However, a newer shape model with rounder lobes aligns better with the simulated population, reducing the need to invoke drastic, post-formation reshaping.
Beyond a single case, the simulations hint at more complex family structures. Four modeled contact binaries ended up with orbiting satellites, and two appeared as satellites within multicomponent systems.
In short
- The “snowman” forms in the Kuiper Belt may emerge naturally from the earliest stages of gravitational collapse, without requiring later events.
- A realistic treatment of solid-body contacts (as opposed to smeared, merging blobs) is crucial to reproducing these shapes.
- The observed properties of Arrokoth are broadly compatible with a gentle formation and subsequent surface processing, though some details (like exact spin and precise shape) point to additional, later evolution.
Why this matters
If this early-collapse pathway holds up, it reframes how we think about the formation of small, bilobed worlds beyond Neptune. It suggests a surprisingly efficient channel to create contact binaries directly from the primordial pebble cloud, shedding light on why such objects are not rare. It also invites us to revisit other bilobed bodies in the solar system with fresh expectations about their origins.
What do you think? Does tying the bilobed shapes to the very start of collapse feel more convincing than theories that require later interactions? Could future data on Arrokoth’s surface, composition, or spin help us nail down when and how these lobes formed — and would you expect similar processes to operate in other planetary systems beyond our own?
