Arrokoth formation simulations favor gentle pebble-cloud collapse over violent impacts

NASA’s 2019 flyby image of Arrokoth—an ultra-red Kuiper Belt object shaped like a snowman—has long been treated as a postcard from the Solar System’s infancy. Now, new computer simulations bolster the increasingly unfashionable idea that some of that architecture emerged from gentle physics rather than cinematic collisions.

The Guardian reports that researchers led by Jackson Barnes at Michigan State University ran 54 simulations aimed at reproducing “contact binaries” like Arrokoth: bodies composed of two lobes that appear to have formed together and later merged at low speed. Astronomers estimate 10–25% of Kuiper Belt planetesimals are double-lobed.

In the favored scenario, Arrokoth formed inside a primordial “pebble cloud” in the protoplanetary disk beyond Neptune. Instead of incremental growth through violent impacts, gravity causes the cloud to collapse into clumps—small planetesimals that can end up in mutual orbit and spiral together. Barnes’ team reports that in some runs, two bodies merged at velocities of about 5 meters per second or less, producing shapes “strikingly like Arrokoth.” The work appears in Monthly Notices of the Royal Astronomical Society.

A key technical claim is methodological: earlier gravitational-collapse simulations often produced a single rounded body because they did not model how particles rest and pile when they touch. Barnes’ group incorporated contact physics, allowing aggregates to preserve lobes rather than “relax” into a sphere. Alan Stern of the Southwest Research Institute, principal investigator for New Horizons, welcomed the result as consistent with the view that Arrokoth is a product of “gentle formation processes,” the Guardian writes.

But even in a field that runs on inference, the devil sits in the numbers. The Guardian notes the simulations used a pebble cloud of 10^5 particles, each around 2 km in radius—an admitted low-resolution stand-in for a hypothesized real cloud of ~10^24 millimeter-sized pebbles. That is not a minor detail: when you coarse-grain by 19 orders of magnitude, “physics” becomes “model choice,” and model choice becomes narrative.

There is also a tension between simulated and observed fractions. Queen’s University Belfast astronomer Alan Fitzsimmons cautions that the new simulations yield only about 4% contact binaries, while telescopic surveys imply a higher proportion. That gap is not merely academic; it determines whether pebble-cloud collapse is a niche pathway or the dominant assembly line.

Falsification would not come from another press release about “agreement with previous work,” but from constraints that force the model to fail: distributions of lobe mass ratios, neck geometries, spin states, internal porosity, and crater retention ages across a statistically meaningful sample of Kuiper Belt objects. Arrokoth is one data point—glorious, but still one.

Centralized storytelling is cheap; decentralized measurement is hard. Planet formation models, like policy models, survive by absorbing anomalies as “complexity.” The antidote is more independent observations—more flybys, better occultation data, and less reverence for simulations that can be tuned until the universe politely agrees.