Deep within the Kuiper belt, some small worlds look like they were assembled from two mismatched snowballs pressed together. The poster child is Arrokoth, the “contact binary” visited in 2019 by NASA’s New Horizons spacecraft. Its twin lobes share similar colors and volatile ices, and its surface shows relatively modest cratering.
What still needed tightening was the how. Did Arrokoth start as two separate bodies that spent eons spiraling together under later nudges such as gas drag, orbital resonances, chance encounters, or did it emerge already “two-lobed,” shaped during the solar system’s formation itself?
A new paper in Monthly Notices of the Royal Astronomical Society argues the simplest answer may work: contact binaries like Arrokoth can form directly during the gravitational collapse of a dense cloud of pebbles in the Solar System’s protoplanetary disk.
“When we first saw the results of our simulations, we were very excited,” Jackson Barnes, a graduate student at Michigan State University (MSU) and lead author of the paper, told Scientific American.
A bizarre relic from the dawn of our solar system
The international study, led by MSU, focused on a long-running idea in planet formation. In the early solar system, when most planets had yet to form, millimeter-to-centimeter “pebbles” could clump together through processes tied to gas–solid interactions, then collapse under their own gravity into kilometer-scale planetesimals.
This avoids growth stages where sticking gets inefficient. Earlier collapse models often produced binary pairs: two bodies that form together and remain separated. The new study focuses on whether collapse can also yield contact binaries — pairs that end up touching — without needing a long later history to bring them together.
Using 54 numerical simulations, the researchers found that contact binaries do appear as a natural outcome of collapse. Across the simulations, the authors identified 29 contact-binary planetesimals out of 834 sufficiently resolved bodies, about 3%. Many showed clear bilobate shapes, and some looked strikingly Arrokoth-like in overall proportions.
Equally important is how gently the lobes come together. Observations and modeling of Arrokoth’s geology have long suggested a low-speed union, so more of a slow drift into contact than a crash.
The new simulations seem to agree, as nearly all contact events occurred at very low relative speeds, typically below approximately 20 feet (six meters) per second.
If Arrokoth didn’t need a long chain of outside nudges to bring its two halves together, then what did the job? In these simulations, the answer is surprisingly straightforward: the “closing scene” can play out right inside the collapsing swarm. In the early years of a solar system, two clumps often form as a natural pair early on, circling each other while everything around them is still messy and crowded. As other newborn bodies drift past, their gravity tugs on the pair just enough to change the pair’s motion. Over time, those repeated flybys act like gentle taps that shrink the gap between the two partners—until the lobes finally meet and stick.
The spin of these objects gives another reality check. A two-lobed body can’t whirl too fast without pulling itself apart, and the simulated contact binaries usually end up rotating at safe speeds. Even in the far-off Kuiper Belt, where collisions are rare, tiny impacts over immense stretches of time can still nudge an object’s spin, little by little.
One mystery hanging out there is how many Arrokoths are floating in the void. Telescope surveys hint that two-lobed “snowman” shapes might be fairly common in some Kuiper Belt groups, but the numbers are slippery. From Earth, astronomers usually can’t see the shape directly. They infer it from how the object’s brightness rises and falls as it rotates, though brightness pattern can be misleading if the object is angled toward us.
“This was something that had been hypothesized ever since the flyby of Arrokoth in 2019,” Barnes said. “We’re rewarded with a variety of shapes including contact binary shapes just like Arrokoth.”