Technically, these objects are known as contact binary planetesimals. Planetesimals include asteroids, comets, and Kuiper belt Objects (KBO’s). Since the imaging of Arrokoth in 2019, the term “snowmen” has become common when referring to these binaries collectively. They don’t all look like snowmen, but they share the features of being roughly spherical objects held together by gravity and cohesive forces.
The existence of contact binaries was long suspected, and finally confirmed by Japan’s Hyabusa flyby of Itokawa in 2005. Astronomers believed that the same process that formed the planets (accretion) should still be happening in regions sufficiently populated by planetesimals — like the asteroid belt and Kuiper belt. But they had no idea how rare (or common) contact binaries might be.
For a contact binary to form, the two progenitors must impact at a gentle relative speed that depends on their masses, compositions, and trajectories. If they hit too hard, they both get pulverized into smaller particles that scatter in all directions. The most likely accretion event would looks something like this:
The animation was central to a previous post, back when Arrokoth was first imaged and provisionally named “Ultima Thule.”
Only 30 planetesimals have been directly imaged to date. Most remain just dots a few pixels in diameter. So it’s interesting that 4 of those turn out to be contact binaries. Computer modeling had suggested that around 10% would be contact binaries, but the data we currently have shows 13% are binaries. It’s a statistically low-σ measurement, but still an intriguing find.
The significance of this research is that planet-building may still be happening in our solar system, and that informs theories about how the planets already here were formed. That knowledge carries over to exoplanets and systems around distant stars, and ultimately impacts the parameter fₚ in the Drake Equation.
Of course, once the accreting protoplanet reaches a sufficient size, details of the impact becomes less relevant — the smaller impactor will more easily be assimilated by the larger body, growing the planet.
Are there any 3-tier snowmen out there? The linear accretion of three spherical planetesimals in decreasing order of size would require very precise dynamics — two are relatively easy, three are nearly impossible. But astronomers estimate there are trillions of planetesimals out there, so maybe some day Frosty will come dancing into our view.
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