We don’t know for sure how any of this stuff formed from the solar nebula in the first place. One currently favored hypothesis is pebble accretion. I won’t go into it here. Augusto Carbillado wrote an excellent blog about the process of planetary accretion for us a few years ago, if you want to learn more.

A recent paper by David Nesvorný, Joel Parker, and David Vokrouhlický that explains how 67P/Churyumov-Gerasimenko may have formed could also apply to MU69. Summarizing other peoples’ work, they wrote that the comet may initially have been a separated binary that formed when a clump of pebbles collapsed under its own gravity. Simulations show that this works physically, and moreover that it’s very common to end up with a binary pair where one object is about 75% the diameter (and therefore about half the mass) of the other. That ratio is common in the Kuiper belt, and is the ratio of the two lobes of 67P. Lo and behold, it doesn’t look far off of the value for MU69, either.

So how does a separated binary become a contact binary? One thing that needs to happen, obviously, is that the two components have to get closer together. In physics terms, their orbit needs to destabilize. Nesvorný and coauthors say the orbit could destabilize in several ways. Small impacts on either object could cancel out some of their orbital energy, allowing them to draw closer to each other over time. The same process might have the opposite effect on other binaries; random chance would separate some wider, while it would bring others closer. If the binary orbit was significantly tilted with respect to the plane of its orbit around the Sun, a kind of orbital dynamic called Kozai cycles could draw them closer together.

Nesvorný, Parker, and Vokrouhlický find that it’s quite possible to collapse binaries extremely gently. They touch at about 80 centimeters per second. That’s slower than human walking speed. I looked around for another creature (besides “a child”) that walks at 80 centimeters per second, and thanks to Michele Bannister I know that geckos climb vertically at 77 centimeters per second. So the two members of a contact binary collide with the speed of a gecko climbing a wall. (That’s adorable.)