Pluto is a weird little thing, isn’t it?

Yes, it is. And we’ve known this ever since it was discovered in 1930. At first astronomers thought it was a big planet, since they also thought it was discovered due to its gravitational influence on Neptune. That turned out to be incorrect (it was discovered essentially by accident; if you want to know more, including what it would be like to actually stand on the surface of Pluto, I suggest reading my book Under Alien Skies), and in fact in the ensuing decades it turned out Pluto was far smaller than expected. It’s only about 2,400 kilometers in diameter — much smaller than our own 3,500-km-diameter Moon. It orbits the Sun on a highly elliptical and tilted orbit, and while it was thought to be the smallest of the major planets it’s far more like the billions of other icy, rocky bodies that orbit the Sun out past Neptune.

It also has several moons, the largest of which is Charon, and it is large indeed… at least compared to Pluto. At a little over 1,200 km in diameter, Charon is half the width of Pluto itself, the highest known ratio by far of any large objects in the solar system (our Moon is about one-quarter Earth’s diameter).

This makes Charon a mystery. How did it form?

It’s possible Charon was captured by Pluto. It may have been an independent body that wandered too close to Pluto and got snagged by the wee minor planet’s gravity. This sort of thing is possible but only under special circumstances; as I wrote in a recent issue of BAN it can occur if the incoming object is actually a binary; one of the two bodies is ejected from the system while the other is captured.

However, Pluto and Charon have a lot of similarities (such as their densities) that makes it more likely they are the result of a parent body (protoPluto, if you will) that got whacked by a smaller object. The collision would have blasted a lot of material into orbit around Pluto, which would then coalesce over time to form Charon. This is a very similar idea to how we think Earth’s Moon formed, in fact. It’s very common, even in the outer solar system, so it’s a good place to start when thinking of formation mechanisms.

This sort of event can be simulated in computers using the known physics of collisions. However, most simulations have a hard time reproducing the current Pluto-Charon system. The models don’t get things right, like their current separation (about 20,000 km, or 8 times Pluto’s diameter) or their compositions. That last part is a good test of a model. For example, Pluto is big enough to have decent gravity, and that means it’s differentiated: heavier materials like rock and metal sank to its center while ice floated to the surface after it formed. A grazing collision would create debris lower in rock and higher in ice, which is something the models should be able to reproduce.

Critically, the sims also rely on the material strength of Pluto. If the upper layers of Pluto are made of tougher material like rock the debris generated will be different than if they’re made of ice. The actual strength hasn’t been well known, limiting the sims. Previous simulations have also used the same physics in the giant collision that formed our Moon, but the situation there was different. Earth and its impactor were far larger, and the velocity of the collision much much higher than what you’d expect for something to hit Pluto, so the physics is different.

Earlier simulations assumed Pluto and its impactor to be strengthless, like a fluid. A team of scientists made a new simulation, however, that used reasonable estimates for the material strength of both Pluto and the impactor to see what would happen [link to journal paper]. What they found is fascinating.

Here’s a video showing their results, which I explain below.

Instead of a giant impact blasting small chunks of debris everywhere around Pluto as you’d get in a strengthless collision (like throwing a stick of dynamite in a box full of feathers), the new simulation shows that the two bodies remain largely intact after they slam into each other. A lot of the energy of the collision is absorbed by the two bodies, so they don’t shatter. Then, the impactor and Pluto merge temporarily, forming a dumbbell-shaped object. About 2.5 hours after impact they stretch apart a bit from the rebound, but by 5 hours they come back together to form something like a squat bowling pin. This is where things get interesting.

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