Something I understand really poorly: why the planets are so close to all being in the same plane.
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My first thought was this as well, but I couldn’t initially think of a source of dissipation.
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Replying to @michael_nielsen
A good general heuristic for understanding this (and also many other astrophysical phenomena such as e.g. why Saturn's rings have a thickness measured in meters, or are almost perfectly circular) is that it is easy to dissipate energy, and hard to dissipate angular momentum.
I immediately thought of the explorer 1 mission, which failed because it was spin-stabilized about the long axis, lost energy due to structural losses in flexible parts, and went into a flat spin. Solid body example of same effect.
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Inelastic collisions (which will dissipate energy through thermal radiation, among other effects).
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