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Through the atoms of progenitor stars long since gone supernova. Probably neutron stars too!

Or also by chance?

I know tidal curves have increase the moons orbit, I’m struggling to think of anything (outside massive collisions) that would cause planets to move closer to their stars, or move to new ones.

Orbits can be quite unstable, for example read (one theory) or the early dynamics of the solar system: https://en.wikipedia.org/wiki/Nice_model

If two planets were to pass close enough together, their orbits could become destabilized enough to essentially fling them.

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