...When Jupiter Aligns with Mars...

The equations of motion that govern the Solar System are chaotic. So a kind of important question is that of whether planetary orbits are stable. The best available answer seems to be maybe.

When simulations are run for a few billion years, some funky stuff sometimes - but only rarely - happens. Sometimes Mercury collides with Venus. Sometimes with the Sun - or, more usually, neither. Sometimes Mars gets ejected from the Solar System.

Gravitational micro-lensing observations seem to show that there are a certain number of orphan planets out there, planets that apparently formed in stellar systems but subsequently hit the road after getting ejected from their birth systems. There are even hints that such an event might have occurred early in the history of our Solar System. One puzzling event of early solar system history is the so-called Late Heavy Bombardment. One theory sanctioned by dynamic simulation is that this asteroid bombardment, some 300-500 million years after the formation of the planets, might have been due to the shuffle accompanying the ejection of a large planet from the system. Versions of the Solar System with another substantial planet seem to be unstable.

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