Planetary Migration
We like to think of our solar system as being stable, and there is plenty of evidence that the Earth's orbit has not changed a lot in the several billions of years of life's existence. It was slightly shocking then, that extrasolar planets discovered to date don't necessarily seem to have behaved this way. In particular, a number of so-called hot Jupiters have been discovered - Jupiter mass planets in very close orbits around their stars. It seems unlikely that they could have formed there, since the environment wouldn't have been cool enough for rocks and metals to condense into planetesimals. Planetary migration had been considered in theory before the discovery of the hot Jupiters, and some migration is believed to have occurred in the outer planets of our own solar system, so the discoveries were hardly impossible to fit into our theories of planetary formation. The mechanisms are somewhat complicated, and include the interactions with density waves induced i...