@logy5000 said:
@Mirabel: If that's the case, why don't large asteroids knock planets out of their orbit?
Because the asteroids around today don't have anywhere near enough force to provide enough excess force to move a planet noticeably, the planet will be moved yes, but on an astronomical scale it's only a tiny nudge at best. The ones big enough to hit with that much force at standard orbital velocities are ridiculously far away in the kuiper belt, scattered disc, and oort cloud or collided with other planets or the sun long ago in the formative period of the solar system. The ones that move fast enough to hit with this much force without being planet sized in of themselves are not native to our solar system and our chances of bumping into one of those are so low as to be nearly nonexistent. You'd have more chance of getting hit by every bolt of lightning a thunderstorm has to offer at once than of either event happening.
When the Solar system was young and there were hundreds or maybe even thousands of planet sized objects, it was essentially one big game of cosmic pinball played out over millions of years until only the present eight planets, the remaining dwarf planets, and large moons were left. They were knocking into each other, yanking each other out of orbit with gravity, and there were a crap ton of smaller asteroids everywhere too. Essentially it was all one big clusterf*ck. During this period, yes you definitely could have impacts that would knock a planet's orbit around, and they'd be quite common. But these days the Solar system is too neat and tidy for this to occur anymore, which is a good thing because asteroids as big as the dinosaur killer hit Earth during it's first few million years of life at least once a minute, not to mention metric buttloads of smaller and bigger asteroids that would pummel the crust into molten slag within hours.
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