Quote:
Originally Posted by parallaxicality
I don't get this. How can gravity cause something to move away from something else?
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Our moon drifts
away from Earth, at a measured rate of about 1.5 inches per year. Imagine a string that connects Earth to the moon. As Earth spins on its axis once every 24 hours, but the moon moves around Earth once every 29 or so days, Earth will whip the moon forward by the tension in the string. Well, the string is gravity. Earth transfers energy to the moon through the gravitational field, the result being a gain of energy by the moon and a loss of energy from Earth. The moon responds by moving into a higher energy orbit farther from Earth, while Earth responds by slowing its spin rate by about 0.0015 seconds per day per century (see
The Recession of the Moon and the Age of the Earth-Moon System).
Now transfer this idea to something else you have heard of: shepherd satellites in the rings of Saturn. The ring material inside the shepherd satellite, closer to the planet, moves faster than the shepherd does. It whips the shepherd in the same way, transferring energy to the shepherd via gravity, just as Earth transfers energy to the moon. The inner ring material responds by dropping down into a lower energy orbit, falling away from the shepherd. The shepherd responds by moving up into a higher energy orbit, moving away from the inner material. Now do the same thing for the material outside the shepherd, which moves slower than the shepherd does. Now it's the shepherd which transfers energy to the outer material, with similar results; the shepherd falls back, the outer material move out. So the shepherd acts as a gravitational catalyst, transferring momentum in a way that will trap material between two shepherds, or clear a gap around one shepherd.
Now replace Saturn's rings with the accretion disk around the sun, and the shepherd with a planet. Same thing, the planet clears a gap in the disk. The result is that the planet stops growing (it has cleared a gap around itself), and the planet will stop migrating because the gap it cleared eliminates viscous drag.
For a more detailed explanation, look up the book
Solar System Dynamics by Murray & Dermott, Cambridge University Press 2001, chapter 10. In general, this is the book I recommend you check out first for questions relating to the title topic, although it does not address the topic of planet or solar system formation.