I swear, if and when my Wikipedia Solar System project is finally finished, I will give each and every one of you a big hug for all the help you've given me in the close-on three years I've been slogging through it. But for now I'm still stuck.

This is a quote from the May issue of Scientific American, about the formation of planets. This is discussing the formation of a gas giant like Jupiter:

I don't get this. How can gravity cause something to move away from something else?

Also, I can't get the whole "nebula" thing straight in my head. Should the giant molecular cloud be referred to as a nebula? Or should that term only be applied to the small fragment (the pre-solar nebula) that became our Sun? To make things even more confusing, some articles refer to the pre-solar nebula as the "solar nebula", while others give that name to the disc of gas and dust that formed the planets.

I don't get it.

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It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

To use the rubber sheet analogy, imagine Jupiter on the sheet.
it creates a dent in the sheet.
Now, roll another object on the sheet so that it "bums" against the edge of the dent made by Jupiter.

Because nature balances her books, as the small object hits that dent, it will take a tiny portion of Jupiter's momentum in the form of an acceleration. As it moves, with that acceleration from the dent- it will have been "bumped" by Jupiters gravity well.

I don't get this either.
The phenomenon may be somewhat poorely explained in the article .
Sure Jupiter is capable to clear a gap in a molecular cloud or asteroid belt around him .
Sometime ago I made a simulation of Jupiter , originally surrounded by lots of space debris .
The effect of Jupiter may be seen here :
http://www.orbitsimulator.com/cgi-bi...num=1189853529
( see reply nr #7 in this topic )

Ah. I think I understand what you're saying; it's a slingshot effect, yes?

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I think fish is nice, but then I think that rain is wet, so who am I to judge?

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

Like a lot of slingshots in succession, yes. It's the same mechanism as ring shepherding.
Stuff interior to Jupiter overtakes Jupiter in its orbit, and they exchange energy gravitationally: Jupiter slows the stuff down, the stuff speeds Jupiter up.
Stuff outside Jupiter's orbit is overtaken by Jupiter: Jupiter speeds the stuff up, the stuff slows Jupiter down.
The interior stuff, losing energy, moves into a lower orbit; the outer stuff, gaining energy, moves into a higher orbit. Jupiter mediates the exchange and stays pretty much where it is, unless there's a density gradient in the disc.

Grant Hutchison

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.

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