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# how did the planets get their positions

Posted on 2012-03-28
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Hi

I'm not physics expert so I would appreciate an answer in simple terms please.

As i understand it the force of gravity on the planets is lower the further away you get. The mass of the planets has no real affect on the gravity as they are so small relative to the sun.

My understanding of planetary orbits is limited to a basic understanding on the cannonball model such that planets are orbiting when the force of gravity balances their momentum. So were the planets travelling at different speeds towards the sun and then 'stopped' (in terms of distance from the sun) at the distance from the sun where the gravity matched their momentum (from their speed) or is there more to it than that. I suppose the question is which came first: the speed of the planet or its distance from the sun.

Did their speed determine their distance from the sun or did their distance from the sun determine their speed? If it is the latter I don't understand how the planets got their positions in the first place.

Thanks
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Question by:andieje
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LVL 27

Expert Comment

ID: 37779182
It is much more complicated than that.
But this gives a good outline....

http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System
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Expert Comment

ID: 37779253
The angular momentum of the cloud is conserved in the rotation of the sun and the revolution of the planets around the sun.

The sun forms first, gas collects and then ignites (via gravitationally confined nuclear fusion).

The remaining gas and dust form a disk around the sun, with heavier elements closer in and lighter ones further out.

Irregularities in the disk allow mass to concentrate to form planets.  The planets eventually sweep up all the mass in their radial region.

Note that the rocky (heavy) planets are close to the sun and the gas (light) planets are further out.

Note that rocky/icy Pluto is no longer considered a planet.
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Accepted Solution

d-glitch earned 500 total points
ID: 37779266
Once the sun has formed, and the mass of the sun is relatively fixed, the orbital velocity can be calculated as a function of distance from centripetal acceleration.

So the distance from the sun determines the speed.
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Author Closing Comment

ID: 37779506
thankyou
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