# Phenomenon of rotation of planets

1. Jan 25, 2012

### dpa

is the phenomenon of rotation of planets best explained based on frame dragging due to sun?

If so does universe rotate as a whole. What causes it.

I am really beginner. Forgive my ignorance.

2. Jan 25, 2012

### netgypsy

When you think the state of planets as they were formed it would be most surprising if they weren't rotating.

3. Jan 25, 2012

### dpa

do you mean dust cloud swirling around itself to give sun, and planets.
Does a satellite rotate as well if placed freely in a orbit of earth or sun.Should it not.

4. Jan 25, 2012

### Drakkith

Staff Emeritus
Yes, the gas and dust cloud had angular momentum as it collapsed which was conserved as the sun and planets formed.

Frame dragging from the sun is very weak and isn't noticeable for objects this far away from the Sun.

5. Jan 25, 2012

### netgypsy

There's no friction in empty space so any rotational motion and translational motion an object has from a launch (and in formation from highly energetic particles) would continue unless it could be corrected as in the space station or satellite that has on board engines to allow adjustments remotely.

6. Jan 25, 2012

### D H

Staff Emeritus
Simple, but wrong. That's not how planets form.

Friction is the answer. The planets didn't form in empty space. They formed in a protoplanetary disk.

Imagine a little spec of dust in the disk orbiting more or less circularly at some distance r from the protosun. As this spec has negligible mass, its orbital velocity is $\sqrt{GM_{\odot}/r}$. Now imagine a protoplanet of mass m orbiting more or less circularly at the same distance. Its orbital velocity is $\sqrt{G(M_{\odot}+m)/r}$. The protoplanet orbits a tiny bit faster than the dust co-orbiting with it.

This means the protoplanet will sweep through the dust that forms the bulk of the protoplanetary disk and pick up mass as it goes. That dust through which the protoplanet is passing slows the planet down a bit ("friction"), dropping the protoplanet into a slightly lower orbit. (Note: This is the currently accepted explanation for those hot jupiters astronomers have found.) Over time this makes for a non-uniform density profile of the dust. There's more dust sunward than antisunward. This uneven density profile exerts a torque on the protoplanet. This is a very apropos explanation of why the gas giants are rotating, but not quite as apropos for the rocky planets.

7. Jan 25, 2012

### netgypsy

Very interesting. But there's just a greater probability for motion than no motion. It would be really unusual for all the interactions of all the particles and all the forces present during the formation of a planet to produce a lack of rotation regardless of the mechanism.

8. Jan 25, 2012

### Drakkith

Staff Emeritus
Was the OP asking about rotation of each planet around it's axis, or about the direction of the orbits? I took it to be asking about the orbits. I guess the rotating satellite question should have tipped me off. Planets orbit a certain direction because of conservation of angular momentum correct?

9. Jan 25, 2012

### netgypsy

I"m thinking it's in Wikkipedia where there is an explanation of how to calculate if the universe is rotating and the conclusion based on current data is that it is but not a lot. I think you'll find it under a discussion of frames of reference. If I find it again I'll link to it. Just happened to read it when researching something else.

10. Jan 26, 2012

### D H

Staff Emeritus
Good question. Looking back, it isn't clear what dpa meant.

@dpa: Rotation typically refers to the rotation of a planet about its axis. If you are talking about orbits, the correct term is orbit or revolution. The Earth, for example, has a rotation rate of one rotation per sidereal day, but an orbital rate of one revolution per sidereal year.