# I Why are the planets revolving around their own axis?

1. Jul 13, 2017

### Neolight

As we know that most planets has two motions around the sun , one revolving and one rotation on its own axis. So what exactly is the reason for this motions to come into play.

2. Jul 13, 2017

### andrewkirk

Planets start small and grow bigger by other matter - gas, dust, rocks - being attracted to them by gravity.

Consider a rock - a meteorite - hitting a planet. Unless it hits the planet while its velocity points directly towards the planet's centre of mass (which will almost never happen), its velocity will have a component of momentum perpendicular to the line connecting the collision point to the centre of mass (circumferential momentum). That momentum has an associated angular momentum, equal to the circumferential momentum times the distance from the centre of mass. That angular momentum is transferred to the planet by the collision.

The spin of a planet reflects its angular momentum around its axis of rotation, which is the sum of the angular momentum received from all collisions of matter with the planet.

3. Jul 13, 2017

### rootone

A star and the planets which form with it are formed from a collapsing nebula of dust and gas.
That nebula has an intrinsic angular momentum.
That momentum (it can't go away) ends up being mainly distributed in the star and the other large bodies.
Hence the Star and the largest planets end up spinning in a similar way.
Although that nice arrangement can easily be disturbed by gravity of other stars which could pass nearby.

4. Jul 14, 2017

### A.T.

There is an infinite number of values their angular momentum could have, so the probability of it being exactly zero goes towards zero. And even if a planet would end up with fixed orientation somehow, the tidal torque would make it spin until it reaches the tidally locked state:

https://en.wikipedia.org/wiki/Tidal_locking

5. Jul 14, 2017

### sophiecentaur

It can be very hard to accept that angular momentum is conserved in ALL situations. It's easy enough with two large solid bodies colliding but it's harder when thinking of a cloud of widely spaced dust particles which are part of a spinning cloud and never touching. But the principle has been shown to apply without any exceptions, when measurable. (Same as with linear momentum)
Imo it's one of those things that can be treated as an 'article of faith' in Physics and there aren't a lot of those about! And it's not just one of those "Nature abhors a vacuum" type statements as it's based on real experience.
The process of tidal locking between two bodies, never in contact with each other, is interesting. It works only when the bodies are not perfectly symmetrical masses (spheres etc.) The Oceans are 'dragging' the Moon around and causing the Earth to slow down and the Moon to speed up, moving to a higher orbit - over billions of years - until the two rotation rates are the same. And, of course, the (non-symmetrical) Moon is already locked to face the Earth all the time.

6. Jul 14, 2017

### rumborak

I think what makes the angular momentum one complicated is that its definition includes an axis in space. Whenever a new object gets included into the system, that axis changes position and orientation. That is, a new incoming body can entirely turn the resulting mass rotating around the other direction, but that's because one was considering the "wrong axis" to begin with when not considering the new body. With the new body in consideration, the system had always been rotating in that new direction.

7. Jul 14, 2017

### hilbert2

It's not just planets, even the oxygen and nitrogen molecules in air are rotating around two axes (even though this has to be described with quantum mechanics). It's a statistical fact that the kinetic energy of a system tends to get evenly distributed among all the degrees of freedom where it can go, and rotational motion is one of them.

8. Jul 14, 2017

### snorkack

But the angular momentum value is, in fact, finite. And the probability of it being exactly zero is nonzero (but small). After all, everybody, including planets and stars, possess an angular momentum which is either half-integer or integer... the latter including zero.

9. Jul 14, 2017

### A.T.

So how small is it, given an infinite number of possible values?

Why integer?

10. Jul 15, 2017

### snorkack

Finite number of possible values, for a finite value.
Earth consists of finite and countable number of protons, neutrons and electrons. Obviously the total number of protons, neutrons and electrons in Earth must necessarily be either even (in which case Earth is a boson) or odd (in which case Earth is a fermion).
Integer or half-integer. Because spin is quantized, in steps of half Planck constant.

11. Jul 15, 2017

### jbriggs444

The angular momentum of a planet cannot be obtained by adding the spins of each of its constituent fundamental particles. You'd get a number that is not even in the right ballpark.

12. Jul 15, 2017

### andrewkirk

To put it crudely, all spin is angular momentum, but not all angular momentum is spin.

13. Jul 15, 2017

### snorkack

The angular momentum of a compound system - whether nucleus or a galaxy - consists of the spins of constituent particles, the mutual orientation of those spins and the orbital movement relative to each other.
Of these components, mutual orientation of spins and orbital movements have angular momentum quantized as integer multiples of Planck constant. The only component which is quantized as halves of Planck constant is spins of constituent particles.

14. Jul 23, 2017

### Timvanhoomissen

The spin of a planet reflects its angular momentum around its axis of rotation, which is the sum of the angular momentum received from all collisions of matter with the planet.[/QUOTE]

Does this mean that if a massive meteoroid were to strike Earth, the length of a day on Earth would change?

15. Jul 24, 2017

### andrewkirk

Yes. If the meteorite came from the West, the day would shorten. If from the East, it would lengthen.

16. Jul 24, 2017

### sophiecentaur

I guess that an impact big enough to cause a significant change would produce such changes in the atmosphere as to make a minute or two of day length the least of our probs.

17. Jul 26, 2017

### snorkack

Simple note about the chance of not revolving:
The angular momentum of Earth orbit around Sun is about 1074
The angular momentum of Earth rotation around axis is much smaller... around 1068
Venus is distinguished for extremely slow rotation - rotation period of over 200 days.
Yet even so, the angular momentum of Venus´ rotation is around 1065.
Angular momentum 0 has 1 state of non-orientation. Angular momentum 1 has 3 states of orientation. Et cetera.
Thus Venus, with angular momentum of mere 1065, has 10130 distinct states of rotating slower than it does.
The probability of a Venus-like planet not rotating is nonzero... but it´s below 10-130.

18. Jul 26, 2017

### jbriggs444

As has been pointed out previously, this is not even wrong.

19. Sep 1, 2017

### DanMP

So, planets start small and grow bigger by other matter - gas, dust, rocks - being attracted to them by gravity AND the planets are formed from a collapsing nebula of dust and gas AND that nebula has an intrinsic angular momentum.

Now, reconsider the rock (/gas/dust) going towards the growing planet, attracted by its gravity. Due to the intrinsic angular momentum of the nebula in which the accretion takes place, the rocks (/gas/dust) are deflected by the Coriolis force, so they will hit off-centre, transferring in this way the angular momentum of the nebula to the new planet/star. It's like the whirlpool in the sink. This nice arrangement can be disturbed by big, late, random collisions that can tilt the initial axis of rotation.

20. Sep 5, 2017

### nikkkom

I think he meant Planck unit system.

21. Sep 12, 2017

### snorkack

Derivation of Earth angular momentum around Sun:
Mass of Earth - 6*1024 kg
Speed of Earth on orbit - 3*104 m/s
Lever arm of the speed - 1,5*1011 m
So angular momentum: 2,7*1040 J*s
Planck constant: 1,05*10-34 J*s
Thus the quantum number of Earth orbit - about 2,5*1074, as stated before.

22. Sep 12, 2017

### jbriggs444

This assumes that angular momentum is always quantized. If linear momentum and position are not quantized then there is no reason to expect that angular momentum is quantized. As @andrewkirk pointed out: