Take the 2-minute tour ×
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's 100% free, no registration required.

Referring to the mechanisms explaining the solar system formation and to the initial rotation of the gaseous cloud that collapsed, I understand easily why the planets orbit the Sun the same way this one rotate (say counterclockwise) but I can't figure out why this apply to planets rotation too. Thinking about that from Kepler's laws and angular momentum conservation point of view, I might conclude that the planets should rotate clockwise because the velocity of the particles that aggregated during the planets formation was higher closer to the Sun...

Apart from a short explanation, I would like to have a good reference from the literature if possible.

Edit, to make my reasoning more explicit: following Kepler's laws, the particles that aggregate on the "day side" of the proto-planets in the east-west direction relative to the ground are faster than the ones hitting on the "night side" in the west-east direction. If we add all of these contributions, the planets should rotate in the opposite direction relative to the initial cloud (i.e. relative to the actual Sun rotation). I guess something is wrong or missing there (to counterbalance the phenomenon I just described) but I can't see what it is...

New edit: References I found some published articles dealing with this kind of question but I don't have the time right now to read them carefully. If someone is motivated to do so, do not hesitate ;-) If I find the answer to my question amongst these papers, I will post it there later. Of course, you may need to use the network of an institution with a subscription to these editors to access them:

R.T. Giuli (1968a) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103568900821

R.T. Giuli (1968b) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103568900122

A.W. Harris (1977) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103577900793

J.J. Lissauer, D.M. Kary (1991) in Icarus: http://www.sciencedirect.com/science/article/pii/001910359190145J

share|improve this question
1  
Can you explain the conservation of angular momentum point? –  HDE 226868 Aug 21 at 18:19
    
@HDE226868 Actually I realize I don't really use it in my reasoning (see my edit)... or possibly I just forgot how I used it :-S Maybe I alluded to the conservation of angular momentum to explicit the fact that the planets should have kept until today their clockwise rotation if my demonstration were correct. –  Arroway Aug 22 at 9:32
    
I appreciate the edit, but I still don't quite understand. Why would the particles on one side move faster than the other? Also, the planets as we know them formed over millions of years after loads of collisions (not exactly a great place to live!) - I don't know how the protoplanetary disk plays into it. –  HDE 226868 Aug 22 at 16:14
    
@HDE226868 Sorry for the late reply. From my point of view, the different velocities are explained by the different trajectories of the particles. The idea was that particles coming from smaller orbits are faster (Kepler's law), so the impacts are stronger on average on the "day side". But this explanation is probably false anyway, or at least not significant, considering that others processes may be at play at the same time and that some particles may come from random directions too. –  Arroway Aug 28 at 14:27
    
Thanks. Unfortunately, I can't quite help you there, so I wouldn't be too quick to rule out that argument. Good luck. –  HDE 226868 Aug 28 at 16:20

3 Answers 3

Even though I'm a professional astronomer (but not an expert on this field), I don't have a good answer. AFAIK, this question is still open. Actually, the fact that most planets in the Solar system rotate in the same sense as they orbit the Sun is an important constraint on possible planet formation scenarios (of which still several are presently discussed by scientists).

For example, the idea that planets form out of dust collected at the centre of vortices in the proto-planetary disc can be almost ruled out, because retrograde vortices are much more stable and long-lived than prograde ones (Jupiter's red spot is a retrograde vortex), and hence planets should be retrograde, but aren't.

share|improve this answer
1  
Thank you for this "open answer". I thought the answer would be easier but the more I look for a good explanation, the more I become aware how complicated it is... So, rather than saying that the way the planets rotate is explained by any "widely accepted" planet formation scenario, we should say the contrary, i.e. the rotation is used as a constraint on the possible scenarios? –  Arroway Aug 28 at 14:08
    
Well, I'm not actually very well informed, so don't quote me on this. –  Walter Aug 28 at 18:41

The reason most of the planets in our solar system (with the exception of Venus and Uranus) rotate in a anti-clockwise direction is due to the proto-planetary disk that formed the Sun and all the planets around it. Since the proto-planetary disk was spinning in a anti-clockwise direction any objects that formed out of the same disk would have shown the same rotation. The fact that it is rotating in a anti-clockwise direction is nothing special. It all depends on the initial spin of the disk.

The reason Venus and Uranus have a spin opposite to most other planets is most likely because they were hit by another large object some time during their formation, causing their spin to change directions. This also explains why Uranus' axis is tilted to such a severe extent.

If you would like to read more here are some links: http://scienceblogs.com/startswithabang/2010/10/07/counterclockwise-but-there-are/ http://hubblesite.org/hubble_discoveries/discovering_planets_beyond/how-do-planets-form http://www.astronomycafe.net/qadir/q50.html

share|improve this answer
4  
Actually for Venus and Uranus it's not the spin direction that has been changed, it's the planet's axis which has been turned upside down for Venus and 90º for Uranus. –  Joan.bdm Aug 22 at 8:49
    
Yes thats true. Because a object hit the planet it changed its axial tilt, making it appear as if it is spinning anti-clockwise –  Nirvik Baruah Aug 22 at 8:57
2  
@NirvikBaruah Thank you for the answer but from my point of view, without further explanations, it only hold for the way the planets orbit the Sun, not the way they rotate. I've just added a paragraph in my initial question to be more explicit. –  Arroway Aug 22 at 9:42
    
@NivikBaruah, do you mean "revolution" here? What you're talking about has nothing to do with the question. –  HDE 226868 Aug 22 at 16:12

I can't give you an explicit ant mathematical explanation but maybe this simulations give you a hint.

If you look at the bottom one with beta=0.01 (faster initial rotation speed) you will see how possibly a binary or even ternary system is formed. I guess this could be picked as an example for planetary formation.

http://www.astro.ex.ac.uk/people/mbate/Animations/prestellar_discs.html

share|improve this answer
    
Why did I get a negative vote? –  Joan.bdm Aug 25 at 7:17
    
It's not me! Your answer is interesting but I am not convinced the analogy between binary system and planets formation is so straightforward. Anyway, maybe it is but it does not help me so much thinking about my initial question, unless I miss something... –  Arroway Aug 28 at 14:13

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.