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How did planets start to spin in the first place?

Some say that gases which condense and form planets start spinning and hence planets acquire a spin. And some say that they start to spin to conserve angular momentum. Can someone give an explanation about this?

Also, do all planets spin in the same direction? If they do, then why?
Also, why do planets have the same chosen rotation direction?

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Q1. The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.

Q2. No, Venus is quite a famous example.

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    $\begingroup$ Why do planets have the same chosen rotation direction? $\endgroup$
    – Sathyaram
    Commented Oct 26, 2015 at 16:15
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    $\begingroup$ It is a statistical reason, that the turbulent disc from which the solar system formed would have been chaotic, some objects moving in one direction and some in another. However, there may be a slight bias in one direction, i.e., more objects moving in one given circular direction (say, for example, we have more objects moving clockwise). Once all of the anti-clockwise objects collide with the remainder of the disc they will become knocked out. Best way I can describe it is by this video: [youtube.com/watch?v=MTY1Kje0yLg] $\endgroup$
    – DarthPlagueis
    Commented Oct 26, 2015 at 16:20
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The possible answer can be this: During the big bang, a large amount of energy was given out. That energy till makes the planets rotate( as there is no friction in space).

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  • $\begingroup$ Why do planets have the same chosen rotation direction? $\endgroup$
    – Sathyaram
    Commented Oct 26, 2015 at 16:15
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There is no force that causes the planets to rotate. Most of the rotation comes about from the conservation of angular momentum. Angular momentum is given by $L=m\omega{r^2}$ where $m$ is the mass, $\omega$ is the angular velocity in radians per second, and $r$ is the radius of the circular motion. Due to conservation of angular momentum, if the radius of the orbit decreases, then its angular velocity must increase (as the mass is constant).

All planetary and stellar systems are born from the collapse of dense interstellar clouds. The clouds may originally be very large (even thousands of light years across). Consider a portion of the cloud the collapses from a size of a light year or so to the size of the solar system. That is a huge change in the size of the system. So, the very slight rotation that the cloud has in the beginning is increased dramatically when the collapse takes place. In fact, this is one of the barriers in star formation: there is excess angular momentum and there has to be a way of losing angular momentum before you can form a star.

Anyway, the bottom line is that stars like the Sun spin from the original angular momentum that was there in the solar nebula from which it formed. Not only that, all orbital motion of the planets (including the spin) is due to this orginal angular momentum.

In addition, they all rotate in the same general direction, with the exceptions of Venus and Uranus. These differences are believed to stem from collisions that occurred late in the planets' formation. (A similar collision is believed to have led to the formation of our moon.)

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  • $\begingroup$ The last paragraph should come with a disclaimer. $\endgroup$
    – DarthPlagueis
    Commented Oct 26, 2015 at 16:10
  • $\begingroup$ Why do planets have the same chosen rotation direction? $\endgroup$
    – Sathyaram
    Commented Oct 26, 2015 at 16:11
  • $\begingroup$ The theory is it came from the original angular momentum of the gas clouds that formed the planets. $\endgroup$
    – StarDrop9
    Commented Oct 27, 2015 at 12:26

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