Question:
why do planets rotate?
V V
2010-08-15 08:34:38 UTC
why do planets rotate?
Fifteen answers:
campbelp2002
2010-08-15 12:33:27 UTC
They are coasting forever in the frictionless vacuum of space on the angular momentum they got when they formed.
poornakumar b
2010-08-15 21:56:02 UTC
At the time of formation it is highly improbable for each of them not to pick up any energy in the form of 'rotation'. Two attracting particles moving towards each other may not meet head on, as the probability for that is very low compared to any other. So on their closest approach the attracting pair form a 'couple' to rotate. Other particles too slowly move closer but execute the already formed couple, this time rotating around the couple's centre. Any particle that is discordant would be expelled by giving it extra energy to escape. In essence all particles of similar movement form into a non-obstructing cloud that rotates. the process builds up till it becomes the general norm and the mass gets compacted, with a rotary moment of inertia, facing no braking (friction). So, it continues rotating.
rutu
2010-08-19 05:15:02 UTC
Bodies (like planets) become larger and larger as their materials coalesce out of the accretion disks of rubble and dust that form them, and crash into the growing body. Materials falling out of the disk don't fall directly into the body, but tend to make glancing blows, or go into rapidly decaying low orbits for a while. The disk material orbits the sun, because material that wasn't going fast enough to orbit crashed toward the sun. The orbiting material farther out from the sun is moving a little more rapidly than material between the body and the sun. So the material falling toward the body from the night side imparts more rotational energy when it hits, adding to the rotation of the body.
?
2010-08-15 16:00:35 UTC
Bodies (like planets) become larger and larger as their materials coalesce out of the accretion disks of rubble and dust that form them, and crash into the growing body. Materials falling out of the disk don't fall directly into the body, but tend to make glancing blows, or go into rapidly decaying low orbits for a while. The disk material orbits the sun, because material that wasn't going fast enough to orbit crashed toward the sun. The orbiting material farther out from the sun is moving a little more rapidly than material between the body and the sun. So the material falling toward the body from the night side imparts more rotational energy when it hits, adding to the rotation of the body.
Lilac
2010-08-16 10:56:09 UTC
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*w*r2 where m is the mass, w 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.

You are saying that original angular momentum of the cloud causes orbital motions and rotations of the planets(mostly). But in the case of orbital motions we have gravitational force that gives us some restrictions of movement(Kepler laws,for example).

What I am saying is that there will be no planets if there was no initial angular momentum in the primordial solar nebula. If a nebula with absolutely no rotation collapses, then there will only be a central non-rotating star and there will not be any planets. Planets form out of a protostellar disk, which itself forms only because of the initial angular momentum of the cloud. The dynamics of a rotating body is of course controlled by forces like gravity. Kepler's laws are a direct consequence of gravity.

The only thing that has to be kept in mind in rotation is that it results in a centrifugal acceleration that points radially from the center of motion. Hence, there has to be some force that conteracts this acceleration; otherwise the body will fly away (in case of orbital motion) or will disintegrate (in case of spinning). In the case of orbital motion, the counteracting force is gravity; gravity causes the body to continually fall towards the center, and this exactly conteracts the force resulting from the centripetal acceleration. In the case of a spinning object, it is the self-adhesion of the body itself that keeps it together. This results in a limit for how fast an object can rotate and still keep itself together. If it rotates too fast, the outward acceleration felt by the elements in the body may be more than the force that keeps them bonded together, and if this happens, the body breaks up. Other than this, there is no real law concerning rotations. (Note that rotational motion involves conservation of angular momentum just like linear motion conserves linear momentum).

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One of the most remarkable features of our solar system is that nearly all of the revolutions and rotations are in the same direction. From a point high above the north pole of the solar system the planets are revolving about the sun and rotating about their axes in a counterclockwise direction. This holds true also for the asteroids. If the planets and asteroids were formed from merely random accretions the would be an even mixture of the directions of revolution and rotation. The sun itself also rotates in a counterclockwise direction. The satellites of the planets also generally revolve and rotate in a counterclockwise direction. Of the thirty something satellites only six do not do so; they are said to have retrograde motion. Of the six exceptions five are outer satellites likely to be captured asteroids. More information will be given later about these exceptions.

An Explanation for Planets Having the Same Direction of Rotation as Their Direction of Revolution

Consider the sun at some incredibly ancient time surrounded with a planetary disk much as Saturn is now surrounded by rings. The disk would have some direction of spin, say counterclockwise as shown below.

The disk would be spinning but not turning as a whole. The equilibrium distribution would have the tangential velocity proportional to the reciprocal of the square root of the disance from the sun. This follows from Kepler's Law as is shown in Orbital Velocities of the Planets.

v = α/r1/2

thus

v2 = v1/(r2/r1)1/2
?
2010-08-16 10:19:16 UTC
Planets move because the gas cloud that originally formed the solar system was spinning. The gas that wasn't spinning fell into the proto-Sun, and the planets formed from gas and chunks that were spinning enough that they were in orbit. Once the planets formed and were in orbit about the Sun, there was no reason for them not to stay there (objects in motion tend to remain in motion). The expansion of the universe is essentially negligible on the scale of the solar system, that is, it appears to have only a very, very small affect on the solar system.
-x-vitu-x-
2010-08-16 11:00:20 UTC
gravity keeps them in a rotational orbit. It's basically a combination of the planet's own force moving in a straight line and the sun's inward pulling force which makes the orbit spherical - and slightly elliptical as well.. wait you did mean 'why do planets rotate around the sun' , right? if you meant revolve , then i have no idea.
Gopikishore S
2010-08-15 15:47:34 UTC
The star revolves on its axis and attracts the planets. The planets rotates due to the resultant of these two forces.
lithiumdeuteride
2010-08-15 15:41:53 UTC
Because they started out rotating, and angular momentum is conserved. An outside torque would have to act upon a planet for it to stop spinning.
chinmay s
2010-08-16 07:44:30 UTC
Let me explain it to you by giving the example of our solar system it self.When our universe was formed the temperature was very high.Most of the things were in plasma state.after that gases were formed.This gases got contracted formed liquids,further solids.In the mean while,in the centre on the solar system,there was a lot of mass getting closer. this causes everything to revolve around it's gravitational pull.when this gases got solidified they became planets.And by the law of conservation og momentum they are still revolving around the sun.
deepak57
2010-08-15 15:56:17 UTC
Presently as per newton's law; thing in motion continues to remain in motion till external force is applied. Regarding starting of rotation, it is self explanatory.
anonymous
2010-08-15 15:46:00 UTC
Everything in space spins. It's leftover from the initial rotation of the primordial nebula from which

the solar system formed.
Asker
2010-08-15 16:54:30 UTC
Because they follow the inertia that formed the bodies when the matter from the leftovers of the original nebula got condensed.
anonymous
2010-08-15 18:05:41 UTC
all those answers were the old way of thinking but from 2004 we now know it is from the geodetic effect a planet has,just as a black hole the farther you go down the warp-space spins faster(frame dragging) but time slows down
Pradyumna
2010-08-15 16:59:30 UTC
because of gravitational pull of the sun


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