# Why do planets rotate? **URL:** \ **Category:** Factual Questions\ **Created:** [January 19, 2001, 6:56am UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829 "2001-01-19T06:56:28Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Jukeball](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@Jukeball](https://boards.straightdope.com/u/Jukeball)\ **Post date:** [January 19, 2001, 6:56am UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/1 "2001-01-19T06:56:28Z")
?? ---
**Author:** ![Badtz\_Maru](https://avatars.discourse-cdn.com/v4/letter/b/e36b37/32.png) [@Badtz\_Maru](https://boards.straightdope.com/u/Badtz_Maru)\ **Post date:** [January 19, 2001, 7:02am UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/2 "2001-01-19T07:02:37Z")
Inertia, and this was asked about a week ago. Try using the Search function before posting, most of these obvious questions have already been asked and answered. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [January 19, 2001, 7:10am UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/3 "2001-01-19T07:10:44Z")
That’s a tough question. The last I heard on it was that computer models of planets coalescing from a rotating nebula end up with planetary systems that have little or no spin to them. It turns out that roughly as many planetismals collide on one side of the protoplanetary disk as the other, which results in no net spin. Of course most of the planets in the solar system do actually rotate in the same direction, and along roughly parallel axes so something is probably causing it. ---
**Author:** ![zub](https://avatars.discourse-cdn.com/v4/letter/z/a6a055/32.png) [@zub](https://boards.straightdope.com/u/zub)\ **Post date:** [January 19, 2001, 10:13am UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/4 "2001-01-19T10:13:21Z")
this is an interesting fact: The rotation of Venus is unusual in both its direction and speed. Most of the planets in the solar system rotate in a counterclockwise direction when viewed from above their north poles; Venus, however, rotates in the opposite, or retrograde, direction. Were it not for the planet’s clouds, an observer on Venus’ surface would see the Sun rise in the west and set in the east. ---
**Author:** ![Milton\_De\_La\_Warre](https://avatars.discourse-cdn.com/v4/letter/m/e480ec/32.png) [@Milton\_De\_La\_Warre](https://boards.straightdope.com/u/Milton_De_La_Warre)\ **Post date:** [January 19, 2001, 11:33am UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/5 "2001-01-19T11:33:59Z")
A related(?) question: Why do the plantes all orbit on approximately the same plane? ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [January 19, 2001, 1:50pm UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/6 "2001-01-19T13:50:47Z")
First there was a nebula containing hydrogen, helium, carbon, etc. (all the elements that make up our solar system). Over time, this nebula collapsed due to its own gravity (although some event may have started it off like passing by another massive object). The material did not come together perfectly symmetrical, so there was some net rotation to the collapsing cloud. Like a figure skater who pulls his/her arms in to spin faster, the cloud of material spun faster as it came together. The faster spin also caused the cloud of material to spread out into a disk shape. (Lots of conservation of angular momentum going on here, plus some gravitational effects, and a smidgeon of friction). The central bulge, where most of the material had accumulated, became dense enough to start fusion reactions in the hydrogen and helium…thus our sun was born. The material in the outer portions of the disk also started to clump together due to collisions and gravity…thus our planets were born. All the comets and asteroids in the solar system nowadays are leftovers that have yet to collide with and gravitationally join the planets. So, things started out with a spin, and stayed that way because there is little to stop it (inertia). And the planets all orbit in about the same plane because they formed from the same disk of material. ---
**Author:** ![Clairmont](https://avatars.discourse-cdn.com/v4/letter/c/e47774/32.png) [@Clairmont](https://boards.straightdope.com/u/Clairmont)\ **Post date:** [January 19, 2001, 4:18pm UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/7 "2001-01-19T16:18:18Z")
Good answer, Phobos. But I think our friend jayshah is not looking for answers. This is his idea of a pop quiz. Every question he’s asked could have been answered with a simple click. I think it’s called “search.” Why is the sky blue? I think that one is next. If I’m not saying what most people are thinking, then I apologize. ---
**Author:** ![furnishesq](https://avatars.discourse-cdn.com/v4/letter/f/977dab/32.png) [@furnishesq](https://boards.straightdope.com/u/furnishesq)\ **Post date:** [January 19, 2001, 4:39pm UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/8 "2001-01-19T16:39:54Z")
Now that you mention it, why is the sky blue? ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [January 19, 2001, 5:03pm UTC](https://boards.straightdope.com/t/why-do-planets-rotate/50829/9 "2001-01-19T17:03:51Z")
**jayshah** , you’ve asked a few good questions here, but candidly you’ve asked a few stupid ones, too. And, as mentioned, you’ve asked some that have been answered in this forum quite recently. Please exercise discretion when asking a question, and please search the GQ archives and **Cecil** ’s old columns before opening a new thread. Thank you.