One great thing about astronomy is the variety of sights it can provide. Do you want to see a star nested inside another star? You can see that. How about thousands of galaxies all colliding in an event so monstrously large and violent that the human brain can’t possibly comprehend it? You can find that, too. But try looking within any one solar system — for instance: our own — and you’re bound to find an orderly system: eight little soldiers all marching in lock-step, spinning on their own internal axes in the same uniform direction. But if the solar system was formed by the mostly random accumulation of gasses and astronomical debris, how could we come to such an ordered state of affairs here in our time?
To see the answer, go find a spinning computer chair. To make the principle as visible as possible here, try to find two equally heavy objects and hold one in each hand. Hold them far out to your sides, then start spinning, holding the objects there. Get a good spin going, get a feel for the speed of it, then slowly, still spinning, bring your arms in to your chest. The closer those weights get to your body, the faster you will spin.
This is a principle called the moment of inertia, and it has two major implications. One: the distribution of mass in a system can exert some control over the spinning of that system. Two: the spinning of a system can exert some control over the distribution of mass inside it. This means that as, say, a cloud of gas and space dust destined to become our solar system contracts slowly due to gravity, the angular speed of that spinning system will increase. Put even more simply: as the solar system was first coming together, its parts all started rotating faster and faster.
For the rest of the story: http://www.geek.com/science/geek-answers-why-do-all-planets-spin-and-orbit-in-the-same-direction-1594165/