Planets in our Solar System all go in the same direction around the Sun. This is a likely outcome for a system that starts out with matter in orbit around a star going in random directions. Collisions between objects going in different directions slows them down, and they fall into the Sun. Objects going in the same direction are much less likely to collide and therefore survive as planets.
Related resources:
- Lonely planets wander galaxy – article
- Our Solar System – revolutionary ideas – article
Transcript
To demonstrate the direction of planetary orbits, place a heavy ball on your gravity well.
Roll two groups of marbles in opposite directions around the central mass. The marbles will collide, and the collisions cause the marbles to lose speed and fall into the central mass. Usually, the result is that the remaining marbles will go in the same direction around the central mass.
