Planetary Rotation Speeds (Sidereal Days) Planets rotate because they formed from dynamic, moving material in the early solar nebula. As this material coalesced under gravity, the conservation of angular momentum ensured that each forming planet inherited its own spin. In fact, the larger the planet, the more material was required for its formation—and the greater its rotational speed. This is clearly illustrated by the rapid spins of gas giants such as Jupiter and Saturn | Jock Williams Meteoro