The word "planet" comes from greek, a word "planetas" meaning "wanderer." When folks long ago saw these moving dots of light in the sky, to them, they could really not decide what they were other than what made them different from the stars. they wandered... and so you have planets. "planetary" has also been used to describe things that sort of, move in circles around each other. The first transmissions in cars used a "planetary gear" setup.. multiple gears turning inside one large one, around a central hub.. So a planetary model would really mean at its core level, a model which involved objects that revolved around a central mass of some sort.
Because it shows electrons revolving around the nucleus like planets around a sun.