Normally a scientist who specializes in the study of celestial bodies, including their sizes, features, compositions, motions, relative positions, etc: An astronomer has a knowledge of the laws of the heavenly orbs, or the principles by which their motions are regulated, including their various phenomena.

Someone who studies planets, moons, and planetary systems, in particular those of the solar system and the processes that form them: A planetary astronomer studies objects ranging in size from micrometeoroids to gas giants, with the objectives of determining their compositions, dynamics, formations, interrelations, and histories.