Our Sun has a retinue of eight planets (most with attendant moons) and an uncounted multitude of dwarf planets, asteroids and comets. A rapidly-expanding search program has revealed that a large fraction of all stars have planetary systems.
We know that planets form in protoplanetary disks, thin rotating structures of dense gas and dust which are a crucial final stage in the process of star formation. The image to the right shows a selection or infrared observations of these circumstellar disks, often seen in silhouette against the scattered light from their central stars.
