A representation of how interaction between a protoplanetary disk and planet makes a density wave and affects the disk's structure. A planet in the disk may be one explanation for the formation of the disk's spiral structure. The basis for the simulation is a code in computational fluid dynamics called FARGO that simulates the flow of gases in motion. Colors indicate the surface density of the disk; the darkest colors show the areas with the least density while the white shows those with the greatest density. Credit: NAOJ