This graphic shows a computer simulation of a circumstellar disk that orbits a young star, with each snapshot showing the state of the disk 1,500 years later. This disk is gravitationally unstable, which means that the disk's gravitational attraction for itself causes material to contract into spirals that can also clump up further to form Jupiter-mass protoplanets. The black circles in the above (click picture to zoom in) follow one such clump that first forms at a distance of 300 AU (ie 300 times the Sun-Earth distance) from the central star, which then spirals inwards due to its interactions with the disk. This simulation illustrates one of the difficulties in forming giant planets via gravitational instability, since the clumps that do form by this process also tend to get driven inwards by the disk, where they might accrete onto the central star. For additional details, see the preprint by Zhu and colleagues.