Planets develop from the dusty placental disk of material that surrounds a star after it begins to shine. The dust in that disk,according to most models, starts to stick to itself until clumps develop large enough to attract other clumps gravitationally. Astronomers believe the process of building planets and dissipating the disk takes about ten million years. Many mysteries remain, or however,including the tendency of dust not to stick together, and the likelihood that colliding clumps could demolish apart rather than agglomerate. Recent discoveries of exoplanets beget begun to overlap with studies of planetesimal disks, and enable astronomers to probe the development and evolution of a star's system of planets and their interactions with the disk.
Source: phys.org