A newborn star typically has a disk of gas and dust from which planets develop as the dust grains collide, stick together and grow. Stars older than about five million years lack evidence for these disks, however, suggesting that by this age most of the disk material has either been converted into planets or smaller bodies, accreted onto the star, or dispersed from the system. Transition disks bridge this period in disk evolution: They have not yet been disbursed, and warmed by the star, can be detected at infrared or millimeter wavelengths. Their infrared colors can be used to characterize their properties. They often show inner dust cavities, which astronomers have sometimes interpreted as evidence of the presence of planets that have cleared out their orbits.
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