This warms the dust and the dust begins to radiate infrared radiation. We 'see' the dust because the matter is distributed into small particles. As when a rock is split into many small pieces, the dust has more surface area than if it were collected into the solid ball of a planet. A disk of dust, therefore, emits more infrared radiation than a like amount of dust that has been made into larger clumps. Just as with Hollywood stars who've been made over many times, it can be difficult to tell how old a heavenly star is.
Uncertainties can exceed a hundred million years.
Still, Habing and his colleagues were able to determine that about 90 percent of stars with dust disks lose them between the age of 300 and 400 million years. What's going on in the disks Models of planetary formation show that a dense dust disk cannot remain circling a star for long. NASA's Fajardo-Acosta says the dust will either be pushed out of the system by stellar radiation, sucked into the star by its immensity, or destroyed by heat or collisions with other dust grains. If fully formed planets have developed, they will sweep the disk and clear up remaining dust. So the apparent 100 million-year persistence of the relatively dense stellar dust disks indicates a process of planetary formation.