What percentage of stars have warm disks around them?
This is one of the primary science goals of the new Spitzer Space Telescope. Right now, we don’t know the answer to that question. I’m not sure exactly how many stars have been found to have excess infrared emission (which is how they found the first of these systems), but my guess would be well under 10%. The real number is probably quite a bit higher than that, but no large-scale survey has yet been completed.
