The Age-Dependence of the Detectabiity of Comets Orbiting Solar-Type   Stars
  The outflows from comets in orbit around G-type main-sequence stars can be detected when they produce transient OH absorption lines in the spectrum near 3100 Angstroms of the host star. There is only about a 3 X 10^{-8} probability of detecting an analog to comet Hale-Bopp orbiting an analog to the Sun. However, for young solar-type stars with very large numbers of comets, possibly delivering water to terrestrial planets, there is as much as a 1% chance that any sufficiently sensitive, randomly-timed observation may detect such transient absorption. 
M. Jura