We propose a search for molecular hydrogen emission associated
  with low mass pre-main sequence stars.  We show that the gaseous
  component of a protoplanetary disk presents a column density
  sufficiently large that, for objects in the nearby Taurus and
  Ophiuchus star forming regions, detection of rotational S(0) and
  S(1) emission from H_2 is readily achieved by ISO SWS.  A unique
  opportunity is therefore available to detect gaseous disks
  spectroscopically and measure their temperatures.  We propose to
  use this diagnostic to determine for the first time the timescale
  for dissipation of the disk gas.  To accomplish this, three groups
  of pre-main sequence stars will be observed for H_2 emission:
  classical T Tauri stars, weak line T Tauri stars, and members
  of a young cluster.  Five objects will be observed in each group.
  The results of this mini-survey should reveal whether the disk gas
  dissipates at the same rate as the dust, or if the gas survives
  for longer timescales.  The observed gas survival timescale will
  constrain the formation timescale for jovian planets.  Observations
  in the S(0) line should be sensitive to as little as 10^-6 solar
  masses of gas in the regions studied.