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.