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The gaseous mass of protoplanetary disks is a fundamental quantity in planetformation. The presence of gas is necessary to assemble planetesimals, itdetermines timescales of giant planet birth, and it is an unknown factor for awide range of properties of planet formation, from chemical abundances (X/H) tothe mass efficiency of planet formation. The gas mass obtained from traditionaltracers, such as dust thermal continuum and CO isotopologues, are now known tohave significant (1 - 2 orders of magnitude) discrepancies. Emission from theisotopologue of H2, hydrogen deuteride (HD), offers an alternative measurementof the disk gas mass.  Of all of the regions of the spectrum, the far-infrared stands out in thatorders of magnitude gains in sensitivity can be gleaned by cooling a largeaperture telescope to 8 K. Such a facility can open up a vast new area of thespectrum to exploration. One of the primary benefits of this far-infraredrevolution would be the ability to survey hundreds of planet-forming disks inHD emission to derive their gaseous masses. For the first time, we will havestatistics on the gas mass as a function of evolution, tracing birth todispersal as a function of stellar spectral type. These measurements have broadimplications for our understanding of the time scale during which gas isavailable to form giant planets, the dynamical evolution of the seeds ofterrestrial worlds, and the resulting chemical composition of pre-planetaryembryos carrying the elements needed for life. Measurements of the ground-stateline of HD requires a space-based observatory operating in the far-infrared at112 microns.

The Disk Gas Mass and the Far-IR Revolution