  The gaseous mass of protoplanetary disks is a fundamental quantity in planet
formation. The presence of gas is necessary to assemble planetesimals, it
determines timescales of giant planet birth, and it is an unknown factor for a
wide range of properties of planet formation, from chemical abundances (X/H) to
the mass efficiency of planet formation. The gas mass obtained from traditional
tracers, such as dust thermal continuum and CO isotopologues, are now known to
have significant (1 - 2 orders of magnitude) discrepancies. Emission from the
isotopologue of H2, hydrogen deuteride (HD), offers an alternative measurement
of the disk gas mass.
  Of all of the regions of the spectrum, the far-infrared stands out in that
orders of magnitude gains in sensitivity can be gleaned by cooling a large
aperture telescope to 8 K. Such a facility can open up a vast new area of the
spectrum to exploration. One of the primary benefits of this far-infrared
revolution would be the ability to survey hundreds of planet-forming disks in
HD emission to derive their gaseous masses. For the first time, we will have
statistics on the gas mass as a function of evolution, tracing birth to
dispersal as a function of stellar spectral type. These measurements have broad
implications for our understanding of the time scale during which gas is
available to form giant planets, the dynamical evolution of the seeds of
terrestrial worlds, and the resulting chemical composition of pre-planetary
embryos carrying the elements needed for life. Measurements of the ground-state
line of HD requires a space-based observatory operating in the far-infrared at
112 microns.
