  The relative abundance of deuterium and hydrogen is a potent tracer of planet
formation and evolution. Jupiter and Saturn have protosolar atmospheric D/H
ratios, a relic of substantial gas accretion from the nebula, while the
atmospheres of Neptune and Uranus are enhanced in D by accretion of ices into
their envelopes. For terrestrial planets, D/H ratios are used to determine the
mechanisms of volatile delivery and subsequent atmosphere loss over the
lifetime of the planet. Planets and brown dwarfs more massive than 13 M_J
quickly fuse their initial D reservoir. Here, we simulate spectra for giant
exoplanets and brown dwarfs (2 M_Neptune to ~10 M_Jupiter) from Teff=200--1800
K including both CH3D and HDO to determine the observability of these dominant
deuterium isotopologues in mid-infrared thermal emission spectra. Colder
objects have stronger molecular features in their spectra, due to the
temperature-dependence of molecular cross sections. CH3D is easier to observe
than HDO at all temperatures considered, due to the strength of its absorption
bands and locations of features at wavelengths with few other strong absorption
features. We predict that for nearby cool brown dwarfs, the CH3D abundance can
be measured at high signal-to-noise with the James Webb Space Telescope; for
objects from 200--800 K closer than 10 pc, a protosolar D/H ratio would be
readily observable in 2.5 hours. Moderately young Jupiter-mass planets
(100--300 Myr) and young Neptunes (10 Myr) may be discovered with JWST and
provide the best targets for detecting deuterium on an exoplanet in the coming
decade. Future telescope designs should consider the importance of isotopes for
understanding the formation and evolution of planetary atmospheres.
