  From the masses of planets orbiting our Sun, and relative elemental
abundances, it is estimated that at birth our Solar System required a minimum
disk mass of ~0.01 solar masses within ~100 AU of the star. The main
constituent, gaseous molecular hydrogen, does not emit from the disk mass
reservoir, so the most common measure of the disk mass is dust thermal emission
and lines of gaseous carbon monoxide. Carbon monoxide emission generally probes
the disk surface, while the conversion from dust emission to gas mass requires
knowledge of the grain properties and gas-to-dust mass ratio, which likely
differ from their interstellar values. Thus, mass estimates vary by orders of
magnitude, as exemplified by the relatively old (3--10 Myr) star TW Hya, with
estimates ranging from 0.0005 to 0.06 solar masses. Here we report the
detection the fundamental rotational transition of hydrogen deuteride, HD,
toward TW Hya. HD is a good tracer of disk gas because it follows the
distribution of molecular hydrogen and its emission is sensitive to the total
mass. The HD detection, combined with existing observations and detailed
models, implies a disk mass >0.05 solar masses, enough to form a planetary
system like our own.
