Substantial reservoirs of molecular hydrogen in the debris disks around young stars

Circumstellar accretion disks transfer matter from molecular clouds to youn g stars and to the sites of planet formation. The disks observed around pre -main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago(1). Bu t the 'debris' disks that encircle more than 15% of nearby main-sequence st ars(2-5) appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide(6-8) : these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is abou t 100 (ref. 9). Here we report observations of the lowest rotational transi tions of molecular hydrogen (H-2) that reveal large quantities of gas in th e debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The ga s masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust r atios of the same order as the interstellar value.