Deficiency of molecular hydrogen in the disk of β Pictoris

Molecular hydrogen (H-2) is by far the most abundant material from which stars, protoplanetary disks and giant planets form, but it is difficult to detect directly. Infrared emission lines from H-2 have recently been reported(1) towards beta Pictoris, a star harbouring a young planetary system(2). This star is surrounded by a dusty 'debris disk' that is continuously replenished either by collisions between asteroidal objects(3) or by evaporation of ices on Chiron-like objects(4). A gaseous disk has also been inferred from absorption lines in the stellar spectrum(5-8). Here we present the far-ultraviolet spectrum of beta Pictoris, in which H-2 absorption lines are not seen. This allows us to set a very low upper limit on the column density of H-2 : N(H-2) less than or equal to 10(18) cm(-2). This non-detection is puzzling when compared to the quantity of H-2 inferred from the infrared observations, but it does show that H-2 is not in the disk on the direct line of sight. Carbon monoxide (CO) has been seen in absorption against the star(8-10), yielding a ratio of CO/H-2 >6 x 10(-4). As CO would be destroyed under ambient conditions in about 200 years (refs 9, 11), our result demonstrates that the CO in the disk arises from evaporation of planetesimals.