Deficiency of molecular hydrogen in the disk of beta Pictoris

Molecular hydrogen (H-2) is by far the most abundant material from which st ars, protoplanetary disks and giant planets form, but it is difficult to de tect directly. Infrared emission lines from H-2 have recently been reported (1) towards beta Pictoris, a star harbouring a young planetary system(2). T his star is surrounded by a dusty 'debris disk' that is continuously replen ished either by collisions between asteroidal objects(3) or by evaporation of ices on Chiron-like objects(4). A gaseous disk has also been inferred fr om absorption lines in the stellar spectrum(5-8). Here we present the far-u ltraviolet 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 o f H-2 : N(H-2) less than or equal to 10(18) cm(-2). This non-detection is p uzzling when compared to the quantity of H-2 inferred from the infrared obs ervations, 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 un der ambient conditions in about 200 years (refs 9, 11), our result demonstr ates that the CO in the disk arises from evaporation of planetesimals.