Deficiency of molecular hydrogen in the disk of β Pictoris

Molecular hydrogen (H₂) 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 H₂ have recently been reported towards β Pictoris, a star harbouring a young planetary system. This star is surrounded by a dusty 'debris disk' that is continuously replenished either by collisions between asteroidal objects or by evaporation of ices on Chiron-like objects. A gaseous disk has also been inferred from absorption lines in the stellar spectrum. Here we present the far-ultraviolet spectrum of β Pictoris, in which H₂ absorption lines are not seen. This allows us to set a very low upper limit on the column density of H₂: N(H₂) ≤ 10¹⁸ cm^-2. This non-detection is puzzling when compared to the quantity of H₂ inferred from the infrared observations, but it does show that H₂ is not in the disk on the direct line of sight. Carbon monoxide (CO) has been seen in absorption against the star, yielding a ratio of CO/H₂ > 6 × 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.