Protoplanetary gas disks in the far infrared

The physical and chemical conditions in young protoplanetary disks set the boundary conditions for planet formation. Although the dust in disks is relatively easily detected as a far-IR photometric ``excess'' over the expected photospheric emission, much less is known about the gas phase. It seems clear that an abrupt transition from massive optically thick disks (gas-rich structures where only ~1% of the total mass is in the form of dust) to tenuous debris disks almost devoid of gas occurs at ~10^7 years, by which time the majority of at least the giant planets must have formed. Indeed, these planets are largely gaseous and thus they must assemble before the gas disk dissipates. Spectroscopic studies of the disk gas content at different evolutive stages are thus critical. Far-IR water vapor lines and atomic fine structure lines from abundant gas reservoirs (e.g., [OI]63um, [SI]56um, [SiII]34um) are robust tracers of the gas in disks. Spectrometers on board Herschel will detect some of these lines toward the closest, youngest and more massive protoplanetary disks. However, according to models, Herschel will not reach the required sensitivity to (1) detect the gas residual in more evolved and tenuous transational disks that are potentially forming planets and (2) detect the gas emission from less massive protoplanetary disks around the most numerous stars in the Galaxy (M-type and cooler dwarfs). Both are unique goals for SPICA/SAFARI. Besides, SAFARI will be able to detect the far-IR modes of water ice at ~44 and ~62um, and thus allow water ice to be observed in many protoplanetary systems and fully explore its impact on planetary formation and evolution.