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.
