  Besides the spirals induced by the Lindblad resonances, planets can generate
a family of tightly wound spirals through buoyancy resonances. The excitation
of buoyancy resonances depends on the thermal relaxation timescale of the gas.
By computing timescales of various processes associated with thermal
relaxation, namely, radiation, diffusion, and gas-dust collision, we show that
the thermal relaxation in protoplanetary disks' surface layers
($Z/R\gtrsim0.1$) and outer disks ($R\gtrsim100$ au) is limited by infrequent
gas-dust collisions. The use of isothermal equation of state or rapid cooling,
common in protoplanetary disk simulations, is therefore not justified. Using
three-dimensional hydrodynamic simulations, we show that the collision-limited
slow thermal relaxation provides favorable conditions for buoyancy resonances
to develop. Buoyancy resonances produce predominantly vertical motions, whose
magnitude at the $^{12}$CO emission surface is of order of $100~{\rm m~s}^{-1}$
for Jovian-mass planets, sufficiently large to detect using molecular line
observations with ALMA. We generate synthetic observations and describe
characteristic features of buoyancy resonances in Keplerian-subtracted moment
maps and velocity channel maps. Based on the morphology and magnitude of the
perturbation, we propose that the tightly wound spirals observed in TW Hya
could be driven by a (sub-)Jovian-mass planet at 90 au. We discuss how
non-Keplerian motions driven by buoyancy resonances can be distinguished from
those driven by other origins. We argue that observations of multiple lines
tracing different heights, with sufficiently high spatial/spectral resolution
and sensitivity to separate the emission arising from the near and far sides of
the disk, will help constrain the origin of non-Keplerian motions.
