  The obliquities of planet-hosting stars are clues about the formation of
planetary systems. Previous observations led to the hypothesis that for
close-in giant planets, spin-orbit alignment is enforced by tidal interactions.
Here, we examine two problems with this hypothesis. First, Mazeh and coworkers
recently used a new technique -- based on the amplitude of starspot-induced
photometric variability -- to conclude that spin-orbit alignment is common even
for relatively long-period planets, which would not be expected if tides were
responsible. We re-examine the data and find a statistically significant
correlation between photometric variability and planetary orbital period that
is qualitatively consistent with tidal interactions. However it is still
difficult to explain quantitatively, as it would require tides to be effective
for periods as long as tens of days. Second, Rogers and Lin argued against a
particular theory for tidal re-alignment by showing that initially retrograde
systems would fail to be re-aligned, in contradiction with the observed
prevalence of prograde systems. We investigate a simple model that overcomes
this problem by taking into account the dissipation of inertial waves and the
equilibrium tide, as well as magnetic braking. We identify a region of
parameter space where re-alignment can be achieved, but it only works for
close-in giant planets, and requires some fine tuning. Thus, while we find both
problems to be more nuanced than they first appeared, the tidal model still has
serious shortcomings.
