  Obliquity tides are a potentially important source of heat for extrasolar
planets on close-in orbits. Although tidal dissipation will usually reduce the
obliquity to zero, a nonzero obliquity can persist if the planet is in a
Cassini state, a resonance between spin precession and orbital precession.
Obliquity tides might be the cause of the anomalously large size of the
transiting planet HD 209458b.
