  In a multiplanet system, tides acting on the inner planet can significantly
affect the orbital evolution of the entire system. While tides usually damp
eccentricities, a novel mechanism identified by Correia et al. (2012) tends to
raise eccentricities as a result of the tides' effect on the inner planet's
rotation. Our analytical description of this spin-driven tidal (SDT) effect
shows that, while the inner planet's eccentricity undergoes pumping, the
process is more completely described by an exchange of strength between the two
eigenmodes of the dynamical system. Our analysis allows derivation of criteria
for two-planet coplanar systems where the SDT effect can reverse tidal damping,
and may preclude the effect's being significant for realistic systems. For the
specific case quantified by Correia et al., the effect is strong because of the
large adopted tidal time lag, which may not be appropriate for the assumed
Saturn-like inner planet. On the other hand, the effective Q for any given
planet in exotic circumstances is very uncertain, so the SDT effect could play
a role in planetary evolution.
