  Multi-planet systems face significant challenges to detection. For example,
further orbiting planets have reduced signal-to-noise ratio in radial velocity
detection methods, and small mutual inclinations between planets can prevent
them from all transiting. One mechanism to excite mutual inclination between
planets is secular resonance, where the nodal precession frequencies of the
planets align such as to greatly increase the efficiency of angular momentum
transport between planets. These resonances can significantly misalign planets
from one another, hindering detection, and typically can only occur when there
are three or more planets in the system. Naively, systems can only be in
resonance for particular combinations of planet semimajor axes and masses;
however, effects that alter the nodal precession frequencies of the planets,
such as the decay of stellar oblateness, can significantly expand the region of
parameter space where resonances occur. In this work, we explore known
three-planet systems, determine whether they are in (or were in) secular
resonance due to evolving stellar oblateness, and demonstrate the implications
of resonance on their detectability and stability. We show that about 20% of a
sample of three planet transiting systems seem to undergo these resonances
early in their lives.
