  Cunha et al. (2018) recently reexamined the possibility of detecting
gravitational waves from exoplanets, claiming that three ultra-short period
systems would be observable by LISA. We revisit their analysis and conclude
that the currently known exoplanetary systems are unlikely to be detectable,
even assuming a LISA observation time $T_{\rm obs}=4$ yrs. Conclusive
statements on the detectability of one of these systems, GP Com b, will require
better knowledge of the system's properties, as well as more careful modeling
of both LISA's response and the galactic confusion noise. Still, the
possibility of exoplanet detection with LISA is interesting enough to warrant
further study, as gravitational waves could yield dynamical properties that are
difficult to constrain with electromagnetic observations.
