  We search for extrasolar planets around millisecond pulsars using pulsar
timing data and seek to determine the minimum detectable planetary masses as a
function of orbital period. Using the 11-year data set from the North American
Nanohertz Observatory for Gravitational Waves (NANOGrav), we look for
variations from our models of pulse arrival times due to the presence of
exoplanets. No planets are detected around the millisecond pulsars in the
NANOGrav 11-year data set, but taking into consideration the noise levels of
each pulsar and the sampling rate of our observations, we develop limits that
show we are sensitive to planetary masses as low as that of the moon. We
analyzed potential planet periods, P, in the range 7 days < P < 2000 days, with
somewhat smaller ranges for some binary pulsars. The planetary mass limit for
our median-sensitivity pulsar within this period range is 1 M_moon (P / 100
days)^(-2/3).
