  We use Atacama Cosmology Telescope (ACT) observations at 98 GHz (2015--2019),
150 GHz (2013--2019) and 229 GHz (2017--2019) to perform a blind
shift-and-stack search for Planet 9. The search explores distances from 300 AU
to 2000 AU and velocities up to 6.3 arcmin per year, depending on the distance.
For a 5 Earth-mass Planet 9 the detection limit varies from 325 AU to 625 AU,
depending on the sky location. For a 10 Earth-mass planet the corresponding
range is 425 AU to 775 AU. The search covers the whole 18,000 square degrees of
the ACT survey, though a slightly deeper search is performed for the parts of
the sky consistent with Planet 9's expected orbital inclination. No significant
detections are found, which is used to place limits on the mm-wave flux density
of Planet 9 over much of its orbit. Overall we eliminate roughly 17% and 9% of
the parameter space for a 5 and 10 Earth-mass Planet 9 respectively. We also
provide a list of the 10 strongest candidates from the search for possible
follow-up.
