  A distant, as yet unseen ninth planet has been invoked to explain various
observations of the outer solar system. While such a 'Planet Nine', if it
exists, is most likely to be discovered via reflected light in the optical, it
may emit much more strongly at 3$-$5$\mu$m than simple blackbody predictions
would suggest, depending on its atmospheric properties (Fortney et al. 2016).
As a result, Planet Nine may be detectable at 3.4$\mu$m with WISE, but single
exposures are too shallow except at relatively small distances ($d_9 \lesssim
430$ AU). We develop a method to search for Planet Nine far beyond the W1
single-exposure sensitivity, to distances as large as 800 AU, using inertial
coadds of W1 exposures binned into $\sim$1 day intervals. We apply our
methodology to $\sim$2000 square degrees of sky identified by Holman & Payne
(2016) as a potentially likely Planet Nine location, based on the Fienga et al.
(2016) Cassini ranging analysis. We do not detect a plausible Planet Nine
candidate, but are able to derive a detailed completeness curve, ruling out its
presence within the parameter space searched at $W1 < 16.66$ (90%
completeness). Our method uses all publicly available W1 imaging, spanning 2010
January to 2015 December, and will become more sensitive with future
NEOWISE-Reactivation releases of additional W1 exposures. We anticipate that
our method will be applicable to the entire high Galactic latitude sky, and we
will extend our search to that full footprint in the near future.
