The Atacama Cosmology Telescope: A Search for Planet 9

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 per year, depending on the distance (r). 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 predicted aphelion and most likely location of the planet corresponds to the shallower end of these ranges. The search covers the whole 18,000 square degrees of the ACT survey. No significant detections are found, which is used to place limits on the millimeter-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. These bounds approach those of a recent INPOP19a ephemeris-based analysis, but do not exceed it. We also provide a list of the 10 strongest candidates from the search for possible follow-up. More generally, we exclude (at 95% confidence) the presence of an unknown solar system object within our survey area brighter than 4-12 mJy (depending on position) at 150 GHz with current distance 300 au < r < 600 au and heliocentric angular velocity 1.'5 yr(-1) < nu.500 au/r< 2.'' 3 yr(-1), corresponding to low-to-moderate eccentricities. These limits worsen gradually beyond 600 au, reaching 5-15 mJy by 1500 au.