Planet X in CMB and Optical Galaxy Surveys
  We consider the possibility of detecting and tracking the hypothesized Planet
9 or other unknown planetary-mass distant solar system members, generically
called Planet X, with a combination of CMB and optical imaging surveys. Planets
are detectable via thermal emission in CMB surveys and via reflected sunlight
in optical surveys. Since the flux from reflected light falls off faster with
distance, the signal-to-noise of planetary observations with optical surveys
falls off faster than for CMB surveys. A promising approach to detecting new
solar system planets with future surveys such as the Simons Observatory, CMB-S4
and LSST, is for a detection in CMB data followed by tracking in the synoptic
imaging survey. Even if the parallax were not detected in CMB data, point
sources consistent with thermal spectra could be followed up by LSST. In
addition to expanding the Planet X discovery space, the joint datasets would
improve constraints on key orbital and thermal properties of outer solar system
bodies. This approach would work for a Neptune-like planet up to distances of a
few thousand AU, and for an Earth-like planet up to several hundred AU. We
discuss the prospects for the next decade as well as nearer-term surveys.
