Probing Oort clouds around Milky Way stars with CMB surveys
  Long-period comets observed in our solar system are believed to originate
from the Oort cloud, which is estimated to extend from roughly a few thousand
to $10^5$ AU from the Sun. Despite many theoretical arguments for its
existence, no direct observations of the cloud have been reported. Here, we
explore the possibility of measuring Oort clouds around other stars through
their emission at submillimeter wavelengths. Observations with the 545 and 857
GHz bands of the Planck satellite are well matched to the expected temperatures
of Oort cloud bodies (on the order of 10 K). By correlating the Planck maps
with catalogs of stars observed by the Gaia mission, we are able to constrain
interesting regions of the exo-Oort cloud parameter space, placing limits on
the total mass and the minimum size of grains in the cloud. We compare our
measurements with known debris disk systems -- in the case of Vega and
Fomalhaut we find a significant excess that is in agreement with measurements
from Herschel. We use the measurements around Fomalhaut to constrain a possible
exo-Oort cloud of that system. We explore an observed excess around the
brightest and nearest stars in our sample as arising from possible exo-Oort
clouds or other extended sources of thermal emission. We argue that future CMB
surveys and targeted observations with far-infrared and millimeter wavelength
telescopes have the potential to detect exo-Oort clouds or other extended
sources of thermal emission beyond $\sim 1000$ AU from the parent stars.
