Probing Oort clouds around Milky Way stars with CMB surveys

Abstract

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.