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We have explored a method for finding giant planets in the outer Solar Systemby detecting their thermal emission and proper motion between two far-infraredall-sky surveys separated by 23.4 years, taken with the InfraRed AstronomicalSatellite (IRAS) and the AKARI Space Telescope. An upper distance limit ofabout 8,000 AU is given by both the sensitivities of these surveys and thedistance at which proper motion becomes too small to be detected. This papercovers the region from 8,000 AU to 700 AU. We have used a series of filteringand SED-fitting algorithms to find candidate pairs, whose IRAS and AKARI fluxmeasurements could together plausibly be fitted by a Planck thermaldistribution for a likely planetary temperature. Theoretical studies haveplaced various constraints on the likely existence of unknown planets in theouter solar system. The main observational constraint to date comes from a WISEstudy: an upper limit on an unknown planet's mass out into the Oort cloud. Ourwork confirms this result for our distance range, and provides additionalobservational constraints for lower distances and planetary masses, subject tothe proviso that the planet is not confused with Galactic cirrus. We found 535potential candidates with reasonable spectral energy distribution (SED) fits.Most would have masses close to or below that of Neptune (~0.05 Jupiter mass),and be located below 1,000 AU. However, examination of the infrared images ofthese candidates suggests that none is sufficiently compelling to warrantfollow-up, since all are located inside or close to cirrus clouds, which aremost likely the source of the far-infrared flux.

Searching for giant planets in the outer Solar System with far-infrared all-sky surveys