Up until pretty recently, there were no all-sky surveys done of the Infrared sky (until the 80s or so). When the first antarctic telescopes opened up, and when the first infrared telescope flew in space, the very first project that was done was an all-sky survey...after all, what would be the point of building a telescope if you didn't know what interesting things you could see out there? You are almost guranteed to discover interesting new phoenomena (something that happened almost immediately with every other wavelength band).
So an object like Jupiter actually radiates more energy in the IR than it recieves from the Sun. I'd have to work through the math, but I am pretty sure that the all-sky surveys done with the Spitzer IR space telescope are good enough to see Jupiter in the IR out to about 1 light year or so (that's 60,000 AU). (Alpha Centauri is about 3.something light years out). The Oort clouds are believed to be much much closer, 1000 AU at most. The limit on something like a Uranus sized mass is a bit closer, around 10,000 AU or so, but the gravitational force by such a body falls pretty fast, so as to have much less of an effect. As such a body orbits the sun quite slowly, one wouldn't have to worry about missing it because it moved while you weren't looking for it.