Well, how big things can exist unseen? We did see Pluto - but very importantly managed to not see Eris till very recently. What else may we not see? Eris at 95 a. u. was found recently. The total luminosity of a reflective object decreases with fourth power of distance, so that we could miss an Earth-sized object at about 200 a. u., Neptune sized object at 450 a. u. and Jupiter sized object at 800 a. u. There is stuff out there at 900 a. u., because that is where Sedna´s aphelion is. Can we observationally rule out a brown dwarf star at Sedna´s orbit? Nothing is bigger than Jupiter except red dwarf and bigger stars, something which we do have observational evidence against (a star the size of Proxima Centauri and as far from Sun as Proxima is from Toliman would be a naked eye object, though an inconspicuous one). We know that Neptune has some inner heat. But then, we know where to look. Would a body as big and hot as Neptune (50 000 km and 50 K) located far enough to be missed in reflected visible light (say 500 a. u.) be seen in infrared?