His argument seems sound; I guess you may debate only the numbers. I would assign an a priori distribution of celestial objects on near the solar system concentrated on the the planetary plane, so this effect may compensate a narrow f.o.v. to some extent. Still, for a narrow enough field of view, you can make the argument that the probability of observation will be too low regardless of the a priori distribution details -- leading to the (possibly absurd) conclusion that many more planets probably exist. I&#x27;m just an amateur, but I believe old telescopes had a fairly wide f.o.v.; this is in part because their resolution doesn&#x27;t even allow too much magnification. I assume they also had very flexible directing so you could survey large patches of the sky if necessary (and in particular you could concentrate around the planetary plane if you&#x27;re eager for solar system objects).