Quoted By:
Directly observing something like a potential ninth planet out in the most distant reaches of the Solar System is difficult. Telescopes are limited by the wavelength of light we're interested in and the size of the dish you have. Ground based telescopes are limited because if your dish is big enough then it's own weight begins to deform the thing, defeating the purpose of making a better telescope to begin with (Hence why many observations are done through interferometry). Telescopes in orbit are expensive as hell to send out, let alone make enormous, so they can only be made sparingly. People ask things like "How can we know anything about galaxies several billion light years away but not even if there's a ninth planet in our star system?" For starters, galaxies glow. You can extrapolate plenty of details through light being emitted or reflected by different material within a galaxy of interest, and if it's within a decent enough proximity then you can map out where things are located. Different material congregates differently, such as HI, HII, CO, dust grains, etc, and these are all indicators of various types of phenomena. Using emitted or reflected light to understand astrophysical processes is pretty much the only thing you can do.
This potential ninth planet that is occasionally mentioned would be colossally difficult to observe directly because it would have a tiny angular diameter and almost certainly be tremendously cold. It wouldn't be emitting light strongly even in less energetic wavelength bands. You might be able to hope for a transit of a distant background star but even that would be tough to notice if you weren't anticipating it. The main arguments used are it would be a nice explanation for some orbital behaviors of trans-Neptunian objects but there are alternative explanations that could account for the same results. As for "Every tiny asteroid," those are just too small to really notice, and far too numerous to practically expect to map.
