system
Given the following post, respond as if you're commenting on Reddit:
human
How is it possible that something as large as a possible Planet 9 has completely evaded visual observation?

I've heard that if there actually is a Planet 9, it's more than 10x the size of Earth. I understand that Planet 9 is extremely far away, but it seems that something that large could not avoid being seen for so long. How can it be that all of Earth's astronomers have missed it for so long?
gpt
It's a combination of a few things. First, it really is very far away. The amount of sunlight that it receives, if Mike Brown's estimates about distance being hundreds of AU are right (and he's pretty good at this), is *extremely* low. Then the light it receives is scattered back in every direction, not just towards Earth, so the amount of light we could see from it is again that much smaller again. It's likely so faint that you need a very powerful telescope to see it, and those haven't been around for a long time in the scheme of things. 

Another reason is that the sky is big. There's lots of it to search, and there's a trade off between searching a wide area and searching deeply. It's possible that no one has looked at where it is hard enough to see it.

The last main reason is that you don't just have to detect it, you have to notice that it's unusual. It's so far away, that means taking a few images separated by a few hours or days and checking to see if there are very small movements of any objects between frames. If there are, it suggests that the object is in the solar system, but again it's probably so far away that these shifts are very small, requiring someone searching the right way specifically for these kinds of shifts. Otherwise, it would just look like a star. If you looked again years later and didn't see it, you might just assume it was something else that's much more common, maybe a dwarf nova or other transient object that happened to be in outburst during the last observation and isn't anymore. Actually realizing that it's a planet that far out requires a very particular type of search that simply hasn't been done, because it's hard and takes lots of time and there wasn't a good reason to do it before now.