Why we can accurately detect stars billions of light years away, but we can’t confirm if we have a 9th planet in our solar system?

Question

Why we can accurately detect stars billions of light years away, but we can’t confirm if we have a 9th planet in our solar system?

In: 2123

Answer 1

Those stars radiate light that hit our earth. Planets, don’t normally radiate light (they do a bit depending on if there’s heating in the core for example) like stars that radiate light because of nuclear fusion.

So simply put, whatever planet might exist is probably very dark and far enough away that we wouldn’t be able to detect much anything else. While stars can’t help but to radiate light and show us where they are as long as we’re in the direction their light is traveling.