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

Stars are bright, so they’re always shining. Planets are dark, and the only way we can tell if they’re there is if they block a star that we know is there.