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

The Star is very bright, so it’s very easy to see.

The Planet doesn’t produce light of its own, so it needs to reflect someone else’s light back at us for us to see it.

The sun is basically just an unusually large Star when you’re that far away… so there’s not much light to hit it and bounce back. For us to see it, we’ve got to catch and focus a lot of the returning light.