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 same reason we can see a lighthouse in the dark, but can’t see the rocks. Once is a giant source of light. The other is a dark object in a dark field.