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

Most people have spoken about how it’s easier to detect stars than planets, which is correct, but I’m gonna go in a different direction.

It’s different to prove the existence of a star than prove the non existence of a planet. It is almost impossible to collect all the evidence that we would require to, 100%, without a shadow of a doubt, prove the non existence of a planet. It’s basically proving a negative, which is fundamentally either impossible or incredibly hard.

If at some time we do detect a 9nth planet all that will do is that we will then ask “But is there a 10nth planet?”