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 emit light and other electromagnetic frequencies that we can detect.

Planets reflect those from stars.

So if the 9th planet is too far away or non-reflective, we can’t see it.

We can only know it’s there because it has gravity and it’s gravity impacts other objects.