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

For the same reason it’s easy to count the lights in a city but very hard to count people at night. One emits data and the other only poorly reflects it.