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

It’s the middle of a pitch-black, moonless night. You’re in the middle of a large, empty field. There’s a lit flashlight lying on the ground 200 meters away from you, which shines brightly in the darkness. There’s also a small black ball lying in the grass, within 5 meters of you, but it’s dark, and it’s black, and you can’t see it at all.