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 only time we can detect an individual star that is billions of LY away, is when that star explodes—a supernova. It’s difficult to overstate how bright a supernova is.

Planets, especially the outer planets, are very cold and dark, making them very difficult to see.