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 pump billions of photons in all directions. Planets only reflect light, and how much light they reflect is based on the number of photons that hit them from a star, and what percentage they reflect. It’s much, much, much harder to see a planet than a star.