Eli5: Why can’t we figure out the planet after Neptune in our Solar System but we are able to find out galaxies thousands of light years away from us ?

Question

Recently have been binge-ing on planets and cosmos, and I saw this youtuber mentioned that we might discover the next planet after Neptune.

However from my understanding, humanity has discovered black holes, magnetars, stars and so on that are hundreds, thousands or even more than that in terms of the measurement of light years.

Our solar system is not in the measurement of light years, yet we can’t discover the next planet after Neptune. Why ?

In: Technology

Answer 1

Another thing worth considering is we don’t look for a specific galaxy, we take a picture of the sky, and say “hey, look, there’s a galaxy there!”.