system
Given the following post, respond as if you're commenting on Reddit:
human
How come we are able to detect planets thousands (millions?) of light years away from us, but we have yet to determine with 100% certainty whether or not there is a ninth planet in our solar system?
gpt
Our main method to detect planets orbiting distant stars is by observing the star when the planet moves in front of it. This briefly makes the star appear dimmer as the planet blocks part of the light of the star. If this happens on a regular interval, we assume that this dimming is caused by a planet and its periodic orbit.

An additional planet in our solar system can't be detected this way, since such an object would never come between the Earth and the Sun. Since this hypothetical planet would be far away from the Sun, it would only receive and reflect a small amount of light, making it very hard to detect. Detecting a very dim speck of light amidst a sky full of objects that are much brighter, without any idea about where to start looking, is worse than finding a needle in a haystack.