system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: centuries ago, how did they find the other planets in the solar system? Also, is there a possibility that there are other planets in our solar system that we have not yet found?
gpt
Mercury, Venus, Mars, Jupiter and Saturn are visible to the naked eye and have been known about and studied since ancient times. Early astronomers noticed that these "stars" appeared to change position compared to other stars and so they started to track thier motion across the sky. The word planet actually means "wandering star".

Uranus had been observed but thought to be a star. It took the invention of the telescope to make accurate enough observations to track its movement. Eventually its orbit was calculated with such precision that astronomers noticed oddities that could only be explained by the presence of another planet - which turned out to be Neptune.

Astronomers also thought they saw a similar disruption in Neptunes orbit and so predicted yet another planet. When Clyde Tombaugh pointed his telescope at the patch of sky he expected "Planet X" to be, he discovered Pluto.

We now know that this was a coincidence, and there is no Planet X. Pluto was found because the outer solar system is full of these small icy dwarf planets and Pluto was just in the right place at the right time.