When was it first discovered that there are other planets besides Earth?

Answer 1

Yes, YCZ6 is correct.

The planets "wander" amongst the "fixed" stars. That was noticed thousands of years ago. However, to get from "wandering star" to a planet that orbits the sun just as the Earth does, took forward thinkers from Copernicus, through Kepler and Brahe, and sealed by Galileo who turned the newly discovered telescope on the sky.

Why it took so long for the penny to drop can be mainly blamed on the church, that forced people to take its fixed idea of the immutability of the stars and planets. In the age of enlightenment in the 16th - 17th centuries, you would be executed for going against those beliefs, as was Giordano Bruno, and tortured, as was threatened against Galileo. Galileo sidestepped torture and possible execution by denouncing the new beliefs later in his life (and who can blame him).

Answer 2

I'm sure civilizations before them had noticed bright objects in the sky that didn't move relative to the stars, but the Greeks were the first to really pay attention to them. However, it was only in the 16th century that the heliocentric model began to take sway and planets were seen as what we now consider obvious: as large objects that rotate around the sun as opposed to around the earth, as was previously suggested.

Answer 3

The first time some caveman looked up and noticed a bright thing in the sky at night that moved over time.

Answer 4

I'm sure that the dinosaurs all howled at the moon.
But that isn't a planet however.
Just think of it - a big green lizard howling at the moon. . .