system
Given the following post, respond as if you're commenting on Reddit:
human
When was it first known and/or confirmed that Mars was another planet and not a star?

In everything I've read trying to answer this question, Mars is referred to as either a heavenly body or a planet. When did Mars go from being a bright spot in the sky to being recognized as another place like Earth?

If there's a big dumb obvious answer for this that I'm missing, the same question would apply to Venus.
gpt
This really depends on what you mean by the term *planet*.

The word "planet" comes from the Ancient Greek for "wandering star." It was recognized since antiquity that Mercury, Venus, Mars, Jupiter, and Saturn all moved differently than the background stars, sliding across the fixed constellations. The ancient Babylonians developed relatively sophisticated models to predict their locations perhaps as early as 2000 B.C. In some sense, then, you could say it was known that Mars was a planet 4000 years ago.

However, that may not really get at the spirit of your question, namely that these were not just stars that moved differently than other stars, but actual separate worlds. The odd thing here is that the last planet visible to the unaided eye to be recognized as a *planet* was Earth itself. It wasn't until Copernicus (roughly 1530 A.D.) that we realized that like the rest of these planets, Earth was in motion through the heavens, circling around the Sun just like the other planets. It was only then that the connection started to be made that if Earth was just another planet, so in turn these planets known since antiquity were also worlds onto themselves.

This was further confirmed by Galileo (1610 A.D.) when he first peered through a telescope at Jupiter and Saturn and saw that these objects were not just points of light, but clearly showed a disc, suggesting they too were substantial worlds.