• It is really impossible to say. The recognition that some of the lights in the sky moved with respect to the background of stars was apparently made before the beginning of written history. So, we have to lump Mercury, Venus, Mars, Jupiter, and Saturn together in this. These are the planets that are visible with the unaided eye. So, they were discovered by people who were simply noting the positions of things in the sky and we don't know in what order they first noted those five points of light were wandering among the others. Note that I don't list Earth with the others. This is because it was not until the Copernican Revolution (which began in the mid 1500s) that people really began to accept that Earth was a planet. It was not until the invention of the telescope that we can start saying when planets were actually discovered. This is because telescopes give us the ability to see thing that are too dim to see with the unaided eye. The first planet to be discovered with a telescope was Uranus. It was discovered by William Herschel in 1781.
  • The first planet to be discovered was our own - earth. However - I assume you want to ignore that one. I would guess that Venus was the next one as it is clearly visible with the naked eye (it appears like the "first star" in the sky - of course it is a planet not a star).
  • Apparently, Uranus IS just visible with the naked eye under exceptional conditions and was occasionally spotted prior to 1781 but always mistaken for a star. The English astronomer John Flamsteed spotted it in 1690 but erroneously catalogued it as 34 Tauri.
  • I would assume that Venus was the first planet to be discovered, as it is the one most visible to the naked eye.

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