There must be more to the story to support the idea that Galileo really had a clue he had a possible planet in hand.

It is surprisingly coincidental that Neptue was closer to Jupiter than some of the moons of Jupiter during just the time he was initially tracking the moons. He likely thought that Neptune was a satellite candidate for Jupiter, but would have quickly dismissed it once it moved off along with the stars.

Galileo was not shy, and I don't see why he would have recognized Neptune as a planet and not mention it anywhere. If he did mention it, then I would be interested in hearing about it.

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Lighten up! This is a stellar board!