There are five planets you can see without a telescope, so none of them can be kept secret! We can't say that anyone discovered any of them when people have been watching them for thousands of years. Everyone was so used to having just those five, it didn't occur to most people that there could be more of them.
But on March 13, 1781
William Herschel was out in the back yard looking for double stars, which was his special interest. When he found something vaguely fuzzy, he assumed it was a comet and was a bit excited. But in the end it turned out to be a previously unknown planet. He was the first person ever actually to discover a planet.