It was the planet closest to the sun, orbiting the sun in 88 days. Well, a couple of those facts are incorrect. Mercury is not the smallest planet. Pluto is much smaller. Actually, I am not upset.
Science progresses by correcting old misconceptions.
And this is an opportunity to see actual science at work. Those facts in the first paragraph are kind of dry. But, the reason some were wrong is fairly interesting. Pluto was thought to be bigger than Mercury, because it is fairly bright, for a small planet way out there away from the sun. No one ever saw a visible disk, in any photo of Pluto.