We used to think Pluto was a misfit. -Alan Stern
Eighty years ago, we solidly had eight planets in the Solar System: the same eight we have now.
But in the late 1920s, a young astronomer was looking up at the sky, night after night, searching for tiny moving objects that could possibly be planets out beyond Neptune. Using this technique of looking at a patch of sky repeatedly over the course of a week, Clyde Tombaugh searched for moving objects, finding many comets and asteroids, but — like everyone else — found no signs of a new planet.
Until January of 1930. I’ve managed to dig up Clyde Tombaugh’s slides of those two fateful nights that led to the discovery of Pluto.
See where the white arrow is pointing? That’s what Clyde Tombaugh noticed. This faint, tiny object appeared to move, night after night, against the fixed background of stars. Very quickly, this discovery was announced and confirmed, and on February 18th, 1930 (four score and one day ago), it was declared that our Solar System has nine, and not eight planets!
Today, we know that Pluto is not some long-lost frozen world out beyond the gas giants, but rather the first object ever discovered in the Kuiper Belt, the largest structure in our Solar System consisting of thousands upon thousands of tiny frozen worlds, ranging in size from little comets to huge, Moon-sized worlds!
The IAU may tell us that Pluto isn’t a planet anymore, but don’t let a small group of scientists tell you what is and isn’t of great importance in our Solar System. Pluto was the only known Kuiper Belt object for 48 years, and do you know what the second one discovered was?
Charon, Pluto’s giant Moon! (As imaged here by the Hubble Space telescope, and shown along with Pluto.) We didn’t discover a third one until the 1990s, and now we have a deep-space probe on its way to Pluto to roam among the Kuiper Belt, for the sole purpose of scientific exploration.
So happy 80th birthday, Pluto. I may be in the scientific minority, but I still think of you as a giant in our Solar System, and as the ninth classic planet.