The Science Case for Why Pluto Still Isn’t a Planet
From 1929 until 2006, Pluto lived in the imagination of children and adults alike as the ninth and outermost planet in our solar system. Until 1978, with the discovery of its giant moon, Charon, it was the only known large object in our solar system that orbited beyond the reach of Neptune. But the story began to change shortly after that. In the 1990s and 2000s, a tremendous number of new objects were discovered — including planets orbiting stars other than our Sun (exoplanets) and a wide variety of Kuiper belt objects (trans-Neptunian objects) both large and small — that compelled us to rethink just what it meant for an object to be considered a planet.