For 76 years, every kid learned there were nine planets in the solar system. Then in 2006, astronomers decided there were only eight. Pluto, the smallest and farthest, got demoted to “dwarf planet.”
The reason came down to a new definition of “planet.” To qualify, an object has to orbit the Sun, be big enough to be roughly round under its own gravity, and - this is the new bit - have “cleared its orbital neighborhood” of other similar-sized objects. Pluto fails the third test. It shares its orbit with lots of similar icy objects in a region called the Kuiper Belt.
A lot of people were genuinely upset by the demotion. Pluto had been a planet for as long as most people had been alive. Some scientists still argue Pluto should be reclassified. In the meantime, NASA’s New Horizons probe flew past Pluto in 2015 and sent back stunning photos showing it has mountains, a heart-shaped plain, and an atmosphere. Planet or not, Pluto is still pretty fantastic.
