Until 1992, Pluto was alone out there. Then astronomers found 1992 QB1, a 160-km icy body beyond Neptune, and within a decade they had found a thousand more. Pluto had not been a lonely outlier. It was the largest member of a belt that had been invisible for the entire history of astronomy. The Short … Read more
Pluto was a planet for 76 years. Then, on August 24, 2006, a room full of astronomers in Prague voted to change the definition, and it became something else. The decision took less than a day. The argument about it has lasted two decades. The Short Answer Why is Pluto not a planet? Because it … Read more
In 2006, the International Astronomical Union created a new category of object and immediately put Pluto in it. Sixteen years later, the New Horizons spacecraft flew past Pluto and found mountains, glaciers, haze layers, and a nitrogen ice plain the size of Texas. If that is not a planet, a lot of people asked, what … Read more