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
There is a common picture of the asteroid belt as a crowded minefield where a spacecraft has to dodge rocks. It is wrong. The average distance between asteroids is about a million kilometers. If you stood on one, you would see nothing but black sky and maybe one faint point of light. The Short Answer … Read more
In 1997, Comet Hale-Bopp was visible to the naked eye for 18 months, longer than any comet in recorded history. It was a chunk of ice and dust maybe 60 km across, sublimating in sunlight, and it put on a show for the entire planet. That is what a comet does when it comes close … Read more
Most people can recite the eight planets in order and then run out of things to say. Ask them what is the solar system made of and the answer usually stops at “the Sun and the planets.” That answer leaves out 99.9% of the objects and almost all of the interesting physics. The Short Answer … Read more