About 20 years ago, kids in school learned about nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. These nine planets orbited the Sun, and they were the only planets that mattered. But things have changed, and the reign of the Nine is officially over. Most famously, Pluto was demoted to “dwarf planet” status in 2006. Bigger shifts in our understanding of the planetary population, however, actually began a decade earlier: In 1995, astronomers discovered a planet with half the heft of Jupiter racing around a sun-like star about 50 light-years away. (A light-year is the distance light can travel in a year. If you were able to fly a ship as fast as the Space Shuttle, it would take about two million years to
