Dwarf Planets

Ten years ago today, a team led by Caltech astronomer Mike Brown discovered what seemed to be the solar system’s tenth planet, an icy body bigger and farther away than Pluto. A couple of years later, though, an international group of astronomers decided otherwise: Not only was the newly found object not a planet, neither was Pluto. Instead, both were lumped into a new category: dwarf planets. Today, the official list of dwarf planets names five objects. The list includes Pluto; Eris, the would-be tenth planet; and Ceres, the largest member of the asteroid belt. But astronomers have discovered many more objects that are likely to receive dwarf-planet status in the next few years. A dwarf planet is defined as an object that’s massive enough to have pulled itself into a roughly spherical shape, but not massive enough to have cleared out its own orbit around the Sun. Astronomers had been debating what is and isn’t a planet for quite a while. Many felt that Pluto wasn’t big enough to be a p