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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 planet on its own. What’s more, it shares its orbital zone with many other objects. These bodies all belong to the Kuiper Belt — a wide band beyond the orbit of Neptune. Its members are icy leftovers from the birth of the planets. They were hurled into this region by the gravity of the giant outer planets. That makes them like the leftover raw materials for a house, all swept out of the way when the house is built.
More about dwarf planets tomorrow.