THE picture shows Ceres, the largest object in the asteroid belt, as seen a few days ago from Dawn, an American spacecraft that is en route there. Ceres’s gravity is strong enough to make it round, like a planet. And it probably has a core and a mantle, like Earth (though the core is thought to be rocky, and the mantle icy). It may even have a thin atmosphere.But Ceres is not a planet. It was classified as such in 1801, when it was discovered, but soon after it was spotted astronomers started finding other objects in the junkyard of rock and ice that is now called the asteroid belt. The idea of calling all of them planets began to look silly, and so Ceres was quietly demoted. These days it is classed as a “dwarf planet”, one of at least five in the solar system.The most famous of them is Pluto, which was, in 2006, demoted from full planethood by the International Astronomical Union, amid much wailing and gnashing of teeth. Pluto now is merely the biggest object in the Kuiper Belt, a second group of asteroids, which extends far beyond the orbit of Neptune, the most distant of the true...
from The Economist: Science and technology http://ift.tt/1txtJlM
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