Brown still gets letters and late-night obscene calls from people who miss having a ninth planet, but he has no regrets. Instead of ten planets, the solar system now had eight. But the existence of Eris raised troubling questions, such as: What’s a planet, exactly? And if Eris is a planet, why not also various other small spheres that orbit the sun? In the end, the International Astronomical Union categorized Eris as a dwarf planet-a polite phrase for “not a planet at all”-and, with Brown’s encouragement, Pluto was demoted, too. Eris was about as big as Pluto, which was still a planet back then, and it orbited the sun at a distance nearly three times greater. This was in 2005 Brown, an astronomer at Caltech, had spotted an object that officially became known as Eris (he preferred the nickname Xena). Very briefly some years ago, Mike Brown discovered the tenth planet in the solar system. Astronomers at Caltech believe they’ve found an ice giant out beyond Neptune.
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