Mike Brown still gets hate mail today. That's because his discovery of the tenth planet, which was later demoted to what he affectionately calls "space junk," also led to the demise of Pluto's standing as the ninth planet. Brown, a planetary astronomy professor at Caltech, discovered Eris (first called Xena) in 2005 shaking up the scientific community. "No one had systematically looked across the sky for a new planet since the 1930s, when Pluto itself was found, and even though astronomers knew of almost five hundred bodies in the Kuiper belt, the searches had been, of necessity, piecemeal, and no one had yet mounted a careful search like the one that had uncovered Pluto," he wrote in his fun and humorously geeky memoir How I Killed Pluto and Why It Had it Coming, which was released today. "Now seventy years after the discovery of Pluto, telescopes were bigger and better, computers made searches vastly more powerful, and astronomers simply knew more about what they were looking for. How could it be that if someone went and looked again for a new planet they wouldn't find something that had been just beyond the reach of the telescopes in the 1930s? There had to be a tenth planet."
But when Brown discovered Eris, thanks to work performed on the Hale Telescope at the Palomar Observatory in San Diego County, it threw a simple, but bewildering question out there: "What is a planet?"
In 2006 the International Astronomical Union made a decision, and it was that Eris was not a planet, which meant that similar-in-size, but less dense, Pluto was also not a planet. As Brown explained sitting in his Pasadena office last week, "if you took away a planet, the solar system would be a very different place." That is to say, with or without Pluto, the solar system would not dramatically change.
But for those who grew up knowing there were nine planets in our solar system, the deletion of Pluto may have been shattering and disputable news. For Brown, however--and perhaps much to his chagrin--facts are facts. "That's not controversy, that's just data."