Still mourning the loss of Pluto as a planet? Blame Caltech astronomer Michael Brown. His 2005 discovery of Eris, a new "planet" past Pluto, prompted other astronomers to redefine what makes a planet, and eventually rule out Pluto and Eris and settle on eight true planets. Pluto and Eris now have the new designation of dwarf planet.

Brown will present the 25th annual Bunyan Lecture at Kresge Auditorium on Wednesday, Oct. 25, at 7:30 p.m., telling the story of his discovery and explaining why it prompted changing the definition of a planet. His talk, titled "Pluto, Eris and the Dwarf Planets of the Solar System," is free and open to the public. The Astronomy Program in the Department of Physics will host the event.

Although initially disappointed that his discovery, Eris, was demoted, Brown said, "It was the right thing to do. I think eight makes much more sense."

Brown scanned the sky for seven years before finally finding what he thought was the "10th planet." Eris is slightly larger than Pluto and orbits three times farther from the sun than Pluto-making Eris the most distant object ever seen revolving around the sun.

The discovery of Eris brought up a dilemma: Just what qualifies as a planet? The familiar idea of nine planets around the sun was dear to many. But for years astronomers had protested that the old classification system, based on purely historical considerations, just didn't make sense. With a new object requesting entry to the club, and the possibility of many more, astronomers had to rethink the issue.

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