Few events in recent astronomical history have had the worldwide emotional resonance as the 2006 announcement that Pluto was no longer considered a planet, at least as far as the International Astronomical Union was concerned. The decision was a long time coming, but no person deserves more credit/blame for forcing the astronomical community's hand than Caltech astronomer Michael Brown. He and his team discovered a number of objects in the outer Solar System -- Eris, Haumea, Sedna, and others -- any of which was just as deserving of planetary status as Pluto. Rather than letting the planetary family proliferate without bound, astronomers decided that none of these objects dominated the orbits in which they moved, so none of them should be planets. Now Brown and his colleague Konstantin Batygin have found indirect evidence that there is another real planet far beyond Pluto's orbit -- which they have dubbed Planet Nine just to remind you that there are currently only eight. [smart_track_player url="http://traffic.libsyn.com/seancarroll/mike-brown.mp3" social_gplus="false" social_linkedin="true" social_email="true" hashtag="mindscapepodcast" ] Mike Brown received his Ph.D. in Astronomy from U.C. Berkeley in 1994, and is currently the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech. He shared the Kavli Prize in Astrophysics in 2012 for his discovery of major new objects in the outer Solar System, and in 2007 won Caltech's annual Feynman Teaching Prize. Home page Wikipedia page Blog Twitter How I Killed Pluto and Why It Had It Coming Online course, The Science of the Solar System Download Episode