Thanks to a hot tip via cuddlyeconomist I was able to make it down to Seattle Town Hall tonight after work in time to see Neil deGrasse Tyson speak. He was touring for his new book, The Pluto Files. He summarized the story of how he became identified as the "Man Who Killed Pluto" hated by 3rd graders across the United States, and gave a humorous and scholarly explanation of the history of the planets.

The thrust of his narrative is that the word planet had no formal definition until 2006. It originally meant the 7 wandering stars known in Greek times: Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. Copernicus defining a heliocentric solar system drop the number by 2 (Sun was in the center, the Moon went around the earth so neither are "planets" in the sense of the others) but added 1, the Earth.

Uranus was discovered in 1690 by accident. Dr Tyson then told the story about how it was originally called Planet George for 50 or so years, and how planets and moons are named. There is a reason why the moons of Uranus are named things like Titania and Oberon instead of names from Greek mythology.

Anyhow the orbit of Uranus did not agree to the orbital predictions based on the known planets, so it lead to the belief of a "Planet X" to account for it. During the 1800s a dozen planets were discovered between Mars and Jupiter, and then later reclassified as asteroids in the asteroid belt. Neptune was found in 1846 based on the predictions for "Planet X". In turn Neptune's orbit was wonky, and led everyone to look for another "Planet X". No predictions made from the orbits could find it, but a systematic search in 1930 turned up Pluto which was presumed to be "Planet X" (the second).

It turns out that when looking at the data for the orbit of Neptune, it was later discovered that the data at one particular observatory was suspicious due to some rather extensive maintenance work done in the middle of the very long timescale observations of Neptune's orbit (which is 164+ years). Throwing out the questionable data, it turned out Neptune's orbit was actually completely as expected. There never was another "Planet X".

Instead Pluto is the first Kuiper belt object every discovered. Dr. Tyson feels that the honor of finding the first of an entirely new class of object should be viewed as a greater scientific accomplishment than finding the ninth and puniest of something we already had 8 others of already.

Afterwards he took various questions from the audience, and his responses included some chiding of cult of Stephen Hawking, an impassioned defense of the US space program, and inspired several people to take on the challenge of acquiring a facility with advanced math.

Dr. Tyson is a worthy successor to Carl Sagan's legacy, and carries the mantle of exciting a new generation with the thrill of the scientific endeavor well. Check him out if you get the chance.