Pluto was demoted in fall 2006. This had been on the back burner for many years, but with the discovery of another Pluto-sized object in the solar system (now called Eris) in 2003, astronomers decided poor Pluto was too small to merit the title of "planet." This simple change in labeling had some not-so-profound consequences.
I was teaching solar system astronomy that fall. Normally, we save the Pluto debate until the end of the year, but in 2006, we moved it up. I also ended up giving several talks around the state about it all, including with my presentation a short segment from ESPN's NFL preview show.
Another minor result was the heartfelt cry of a Hattiesburg sixth-grader, as interviewed in this very newspaper on how to remember the planets: "'My Very Eager Mother Just Served Us Nine Pizzas.' What do we do now?"
That young girl should now be of college age. Wherever she is, I invite her to take my astronomy class at Southern Miss, where we can allay her fears.
All this should make clear an important point - the number of planets isn't fixed. Before the advent of modern science, people recognized seven planets - the sun, the moon, Mars, Mercury, Jupiter, Venus and Saturn. The word "planet" comes from the ancient Greek word for "wanderer," and was applied to these objects because they wander across the sky compared to the constellations. Their names survive in part as the roots of our days of the week.
When Copernicus revived the notion of a heliocentric universe, the sun and moon lost their status as planets, but the Earth joined the club. A "planet" was now an object that independently orbited the sun. The number of planets thus dropped to six.
Matters stood that way until the late 18th century. It was then that a few careful observations restored the number of planets to seven.
William Herschel, a German émigré to England, had (with the help of his sister Caroline) become the best grinder of lenses for telescopes in the world. While scanning along the plane of the solar system, he came across a pale blue disk. At first he thought it might be a comet, but further observations showed its orbit to be virtually circular and about twice as far from the sun as Saturn. It eventually became apparent that this was a planet. At first he tried to name it "George's Star," after his patron, George III of England. Few others went for that, and eventually someone suggested Uranus, in keeping with the mythological theme of the other planets.
The number of planets has gone up and down with our discoveries. For a short while in the early 19th century, Ceres was taken to be a planet; we now recognize it as the largest of the asteroids. Neptune was added in 1846, and our current tally (in our solar system) now stands at eight.
Next: In "our" solar system? There are others? See you then!