Pluto Routinely Set Aside in Planet Discussions
While petitions to protest the IAU’s definition of “planet” and “dwarf planet” swirl around the internet, it is worth remembering the Pluto has always been a misfit as a planet. The standard explanation of planet formation in our solar system neatly describes the four terrestrial planets (Mercury, Venus, Earth, Mars) and the four giant planets (Jupiter, Saturn, Uranus, Neptune). When I teach freshman astronomy from a popular astronomy textbook, Pluto is handled with the other icy objects of the outer solar system, not with eight other planets. Today, attending a graduate-level lecture on the formation of the solar system, we reviewed the concept of the minimum mass solar nebula wherein a lower limit to the mass of the disk from which the planets formed is estimated. In this exercise the masses of the planets are augmented by enough Hydrogen and Helium to make their composition the same as the Sun. We looked at a figure from a thirty-year-old paper that presented this calculation - for eight planets. Pluto was not included because it is obviously not in the same category as the other eight planets. It will make for a cleaner presentation for me in the classroom not to have to explain why Pluto doesn’t fit the patterns of the other planets. It deserves to be the leader of its own class, “dwarf planets”, rather than the misfit in the planet family.
August 31st, 2006 at 7:15 pm
We didn’t cover the planets in freshman astronomy in my university. Too bad. We actually spent most of our time with brightness of objects, special relativity, and describing locations on the “celestial sphere”. But the textbook did have the planets, and Pluto was always explained as an atypical planet, but a typical but large Kuiper Belt Object.
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