Bye Bye Pluto (Finally)

Pluto and Charon, mutually attracted rocks, now called "dwarf planets."

I've been teaching Astronomy for four years, and every year I've told my students that Pluto should not be classified as a planet. Smaller than Earth's moon, it has none of the properties that would make it a true planet in terms of size and composition. Pluto's composition matters in terms of classification because of how the solar system formed (see previous post). There are terrestrial planets nearest the Sun and gaseous planets farther out. Pluto is one of thousands of rocks outside of this gas giant region, and should be grouped accordingly. Its orbit and location should put it in a class with other rocks called "Kuiper Belt Objects" in the outer reaches of the solar system. The only shame is that Pluto's orbit, although highly irregular when compared with the other eight planets, does happen to coincide with Neptune's every once in a while, so it is the closest of the K.B.O.'s. Hopefully this "controversy" will make people want to know more about the formation of the solar system, not less. This change reminds me of why I prefer Science to belief; we can change our minds when new evidence arrives. Thanks to Xena and Quaoar and all of the other big rocks out there!