Monday, December 22, 2014

Why I don't feel bad for Pluto

Planets and demiplanets and the whichness of why

In 1596, Johannes Kepler wrote, “Inter Jovem et Martem interposui planetam.”

Translated into contemporary English, “There’s gotta be a planet between Jupiter and Mars that we haven’t seen/discovered yet.”

Johnny said that because the math demanded a planet-sized mass there which was affecting the orbits of the other planets. As it turned out, there was a planetary-sized mass there, it just wasn’t all in one big piece. It was a zillion asteroids (actually about 200 larger than 60 miles in diameter, about 40K known), the largest of which was named Ceres when it was discovered. Ceres’ mean radius is about 300 miles which, by current definitions, qualifies it as a dwarf planet, like Pluto. Speaking of Pluto…

In the same way that Johnny K. knew that the math required there to be a planetary mass between Jupiter and Mars, math also demanded a planetary mass out beyond Uranus. In 1930, Clyde Tombaugh discovered Pluto and astronomers decided that Pluto must be “it”. Their ability to determine specifics about Pluto in those days was not very sophisticated and as the years passed and instrumentation improved, doubt about Pluto’s status seeped in. It was not nearly massive enough to be the “planet” (gravitational mass) astronomers were looking for out there.

Imagine if Johnny K. had seen Ceres in his primitive telescope. Perhaps Ceres would have been called “Phaeton,” the planet between Jupiter and Mars, for a lot of years until careful observation determined that it wasn’t big enough to be what they were looking for. AAMOF, in 1801 the astronomer Giuseppe Piazzi did indeed discover Ceres and declared it that “missing” planet. Piazzi called it “Ceres,” other tried to stick their name(s) on it: Hera, Demeter, and of course the generic Phaeton. It was indeed considered a planet for a while, until more precise observation revealed its more-accurate status as one smallish object among many. That’s parallel to what happened with Pluto. Ultimately, astronomers knew that Pluto wasn’t adequate to be what they were seeking.

Nowadays, we know that Pluto is merely one object in the Kuiper Belt (a formation similar to the Asteroid Belt) and it’s not even the biggest object there. For instance, Eris is about 30% bigger than Pluto. This doesn’t make me feel bad for Pluto, personally. I think it’s nice that Pluto is not alone out there in the deep black. He is surrounded by dark companions. And the entire Kuiper Belt is not alone out there, either. The Oort Cloud lies beyond even the distant Kuiper Belt at the far edge of the Solar System, the leaping-off point to interstellar space.

I think that’s pretty cool.

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