Planned Planethood Exposed
This is getting too confusing.
Do you remember this mnemonic: “Men Very Easily Make Jugs Serve Useful Needs, Perhaps?” How about the even more loaded “Mary’s Violet Eyes Make Johnny Stay Up Nights Plenty?” No matter what oddly risque memory-jogger you were taught to remember the planets, it required nine or less words. And you still forgot it. Proposed new planetary regulations could force your poor grade school teacher to pen the Gettysburg Address of mnemonics: 110 words!
Astronomers have taken to quibbling over what is and what is not a planet. This leaves the rest of us struggling to rewrite our memory whenever some tiny speck with an unpronounceable name is inducted into the club or tossed out. Before I propose a scheme to stop this madness for good, let’s survey the recently proposed new rules. Our own Ross Pomeroy described the situation this week. Here’s a brief recap:
Scheme I (current): A planet is “a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
Scheme II (proposed): “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of orbital parameters.”
That’s much too complicated. I propose a third and simpler way that will keep the numbers and names of planets at bay. There are eight planets, no more and no less. Here’s why.
Earth, Mars, Venus, and Mercury are all similar to one another. And they are also extremely different from everything else around them. Each is a big (nearly) spherical ball orbiting well inside the asteroid belt. There are zero other bodies like them orbiting in this infield region of the solar system. In one sense, they are four siblings. Call them "Planets A" for simplicity.
There are another four siblings in orbit, too. Jupiter, Saturn, Uranus, and Neptune are massive bodies that dwarf everything but the Sun. Jupiter alone has more mass than 144,000 Plutos or 473,000 Haumeas. (You mean don't remember planet candidate Haumea?) The gas giants are completely unique from any other objects circling Sol. They all orbit together in the vast outfield of the solar system and have many similarly strange proclivities. Let’s call them Planets B.
That makes eight planets total: the four Planets A plus the four Planets B. Are there any other groups of large important objects in the solar system? No, everything else is just a sad little piece of flotsam that happens to orbit the sun.
What about Pluto? The discovery of Pluto on photographic plates was an amazing feat of squinting. This faint speck of light escaped detection for more than a decade before Clyde Tombaugh noticed it (try your own eye here). At the time it was a unique object in the solar system. Modern astronomy has since found several more distant lopsided specks. Pluto is now neither unique nor part of a cohesive grouping of planets. It’s one of a bunch of odd “other” things with little importance to the greater solar system.
How about exoplanets? For now we can only detect large exo-Jupiters and occasionally smaller exo-Earths. Maybe we can cross the bridge of what to do with exo-Plutos when we come to it.
Rather than coming up with an increasingly convoluted and tricky set of rules to decide whether every arbitrary rock floating around the sun is a planet or is not a planet, why not keep it simple? Let’s decide to call the eight important objects planets. Everything else is just a rock. Why? Because we’re the parents, because we make the rules, because we said so.