About this dwarf planet Pluto

There has been much hand–wringing of late about Pluto being demoted to “dwarf planet.” The Ephemeris carried a couple of articles recently on the subject. One author even suggested grandfathering Pluto in as a planet. Here’s an alternate view.

Pluto is tiny, even smaller than our Moon by about two thirds. That by itself doesn’t necessarily mean Pluto shouldn’t be a planet, but the perspective is interesting.

Try this: Pull out a dime and a dollar coin (Sacagawea). If we let the dollar represent our Moon, then the dime would be about the size of Pluto.

A 150 pound person on Earth would weigh 27 pounds on our Moon, but they’d weigh only 12 pounds on Pluto. Pole vault anyone? :-)

I find the density even more telling. Your typical garden variety rock is roughly 3 g/cm3. Water is 1.00 g/cm3, that’s why rocks sink in water. The Earth’s density is 5.5 g/cm3, we have a core of iron and other heavy elements. Our Moon’s density is 3.34g/cm3 indicating it is made mostly of crust material (rock) and doesn’t have a significant iron core. This, by the way, is strong evidence for the hypothesis that the Moon formed from the debris of a collision between the primordial Earth and a Mars–sized object.

Pluto, on the other hand, is only 1.1g/cm3. This means Pluto is made mostly of frozen liquids and gases. If Pluto were brought in close to the Sun where the Earth is, it would melt and evaporate. It would become a fantastic comet!

Try this: Get a glass of water, stir in a spoon full of dirt, now freeze it. What you have there is not a bad simulated sample of Pluto.

Pluto wasn’t the first to be demoted

Have you heard about poor Ceres? For 51 long years it was a planet, then in 1851 it got demoted to mere asteroid. There were several “planets” discovered by then and counting. As we learned more, it became clear that either we must get used to a solar system with dozens, possibly even hundreds of planets, or we refine our system of planet classification. Based on people’s reaction to Pluto today, I wonder how much of a hue and cry went up about Ceres.

But that absolutely pales in comparison to what happened to the Earth. The Earth was at the center of the whole Universe since the beginning of time! All things revolved around us. Those were glorious and heady times, weren’t they? Then, horror of horrors, around 1543 the powers that be demoted the Earth to a lowly measly planet. We went from THE center to one of many mere planets orbiting the Sun. It’s our own home too, not some tiny ice ball that we barely know way out at the fringes of the solar system. Oh, the ignominy of it all!

How could they do such a thing to poor Earth? Or for that matter, Pluto? Simple, we have this annoying little habit of learning more and more about the nature of things.

The proposal paper

The January 2007 Scientific American has an interesting article about Pluto’s status. They included a link to the original proposal for planetary definition that got this whole ball rolling (“Planet Classification: Gravity Rules”). The paper is available in PDF format at http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf. It gets into some mathematics, but it’s an interesting read if you gloss over the dense parts.

Intrinsic vs Dynamic

There are a couple of approaches to defining a planet. The argument for an intrinsic definition (considering only the qualities of the body by itself) fails to account for the possibility that the body is a moon. For example: some of the larger moons of Jupiter and Saturn could easily be called planets if they orbited the sun. A dynamic classification takes into account the planet’s orbit and how it affects other bodies.

The definition

So, thanks to the IAU, now we have these three requirements for planethood: (final resolution http://www.iau.org/iau0602.423.0.html)1) Orbits the sun.2) Sufficiently massive that gravity dominates its hydrodynamic equilibrium. (Translation: gravity is primarily responsible for it’s shape. This removes the notion of “how round is round.”) But it must not be so massive that fusion can happen or has ever happened in its core. This excludes brown dwarfs.3) It clears its orbit of anything less massive. That is, a planet is the most massive thing in its particular orbit.

OK, about that last one... I (and many others) have a problem with that. Doesn’t this mean there are no planets at all? After all, even Jupiter has other “stuff” floating around in its orbit.

Well, yes, but that’s not a problem. According to the definition paper, they intended for a planet to be capable of clearing it’s orbit zone, not necessarily that it actually has. That’s an important distinction, (something the news media often misses).

Have you heard about this? The definition paper talks of “überplanets” and “ünterplanets,” (translation: superplanet and subplanet).

Most people agree that third qualification could use some refining, but basically it defines an überplanet as one whose mass is such that it will win any fight (dominate) against something else in its orbit. That means it will (if they get close enough) disrupt the orbit of any “opponent” by either flinging it out or in, absorbing it, or forcing some sort of resonance orbit. An ünterplanet is one that cannot do that.

And THAT is the killer that demoted Pluto - it is forced into a resonant orbit by Neptune. Further, Pluto shares it’s orbit with this wide swath of minor planets and planetesimals called the Kuiper belt. Pluto is one of the largest of the Kuiper belt objects (KBO), but it is not capable of clearing them out. So, there it is.

There are planets and then there are PLANETS

If you have trouble with Pluto being a dwarf planet, you’re going to love this: According to the planet classification paper, the Earth is a dwarf ! The media told us that Pluto is now a dwarf planet and the other eight are planets. Right? As is so often the case with the media, we didn’t get the complete story. (We can’t blame them too much, however, since the final resolution doesn’t go into these details.) The definition paper describes several categories. There are supergiants, giants, subgiants, dwarfs, and subdwarfs. And each of those are subdivided into rocky, icy, and hydrogen.

So, here’s what we’ve got: Jupiter is a hydrogen giant. Saturn is a hydrogen subgiant. Uranus and Neptune are icy subgiants. Most of the KBOs are icy subdwarfs. Mercury, Venus, Earth, Mars, and PSR1257+12C are rocky dwarfs. Pluto, Charon, the largest asteroids, some KBO’s, and PSR1257_12A,B are rocky subdwarfs. Even moons are included if you tack on “satellite,” Saturn’s moon Titan is an icy subdwarf satellite.

I don’t understand why they called Pluto rocky and not icy. It also seems to me the first criterion needs a little refinement too. The shape of a blob of water would be dominated by its gravity, but I dare say no one would call that a planet. Of course, a blob of water couldn’t dominate its orbit, so at least there’s that.

Bottom line

I like to think of it this way: We haven’t lost a planet, we’ve gained a whole bunch of minor planets.

For those who still insist on Pluto being retained as a planet, consider this: Why only Pluto? Why not reinstate Ceres too? After all, Ceres got the shaft before Pluto. Then Ceres would be the fifth planet, between Mars and Jupiter. Not to mention the other large asteroids. And last but not least: Of course, we should also reinstate the Earth to its original glorious status too! Shouldn’t we? After all, what’s that tiny little far away ice ball have over the Earth anyway? :-)