I was not surprised that the International Astronomical Union decided that Pluto isn't a planet. Even immediately following its discovery, there were doubts about assigning it that status. It's too small, the orbit is too eccentric, and so forth. Add in the fact that other, similar objects have subsequently been discovered further out, some of which may be larger, and there is little choice left but to either add possibly dozens of new planets, or remove one iffy one. But the IAU's actual decision to call Pluto a "dwarf planet" just doesn't sit well with me. (After all, it's not named Dopey.)
In astronomy, some breakpoints are easy to define. For example, the difference between a gas giant, a brown dwarf and a star is that gas giants are too small to achieve deuterium (double-mass hydrogen) fusion, brown dwarfs only achieve deuterium fusion (which doesn't last long, since that form of hydrogen is rare) and not regular hydrogen fusion, and true stars are big enough to achieve the fusion of ordinary hydrogen atoms.
The breakpoint between gas giant and rocky planet is more obscure. The best determining factor is that a true gas giant must have most of its mass in elements and compounds which are gases rather than solids. That firmly places Venus in the rocky planet category. Though having far more atmosphere than Earth the total mass of atmosphere is still far less than the mass of the rocky portion.
Part of the problem with determining exactly what Pluto is comes from the fact that we don't know exactly what it is made of. We only recently launched a probe to perform this task. About a year after its launch, early in 2006, it passed Jupiter in a slingshot maneuver. The gravitational boost it received from that gas giant will help it reach Pluto in only eight more years. (The fact that we could have had a probe do this within Clyde Tombaugh's lifetime and didn't is scandalous.) Pluto is thought to be mostly rock with a thick layer of ice and frozen gases (that is, substances which would be gaseous under terrestrial conditions) on the surface. The atmosphere is currently both denser and more active than during most of Pluto's long year because the body (whatever classification it is eventually placed in) has recently made its closest approach to the Sun. However, it is already cooling and precipitating out, so we missed the peak.
We have pretty good estimates of Pluto's physical size from telescope observations, and the periods of its moons gives us a direct measure of the mass. That lets us calculate the density, which supports the conclusion that the mix listed above is accurate. One viewpoint is that Pluto is basically a very large asteroid which is far enough from the Sun that its primordial ice and gas haven't completely evaporated away. That really doesn't fit the image of any of the rocky inner planets, or the gas giant outer planets, or even a comet or asteroid. It does resemble several gas giant moons, though, of which more later.
Then there's the orbit. I heard a newscritter on TV who was talking about this matter smugly announce "Of course Pluto isn't a planet. Planets have cir-cu-lar orbits. Pluto has an e-lip-ti-cal orbit." Ignoring all astronomy from Kepler on and implying that the scientists who hadn't used this "distinction" to settle the matter were all idiots. And, yes, she pronounced those words in that labored fashion, obviously unaccustomed to them.
Now, I can see where she got this misconception. (\T\h\e\ \U\S\ \e\d\u\c\a\t\i\o\n\ \s\y\s\t\e\m\.\) All bodies orbiting another body have elliptical orbits, but for most planets the difference from a true circle is small, something requiring detailed observations to determine. So she ignored that variation, mentally rounding both the orbits and the math. And also ignoring that this small difference drove many astronomers to distraction for centuries until Johannes Kepler finally figured it out.
Pluto, on the other hand, has an orbit which is obviously elliptical. So elliptical that it crosses the orbit of Neptune. One theory - which also explains some of Uranus's peculiarities - is that Pluto was either previously a moon of Uranus or a moon of whatever knocked that gas giant over on its side. (According to Larry Niven, that was a Slaver ship in stasis moving at high speed, but I doubt any of those had Pluto as a moon.)
That orbit is a major blow against Pluto being a planet. Because planets shouldn't cross the orbits of other planets. The harmonic resonances which keep a planetary system stable would almost certainly change this situation after enough cycles. (Pluto's very long orbital period would slow this process, but even so it has probably been gravitationally pumped a significant amount over the past few billion years. In only a few more billion it might not even cross Neptune's orbit. Of course, it's orbit might already be in harmonic stability. Last I heard, the discussion was ongoing. Pluto and Uranus currently have a 3:2 orbital resonance, but this could be influenced by the other gas giants over time.) Add in the steep, 17 degree inclination from the plane of the ecliptic - several times that of any true planet - and you have something which looks tacked on and doesn't really seem to belong with either the classical planets or Uranus and Neptune.
But "dwarf planet"?! No, thank you. Don't you people have any romance in your souls? Not that I object to the classification per se; it's just that "dwarf planet" and "minor planet" have long been used to describe the largest asteroids, such as Ceres. Which itself was one of those discoveries which challenge our preconceptions. As Pluto did after it, the asteroid Ceres confounded the notions of astronomers at the time of its discovery, disrupting their neat and orderly classification system. Pluto isn't the same as any of the major asteroids in composition or behavior.
Personally, I think we should declare Pluto to be the first-discovered of a new class of bodies. "Kuiper Belt Objects" would work technically, but most of what is out there would be comets, with a far higher percentage of gas than is possessed by Pluto. Also, as mentioned above, Pluto may not be from there. So my suggestion is to declare Pluto to be a "Tombaugh Object." We can then use the opportunity when someone asks "A what object?!" to explain about the modest farmboy who discovered something new in our universe. A very American story which the members of the IAU who changed how Pluto is labeled obviously don't appreciate.
This document is Copyright 2007 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: firstname.lastname@example.org