Image Credit: NASA / ESA /H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team
I get back from a fun week of supernova discussions, and the top astronomy news story I spy is on CNN, which says, "Debate over Pluto rages on." Alas. It's the three-year anniversary of the International Astronomical Union's vote to revoke Pluto's planetary membership card, so I guess these stories are bound to come up. The problem is, there's really nothing new, it's just the Earth has gone around the sun three times since the vote was taken.
So, I thought I'd sum up my opinion on the matter. And this opinion starts with one crucial bit of information: I'm taking as pure of a scientific stand as I can, which means setting aside history, public opinion, and as much human emotion as possible. That's part of my job as a scientist. I am supposed to be able to look at factual evidence and see if that evidence supports a hypothesis, no matter where that evidence or hypothesis came from.
So, let's look at the Solar System and start with something easy. Our Solar System is dominated by the sun. Roughly 99.987% of the mass in the solar system is found in the sun. The sun is almost at the dynamical center of the Solar System. The sun is the only thing in our Solar System that produces its own energy by fusion reactions. The sun is clearly different from everything else in our Solar System. So we can set it apart.
Alrighty, let's look at what is left. Jupiter has about 3/4 of the remaining matter in the Solar System. It is a big, almost spherical collection of gas (with maybe some rocks in its core, but we can't see those). It's mostly made out of hydrogen and helium, but has never performed nuclear fusion. But Jupiter has a smaller sibling, Saturn, and a couple of similar cousins, Uranus and Neptune. So, although there are some differences between these four objects, they are all similar enough that we'll lump them together for the time being. For the lack of a better term, let's call them "gas giant planets".
Okay, now we are getting to the hard stuff. There remains about two or three Earth masses of material left in the Solar System, and this material is distributed among a bunch of rocky things. Ordered by decreasing diameter, Earth is the biggest, Venus a close second, and Mars a distant third. These rocky bodies are able to hold on to an atmosphere. Next, in radius, comes Ganymede, Jupiter's largest moon, which doesn't have an atmosphere. Then we have Titan, Saturn's largest moon, which has an atmosphere as thick as Earth's. Now we get to Mercury. Next come a bunch of moons: Callisto, Io (Jupiter), the Moon (Earth), Europa (Jupiter), and Triton (Neptune). Then comes Eris, the largest known member of the Kuiper Belt, a collection of icy bodies further from the Sun than Neptune. Finally, in 13th place among the rocky/icy things in our Solar System, we get to Pluto.
If we keep going down in size, we quickly pick up many more icy objects in the Kuiper Belt and the largest asteroids in the asteroid belt between Mars and Jupiter. Ceres, the largest asteroid, is less than half the diameter of Pluto, and only about 1/6000th the mass of the Earth. As we keep looking smaller, we find more and more asteroids down to the smallest sizes we can see -- objects only a hundred meters across or even less!
There's an important point here. Once we start looking at rocky things in our Solar System, there's no obvious demarcation between types of objects. The bigger "terrestrial planets" all have atmospheres, but so do some moons. A couple of moons are larger and more massive than the classical planet Mercury. Some things are icy, some are rocky. Things the size of the largest asteroids and bigger are all round in shape. So, from considering the size, mass, shape and compositions, there are some lines that could be drawn, but none of these classifications seem obvious.
Okay, so let's look at something else. Let's look at the structure of the Solar System. Sun near the middle, all by itself. Then comes some of the small rocky things: Mercury, Venus, Earth (with its moon) and Mars, with a few tiny asteroids scattered about in there. In this case, the asteroids in this region are so small compared to the big five rocky things that they look fundamentally different. Continuing outward, we next find the asteroid belt, the collection of most asteroids, including the biggest ones: Ceres, Pallas, and Vesta. Next come the four big gas giant planets, each of which has a bevy of much smaller moons, and with a few asteroids and other small icy things swimming about. Last comes the Kuiper Belt, which has a lot of icy things of all sizes. Eris and Pluto are both clearly in this Kuiper Belt -- they're the biggest things in the Kuiper Belt, but otherwise they share similar orbits at similar distances from the Sun.
In this structural view of the Solar System, the four gas giants and four of the rocky bodies (Mercury, Venus, Earth and Mars) all stand out as unique, dominating their respective parts of the Solar System. The asteroid belt is its own thing, and the Kuiper Belt is its own thing. In this view, we can call the eight unique things "planets", and everything else "non-planets". It seems a natural division, and we can define mathematical and physics-related definitions that would include the eight planets in the "planet" category and relegate everything else to the "other" category.
But this definition is also not satisfactory. If we were to take the Earth, stick it on a circular orbit in the middle of the Kuiper Belt, and let billions of years go by, the result would be a round ball of ice the size of the Earth in the middle of a bunch of other smaller balls of ice. An astronomer looking at this Solar System would probably just include Earth as a giant member of the Kuiper Belt. Mars or Mercury certainly would just be considered larger Kuiper Belt objects. But if we moved Jupiter into the Kuiper Belt, it would quickly fling every tiny ice ball out into deep space or down into the sun, and the Kuiper Belt would quickly cease to exist, other than the colder version of Jupiter happily orbiting the sun. In similar thought experiments, Jupiter, Saturn, Uranus and Neptune would still be unique objects, but the smaller rocky planets would not be.
So, from a scientific-motivated standpoint, I think it is unclear if even the Earth should qualify as a planet. Certainly the Earth is unique given its current place in the Solar System, but if it were a moon of Jupiter or out in the far reaches of the Solar System, it would be just another ball of ice. In short, I would argue that there is no good definition for a planet, at least one that would include the Earth and Mars and Mercury while excluding Jupiter's larger moons or giant ice balls in the Kuiper Belt. The rocky things in our Solar System form a continuum of sizes and relevance, and any lines we draw are, to some extent, arbitrary. I'd argue that this is true even if these distinctions are motivated by physics (such as whether an object's gravity dominates part of the Solar System, or if its gravity is strong enough to make it round)!
So, back to poor Pluto. What should we call it? I think that, had we not known about Pluto and we were to discover it today, we would not call it a planet. We'd call it a member of the Kuiper Belt. But, as I've described in far too many words, even a scientifically-motivated definition is, in the end, arbitrary, at least among the rocky and icy things in our Solar System. So I'm perfectly happy to allow history and sociology to help us define what we'll call a planet in our Solar System, and I won't argue with anyone who wants to say that there are nine planets, or even thirteen (let's make Eris, Ceres, Pallas and Vesta planets, too!)
More important than which little objects are planets is that we realize there is a hierarchy of objects in the Solar System, and that the Earth is not in the first or second tiers of that hierarchy. So, if we are looking for other Earth-like planets in other Solar Systems, we will have to look past a lot of big balls of gas before we find the insignificant rocks that may be home to other astronomers studying the four planets of our Solar System.
One last note, for those who have read more about these arguments at other places or even been involved in the discussions of definitions of the word "planet" themselves. I'm NOT arguing that concepts like Hill spheres and formation mechanisms are useless; I actually find them very appealing. But, in the end, those are definitions that leave as much to chance as they do to deeper truths. No matter where a Jovian planet is in a solar system, we would call it a planet. The same is not true for the Earth. This bothers me. Maybe I shouldn't think so hard about it.