Moons and Planets: Large Moons or Small Planets?
Published on Aug 18, 2006 at 12:54 am.
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Filed under extraterrestrial moons, planets.
For the last couple of days, I have been posting about the proposed new definition of “planet.” Personally, I think that with a whole year to work on a definition, they could have come up with a better one. But, this sounds like something that they got together and decided the night before the meeting (sort of like how my students to their projects). I am not the only one who doesn’t like the new definition. There’s a growing sentiment among astronomers that I talk to who, upon thinking this through, think that they could have done a far better job.
I’ve already shared several reasons why I don’t like the new definition. For one thing, the provision for declaring binary planets raises questions, as I said in the last posting. Let me explain another issue that this proposal fails to properly address. Under the new definition of planet, a planet is something large enough for its gravity to deform the body into a spheroidal shape. But, there is a caveat. A planet must be such a body orbiting a star, not another planet, unless the barycenter of that orbit is exterior to either of the orbiting bodies, in which case they are defined as binary planets. As we said yesterday, that means that a body whose orbit is quite elliptical might actually sometimes be a planet, and sometimes not.
Under the new definition, Ceres is a planet, and there are about a dozen “maybe planets” that we need to think about. Ceres was first declared a planet when it was found, but it was dropped from the list because it was obviously tiny compared with the rest of the things that we called planets. Now, it is back on the list, along with Pluto, Charon, and 2003 UB313. There are plenty of things in the Solar System that are significantly larger than these bodies and have enough gravity to make themselves round (even more spherical that Ceres!), but they don’t count because they orbit another planet. They are moons.
Our own Moon is an example. In fact, outside of the Pluto-Charon system, the ratio of sizes of the Moon to the Earth is larger than any other moon-to-planet ratio. Planetary astronomers have long thought of the Earth-Moon system as being practically a double planet. This concept if further encouraged by seeing that the Moon is only a little smaller than the planet Mercury, and is similar in surface appearance. But, the center of mass between the Moon and Earth lies just a bit below the surface of the Earth, so it is a moon.
But, there are larger bodies yet. Ganymede, Jupiter’s largest moon, is very large. In fact, it is even larger than the planet Mercury! Ganymede is only a bit smaller than Mars!It has plenty of gravity to pull itself into a spherical shape, and it appears to be well differentiated (something that may not be true of some of the newer members of the planet family under the proposed definition). 
Jupiter’s moons Callisto and Io are only a bit smaller than Mercury. Io is one of the most geologically active places in the Solar System. Europa, another of Jupter’s moons, is believed to even have an ocean of salt water below its icy surface. These four bodies show evidence of surface modifications by gravity and by geologic processes. In short, they seem to be in every way planets, only they don’t orbit the Sun. 
I think that they have far more right to be thought of as planets than Ceres, or even Pluto. They are far bigger than Pluto. In the case of Europa and Io, they are even more like the terrestrial planets in composition than Pluto! Even Ganymede and Callisto are more like the terrestrial planets than Pluto is, though large portions of these two bodies are made of ice. All four likely have more activity and differentiation than Ceres. In short, they are far more planetlike than either Ceres or Pluto. Only their location sets them apart.
Farther out, Titan, Saturn’s largest moon, is also larger than Mercury, like Ganymede. In fact, Titan and Ganymede are virtually the same size. Titan even has an atmosphere. It has clouds, rain, rivers, and lakes. Granted, it’s methane, not water, making these features on Titan, but still these are features that can not exist on smaller worlds like Ceres or Pluto (even if Pluto were closer to the Sun).
Now, I had no problem declaring these bodies to be moons, given the historic definition of a moon as being a smaller body orbiting a larger non-stellar body. They even probably formed in the same manner as planets. But, I always have called them moons. But, that was before this proposal to add tiny things to the list of planets. Already, it was questionable if Pluto should be on the list if these bodies were not. For years, in my planetary astronomy classes, we treated these large moons as worlds, not just as moons. This is all with the understanding that Pluto made the list of planets by mistake. It was wrongly thought to be much larger than it really is. Had astronomers known in 1930 Pluto’s true size, then Tombaugh, instead of discovering a planet, would no doubt have been heralded as the discoverer of the largest asteroid in the Solar System.
In fact, if any of these bodies orbitedthe Sun, no one would quesiton their planet status. Even if they were to orbit together with either Mercury or Mars, it would be clear that they were part of a binary planet. So, why declare them to not be planets simply because they orbit something huge?
My contention here is that if Charon and Pluto make for a binary planet, then why can’t there be small planets orbiting a gas giant? Clearly, if any of the moons that I mentioned here were in orbit around the Sun, no one would have ever questioned calling them planets, even before the current controversy. With Pluto being the sole little thing on the list of planets, then you could just say, “Oops. That was a mistake.” Then, calling these worlds moons and not planets is understandable. But, if you try to add other little things then we need to rethink what makes a moon and what makes a planet.
-Astroprof
(Image Credits: Photos courtesy of NASA. Comparison charts, courtesy of Calvin J. Hamilton)
