Defining Planets (Part VII)
Published on Feb 23, 2009 at 2:46 pm.
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Filed under planets.
In my previous installment of this series, I wrote about the role that structure may have in defining planet. It would seem that differentiated bodies would be different from non-differentiated bodies. However, differentiation is difficult to determine simply by looking at a potential planet. In fact, it has been proposed that even Jupiter may have enough convection in its core to keep the core area from being as differentiated as perhaps Saturn’s core may be. Our best guess it that Pluto is differentiated, which may be one of its biggest differences from many of the smaller Kuiper Belt objects. However, we do not know for sure if Pluto is differentiated, and we won’t know for sure until we land probes on that world and are able to get seismic studies of its interior. So, differentiation is tough to determine. Hydrostatic equilibrium is much easier to determine, and the mass required to achieve hydrostatic equilibrium is not all that much different from that required to differentiate. The greater ease of determination is why hydrostatic equilibrium is more often cited as a discriminator for planetary status rather than differentiation. However, even determining hydrostatic equilibrium is difficult at the low end of the scale without actually sending a spacecraft to the planet to look at it. And, there are degrees of differentiation and equilibrium possible near the cut off at the low end of the spectrum. My biggest objection to using either as a planetary discriminator is that both criteria would leave a large number of objects in dispute, both by virtue of lack of information of those bodies and by virtue of their falling in the gray area of partial differentiation or partial hydrostatic equilibrium. A good definition should not have such ambiguity.
So, what other possible criteria should a definition of planet consider? On my Part III posting of this series, I had suggested a number of possible considerations, and I’ve been going through them ever since. The next one on the list is to consider the method of a body’s formation. Perhaps we can define a planet as forming in some way differently than a non-planet. Unfortunately, we don’t really know how planets form, exactly, so this turns out to not be a very useful suggestion. Still, I think that it is instructive to go over how it would apply to planets, because, in doing so, we may get a better feeling for just what makes a planet something other than a non-planet.
There has long been debate over how planets form. Astronomers have come to accept a proposal now over a half century old that planets form as a consequence of star formation. This is actually an update to an earlier model of planet formation in which planets form from a disk of gas surrounding a star. Such a disk of gas, called a proplyd, is seen in the above Hubble Space Telescope image. The gas coming together to form a star forms an accretion disk. Early versions of this model suggested that the material in the disk clumps up and forms planets. Now, it looks a bit more complicated than that. The young star, once it gets hot enough, stops material from accreting into it. The left over material in the disk then is eventually blown back into space by the star’s stellar wind. But, in the time between the end of the star’s accretion and the dissipation of the disk, planets may form. Naturally, I am greatly simplifying the process for the sake of not making this blog entry far too long. There are plenty of books that you can read about this process.
But, just how do planets form in the proplyd? There are two competing models. One model suggests that planet form from direct condensation out of the proplyd. The other model says that planets accrete from smaller bodies within the proplyd. Let’s look at these two models.
The direct condensation model says that some sort of density instability happens in the proplyd and a local portion of the proplyd becomes dense enough that its gravity is sufficient to pull in material from around it making the dense region larger. Eventually, this dense region coalesces to form a planet. Such a planet would be composed largely of the sort of material that is in the proplyd. In fact, such a planet could form in the accretion disk of a star, itself, without having to have a proplyd. In either case, the planet formed in such a manner would have a composition fairly consistent with the dominant material in the disk. That means that hydrogen and helium would likely be the main constituents of such a planet. Gas giant planets fit the bill for this sort of formation process. A number of planetary scientists do propose that Jupiter and Saturn formed in this manner.
The accretion model says that the material within the proplyd is dense enough and cool enough that dust grains form. These are small bits of silicates and irons that form in the disk. The dust grains then gradually grow larger into tiny nodules. These nodules then coalesce into larger rocks. Chondritic meteorites may originate in such a manner. But, the accretion model goes farther. These meteorites clump up into larger bodies. In the outer portions of the proplyd, it is cool enough that ices are part of the mix, so the clumps formed there are icy in addition to bits of rock and iron. Many of the smaller asteroids and comets are believed by some to be rubble piles and accumulations of material left over from this period of the Solar System. Of course, we don’t really know that for sure, which is why spacecraft to the asteroids and comets are so important. We are still in the process of learning about these very important bodies. But, the smaller asteroids and comets will occasionally run into one another. That could either smash them to bits, or it could lead to larger bodies. Depending upon how the collision happens, bodies could be broken down smaller or built up larger. But, when a few of those bodies become large enough, they take on a different character. When a large enough body forms, then collisions with smaller bodies adds more than it takes away, so they inevitably grow larger. These bodies we then call planetessimals (which simply means little planets). The planetessimals would continue to grow, but it would take a very long time for them to continue to a large size. But, all of those collisions would likely result in some of those bodies having orbits that are rather eccentric. If the orbits are eccentric enough, then they will come close enough together to permit collisions between planetessimals. Such collisions could result in destruction of planetessimals, or it could result in larger bodies forming. These larger bodies are the planets. In the inner Solar System, the planetessimals would be largely silicates and irons, and so the inner planets would consist mostly of those materials (which is what we see). In the outer Solar System, the planetessimals would also contain a number of ices, so the planets of the outer Solar System would have a large abundance of such ices. But, with the ability to pull in not only silicates and irons, but also ices, the outer planets might be able to grow larger. If they grow large enough, then they’d even be able to pull in and hold onto hydrogen and helium in abundance. This model does a good job, therefore, describing Uranus and Neptune.
But, what about the asteroid belt or the Kuiper belt? The larger asteroids, such as Ceres, Vesta, Juno, etc, would be left over planetessimals in this model. The larger Kuiper belt members, such as Pluto, Eris, Makemake, etc, would be left over outer Solar System planetessimals. I’ll speak more to this in a later posting.
Now, we are ready to discuss how this applies to planet formation. Unfortunately, it does very little to clear matters up. First of all, if the models are right, Jupiter and Saturn formed in a different manner than the rest of the bodies in the Solar System, so do they deserve to be called planets at all? Even more confusing, the swirl of material coalescing to form Jupiter and Saturn likely also had accretion going on in it, forming the larger moons of those worlds. So, that would make the larger moons of Jupiter and Saturn more like planets than Jupiter and Saturn themselves! In fact, by most of the rest of the planetary criteria that I’ve discussed, Ganymede, Europa, Titan, etc, fit the bill for planets in terms of composition, differentiation, hydrostatic equilibrium, etc. The smaller moons may be more like captured bodies, or they may be left overs from the early stages of accretion, like the asteroids and comets. But, the larger ones almost certainly formed in the same manner as the planets. The only thing that keeps them from the list is that they orbit a planet instead of the Sun. However, if Jupiter and Saturn form more like the Sun than they did like the planets, and if formation matters in our definition, then it makes sense to take them off the list and to add their larger moons. Now, that is a proposal that would likely make a lot of people pretty upset.
But, what about the low end of the planet spectrum? That is where most of the discussion and arguing comes into play. How small does the designation of planet go? Does this criteria have anything to say for that? Well, perhaps it does, and perhaps it does not. The key comes in defining the planetessimals. Just what counts as a planetessimal? And, does a planetessimal have any significant differences from planets or sub-planets? This is one of the places that this particular definition of planet suffers. It requires defining another term, and that definition is just about as tough as defining the term planet, itself.
So, just how would we differentiate a planetessimal from a meteoroid, asteroid, or comet? One fairly simple definition would be that a planetessimal is a body large enough that collisions with other bodies on average add to its mass rather than subtract from its mass. Unfortunately, that is not a satisfactory definition, as it depends, in part, on the other bodies. It may also depend upon the composition of the planetessimal (small icy ones may be a different size than small rocky ones). It also may depend upon just where in the Solar System the body is located. In regions with a smaller percentage of sizable bodies, the term planetessimal may apply to smaller bodies than in regions of the Solar System with a larger percentage of sizable bodies. There are plenty of other issues, as well. So, it becomes difficult to differentiate between the smallest planetessimals and the largest sub-planetessimals. That gets us back to the difficulty of defining planet, anyway, when we look at the smallest bodies to have that designation.
But, assuming that we can differentiate a planetessimal from a sub-planetessimal, can we differentiate a planetessimal from a planet? In other word, how does a planet differ from a planetessimal? I would argue that there is not clear distinction. Planets would generally be thought of as larger than a planetessimal. But, that would mean that planets, too, are more likely to add to their mass than lose mass in an impact. Clearly, of course, if bodies the size of Earth and Venus smashed into each other, they would both likely be destroyed (though it is possible that the debris could reform into another planet). If a body the size of Earth were to collide with Neptune, though, then Neptune would survive just fine, and the smaller body would be destroyed. So, it becomes just a matter of scale. If that is the case, then planetessimals are simply, as the name implies, small planets.
The only real difference between planets and planetessimals, thus, is that the planetessimals are intermediate forms. Most bodies of that size are absorbed through collision with larger bodies to form the planets such as the terrestrial planets or Uranus and Neptune. A few left overs are still flying around the Solar System that never got a chance to become part of a larger body due largely to the gravitational influence of even larger bodies, such as the gas giants. In the inner Solar System, Ceres is an example. In the outer Solar System, Eris and Pluto are examples. But, perhaps that makes a case for the difference between a planet and a planetessimal: the planets are the limiting case of collisions. Bodies collide until they reach approximately as large as they will be: terrestrial planets in the inner Solar System and Neptune-like planets in the outer Solar System. But, that does not seem to make the planets really much different than the planetessimals.
So, depending upon how you apply this criteria, then considering formation, Pluto would stay on the list of planets (as a planetessimal), and it would be joined by Ceres, Juno, Eris, Makemake, and a whole host of other bodies. The number of “planets” in the Solar System might number in the dozens, or even hundreds (depending on how you compute the collision potentials). But, since Jupiter and Saturn form differently, it kicks them off of the list. They form more like stars, but they fail to qualify as brown dwarfs, so they’d be sub-stellar bodies. Furthermore, if Jupiter and Saturn are not planets anymore, then it leaves the question open as to whether their larger moons should be on the list of planets, since they form just like the objects that we recognize as planets.
This definition, thus, opens a pretty big can of worms that I think most people don’t want to deal with. But, that is not a good reason to avoid it in favor of other definitions. The biggest flaw with this definition is that we are still learning just how planets form, and so all of this may be wrong, anyway. That is the most convincing argument against using formation as a criteria for planetary status, even though it may be more reasonable than some of the other definitions given. So, that means that we need to continue searching for a good definition. There are still several possibilities to consider, and they will be in forthcoming postings.
-Astroprof
Image courtesy NASA, HST
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