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
Defining Planets (Part VI)
Published on Feb 19, 2009 at 11:27 pm.
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The next installment of my series is to look at whether planets and non-planets can have a difference in structure that helps to distinguish them apart. So, that means that we need to know the structure of a planet. That is not as easy as it sounds.
It took a long time before geologists knew the structure of Earth. Our home planet has an iron-nickel core, surrounded by a thick mantle made of dense rocky silicate-rich material, with a thin crust composed of less dense rocky material. The Earth’s core is composed of two parts, a solid inner core and a liquid outer core. The Earth’s core is hot enough that the outer core is liquid. The inner core is solid due to compression resulting from the weight of all the material above. The mantle is also very hot, but the pressure raises the melting point slightly above the temperature, but the rock is hot enough and under enough pressure to be pliable. The actual term frequently used is plastic. The crust is solid. There is an atmosphere outside of the solid portion of Earth. The temperature on Earth permits water to be solid, liquid, or gas. The water vapor is in the atmosphere. The solid water falls from the sky as snow, sleet, or hail, and vast amounts of solid water is located near the planet’s polar regions. Much of the water, though, is liquid, and most of that accumulates in low places on the Earth’s crust, namely the oceans.
So, how much of that (if any) is required for a planet to be a planet? Do all planets require a metallic core? We have reason to believe that all of the Terrestrial planets have iron-nickel cores. Earth’s core may be the largest of these, but Mercury’s core is a greater percentage of that planet’s mass. Mars likely has the smallest core and the smallest percentage of a Terrestrial planet’s mass comprising the core. But, what about the larger asteroids? If a body is large enough, then the heat from its formation will help it differentiate: that is, the heavier material will sink to the middle and the lighter material will move to the outside. I’ll talk more about that in a later posting. Most astronomers believe that the larger asteroids, such as Ceres, Pallas, Juno, etc, are likely differentiated to at least some extent. Probably, the larger the body, the better differentiated that it will be. Thus, each of these bodies will have an iron-nickel core. Even smaller bodies are likely differentiated, as a common supposition is that the origin of the most iron rich meteorites is from the collision and destruction of differentiated asteroids. Bodies the size of Ceres and Pallas are tough to blow apart, so these iron meteorites likely come from smaller bodies. So, how differentiated is enough? Is simply having an iron core enough? Suppose a body is only partially differentiated, with only some of the iron sinking to the middle and the rest still distributed throughout its bulk? How much of the planet must be in the form of this core? Such a definition permits most of the larger asteroids onto the list of planets, but then we get into a gray area where the smaller ones are not quite fully differentiated. That makes for a repeat of the same sort of ambiguities that got us into this whole mess of trying to define planets in the first place.
Worse, the outer planets have different compositions than the inner planets. For Jupiter and Saturn, only a tiny percentage of the planet is composed of a core at all. Most of those worlds’ mass is in the form of hydrogen and helium, and most of that forms a vast ocean of liquid material. Even more disturbing, the hydrogen is at a temperature and pressure above the critical point of that element, so there is no true boundary between the gaseous uppermost regions and the liquid region below. Rather, the material simply takes on more liquid-like properties with depth, eventually being more liquid-like than gas-like. And, of course, since the bulk of the planet is in this liquid state, it does not have the large plastic silicate mantle of the Terrestrial worlds. (For that matter, Mercury probably has the smallest mantle of the Terrestrial worlds: it is mostly core and crust). Under the temperatures present at the center of Jupiter, it is not even clear what state the core material is in, or even if convection near the core allows it to fully settle down to be a solid-like body. The matter is even more difficult when extra-solar planets are considered. For some of the bodies larger than Jupiter, they may have sufficient convection to keep them from differentiating. Of course, some may argue that such a state may be the delineation between a massive planet and a small brown dwarf. But, there are indications that gas giant planets like Jupiter may have formed around stars very low in metallicity. Planets around such stars would likely be very deficient in composition of materials other than hydrogen and helium, so they would fail the differentiation test even if they were otherwise identical to Jupiter or Saturn.
Uranus and Neptune further complicate the matter. We still don’t know what the cores of these worlds may be like. It has been speculated that the cores are iron and silicate rich, but it has also been speculated that they are mixed with ices. In fact, a major part of these worlds may be a vast ocean of sorts of liquid ices. So, they have a different structure than the gas giants and the Terrestrial planets, though they likely are at least differentiated, just in a different manner due to their different compositions.
So, how does this leave Pluto, Eris, and the rest of the Kuiper Belt objects? Again, we simply don’t know. The speculation is that the larger ones, Pluto, Eris, Makemake, etc, are probably differentiated. But, just how differentiated, we don’t know. Likewise, we don’t know if that means a core of iron-nickel surrounded by a mantle of silicates surrounded by ice, or perhaps a core of iron, nickel, and silicates surrounded by ices. It would be nice to have a definition of planet that did not depend upon extensive further study to find out whether something is a planet or not.
And, we would also like to be able to discuss extra-solar planets, too. Unfortunately, we know even less about them than we do about the icy bodies at the outer reaches of our Solar System. About all we know about the extra-solar planets, about 340 so far, is their mass, and even that is not always well known. We infer what they might be like based upon what we see in our Solar System and what we know about physics, but I should point out that those inferences are really just educated guesses. We simply cannot say if they are differentiated or not.
So, it would be best to move on and to look for some other property upon which to base our definition of planet. But, what could that other property be? Sticking with structure, there is another property that is quite promising, and in fact has been jumped on by many who are trying to define planet, and that is the shape of the prospective planet.
All mass has a gravitational field. Larger things have more gravity. Earth is quite large, so it has a lot of gravity. When objects are in the presence of a gravitational field they have a weight. The more their mass, the more that they weigh. So, a large truck weighs more than a small car. A skyscraper weighs more than a tool shed. A mountain weighs more than a mole hill. But, if these large and heavy bodies are placed on the surface of the Earth, they must be supported by the ground beneath them. I grew up in Houston, where the ground was soft. This was a problem for the tall skyscrapers that were built there. The ground could not support their weight, and they would actually start to sink. Worse, they would often try to sink more on one side than the other and become unstable. So, these buildings had to be built with giant weighted sub-basements. In essence, they floated in the ground. Similar building practices are done elsewhere the ground is similar. Mountains are heavier, though. So, they tend to sink deeper. Mountain ranges tend to push down into the Earth’s mantle. Eventually, the buoyant force of the mantle supports the weight of the mountain range. If the mountains are too big, they sink farther. If the mountains have sunk too far, they are buoyed upwards. This process (actually a bit more complicated than I have set forth here) is called isostasy. But, the base of the mountain range can only sink so far into the mantle before the heat breaks it up and causes it to melt. That causes the mountains to lose buoyancy and to sink farther. This means that there is a maximum height that mountains, plateaus, and other protrusions from Earth’s surface can have. Thus, these forces tend to cause the hills and valleys of Earth’s surface to eventually try to flatten out to make the Earth more spherical in shape. The same holds on other planets. There is a maximum degree of variation permitted from a spherical shape due to the forces of gravity and the stiffness of the crust and mantle and the buoyancy of the mantle. The degree of variation, obviously, would depend upon the mass and density of the planet and upon its composition. Icy bodies would be more easily fashioned into spherical shapes than rocky bodies.
It has been argued that we should simply define a planet as being a body whose gravity compresses it into a spherical shape. That, as it turns out, is a bit simplistic. Even our own Earth is not really spherical. Ignoring the mountains, valleys, ocean basins, and other features that are deviations from a spherical shape, Earth’s diameter is greater if measured across the equator than when measured from pole to pole. The deviation is quite a bit more than would normally be permitted under isostatic conditions, so what is going on? The discrepancy is due to the fact that Earth rotates. Rotating bodies tend to bulge at their middles. This posting is already getting long, so I won’t go into such discussion as centripetal and centrifugal forces, or how some of the gravitational forces that would otherwise pull in the bulge are being used to keep the outer parts of the Earth going around an the right speed, or any such thing. You probably already have experience with spinning objects bulging in their middle, and you can read up on why that is the case (or sign up to take my physics class, or one of the many others offered all around the world). We call the ratio of the diameter across the equator to the diameter along the rotational axis the oblateness of the body. Saturn is nearly 11% wider across the equator than it is pole-to-pole. This year, with the rings nearly edge-on, is an excellent time to see Saturn’s oblateness because it is so obvious. (Typically, the rings distract people and they don’t notice how oblate Saturn itself appears.)
Since all bodies in the Solar System rotate, they will all have some degree of oblateness. How oblate they are depends upon their rotational rate and their composition, mass, and structure. Though Saturn and Jupiter have nearly the same rotational periods, Saturn is noticeably more oblate than Jupiter. But, it would be silly to say that Saturn is not a planet because it is simply more oblate than Jupiter. Likewise, if there were two bodies of nearly the same size, mass, and composition but one rotated far more rapidly than another it would be more oblate. Again, it would be silly to say that such a body was less of a planet because it was spinning too fast. So, we need another measure besides just the degree of sphericity of a potential planet. But, that turns out to be easy.
A very closely related concept to isostasy is hydrostatic equilibrium. Despite the “hydro” part of the name, water is not necessarily involved in hydrostatic equilibrium. Putting it into fairly basic terms, hydrostatic equilibrium occurs when the inward pull of gravity is balanced by the outward forces created by the pressure caused by the compaction of something under the force of gravity. Isostasy can be thought of as the mechanism at work in hydrostatic equilibrium. But, the rotation of a body means that gravity not only compacts things, but it also helps keep the outer parts of a rotating body moving around at the same distance from the axis of rotation. The faster that a body rotates, the greater the percentage of the gravitational force is needed to hold the body together, and thus the less compaction happens. That allows the outer edges to pushed farther outward. This also sets a limit as to how fast a body can rotate without flying apart.
So, a possible differentiation between planet and non-planet may be whether or not the body is in hydrostatic equilibrium. That permits bodies such as Saturn, not quite spherical, to be a planet. It even permits Haumea, currently a dwarf planet and even more oblate than Saturn, to be considered.
However, hydrostatic equilibrium is perhaps not quite the end-all definition that some would hope for it to be. For one thing, the size at which a body reaches hydrostatic equilibrium will depend, in part, on its composition. Thus, a body made of more of material some materials may be considered to be in hydrostatic equilibrium while another body of the same mass might not if it is made of some other type of material. In fact, the more fluid a body is, the more easily it will reach hydrostatic equilibrium. Thus, an icy body or a liquid one would be considered a planet while a somewhat larger rocky body would not. That seems silly.
There are other issues, too. The asteroids Pallas and Vesta, and perhaps some of the Kuiper Belt objects also are a thorny issue. These bodies are close to the hydrostatic equilibrium line. A little more mass, and they may count. In fact, it has been speculated that perhaps they were in hydrostatic equilibrium when they formed, but collisions with other bodies after they became more solidified distorted their shape. By that time, however, they had already cooled and solidified to the point that isostasy could not bring them back to hydrostatic equilibrium. Does that mean that they were planets in the past but now are not? Would they have remained planets if they had not been reshaped by collisions? Is that a reasonable definition of planet? And, of course, using hydrostatic equilibrium as a delineator begs further questions. Just how close to hydrostatic equilibrium do the planets need to be to make the list. After all, there are a near continuum of bodies at the small end of the proposed planetary scale. Many of these bodies are large enough to at least partially achieve equilibrium. Again, as I have argued before, does it make sense to call one body a planet and another body, nearly identical in composition, mass, and size, not a planet because it lacks only a few tons or a few kilometers in size to make the cut?
While achieving hydrostatic equilibrium is a much better proposal for planetary status than other suggestions based on structure, I believe that it is still not quite sufficient. Any definition that makes extremely different bodies planets (such as Saturn and Mars) but makes extremely similar bodies different classifications (such as Ceres and Vesta, Pluto and Quaoar) is a poor definition. There needs to be something better.
You can see why the IAU committee that was working on this had such a tough time. You can also see why the final resolution that was passed that only took a few hours to craft was doomed to be flawed.
-Astroprof
Defining Planets (Part V)
Published on Feb 14, 2009 at 4:43 pm.
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In my last posting, I talked about mass and size as possible discriminators for planets. The size issue was one of the first arguments put forth to remove the asteroids from the list of planets. But, as I point out, there are some problems with simply using mass and size. So, perhaps some other physical property is needed to differentiate between planet and non-planet.
One possible difference between planets and non-planets may be their composition. This idea goes back hundreds of years. Remember, at first people knew of the Earth, the Sun, the Moon, and stars. The stars were just dots in the sky. Some of those dots moved. Those were planets. There was no real reason to suspect that some dots were physically any different from other dots in composition. But, when astronomers finally came to the conclusion that the Earth orbits the Sun like the planets, it raised the question as to whether the planets, the Sun, and the stars are different compositions. Telescopes showed that the planets were not just dots. They had disks. Some showed features. They were worlds. The Sun shone of its own light. The planets and the Moon did not. They reflected sunlight. The stars were exceedingly far away since parallax was not visible as Earth moved about the Sun (parallax was, of course, eventually discovered, but it took a while). So, the stars were too far away from the Sun to shine be reflected sunlight, and they were not near anything else, so they had to shine by their own light like the Sun. The Sun is hot and shines. Soon, astronomers realized that the Sun must be composed of a hot gas, much like the flames in fire. That was why it was shining. The stars must be similar. Earth is composed of solid rock (at least the surface of Earth is so composed). The Moon and Mars appeared solid, too. Venus, Jupiter, and Saturn were cloaked in clouds, so there was no telling what the visible surface of those worlds was like, but the astronomers of several hundred years ago had no reason not to suspect solid surfaces beneath the clouds. So, planets are composed of rock, and stars are composed of gases. That seemed to be a good definition.
The problem with that definition is that it failed as soon as spectroscopy was able to show the compositions of the worlds. Now, we know that the inner planets are composed of silicates on the outside and iron in their cores. The proportion of the planet that constitutes iron core versus silicate mantle and crust varies with the planet. Mercury has perhaps the largest percentage of the planet as core, and Mars the least (unless you want to count Earth’s Moon). A similar composition, though, is likely evident in the larger asteroids. In fact, most of the asteroids are composed of silicates and iron. For the larger bodies, melting and differentiation has allowed the heavier material to sink towards the core of the body and the lighter material to rise to the top. Ceres and other of the larger asteroids are expected to have iron rich cores and silicate rich mantles and crusts. So, by the criterion of composition, they would also be planets like Mercury, Venus, Earth, and Mars.
The problem is that we now know that the outer planets have different compositions. The atmospheres of Jupiter, Saturn, Uranus, and Neptune are hydrogen and helium. In fact, there is so much hydrogen and helium in the atmospheres of Uranus and Neptune that those gases make up a major percentage of those worlds masses. They likely have silicate and iron rich cores surrounded by large mantles of liquids (mostly water, but under such pressure and temperature that it does not act like what most people think of the normal properties of water here on Earth). The situation is even more extreme with Jupiter and Saturn. Those worlds have not just a large portion of their mass as hydrogen and helium, but the vast majority of the mass of those worlds is hydrogen and helium. They may have small (comparatively) cores of iron and silicates, but the majority of the planet is composed of hydrogen compressed to such an extent that it is liquid. For much of those planets, the liquid hydrogen is even in such a state as to conduct electricity and have other metallic properties. Hydrogen and helium are normally gases on Earth. Thus, all four of these worlds, Jupiter, Saturn, Uranus, and Neptune, are called “gas giants,” even though they are more liquid than gas in structure. Thus, the four gas giants have significantly different compositions than the four terrestrial planets.
Pluto and the Kuiper Belt objects are composed largely of ice and rock. The ice comprising these works is not all water ice. It also consists of frozen methane, frozen carbon dioxide, and other frozen chemicals that are not normally in a solid state on Earth. That makes Pluto, Eris, and the rest of the Kuiper Belt different in composition from the other planetary bodies.
One argument against admitting Eris and the other Kuiper Belt objects into the family of planets is that they are icy bodies, not like the rest of the planets. But, what are the rest of the planets? Already, we see that they compose two distinctly different classes of bodies. And, even among the gas giants, Uranus and Neptune are different in composition from Jupiter and Saturn. The gas giants all share somewhat similar structures, but they are different enough that one could legitimately argue that they comprise two separate classifications of bodies. Certainly there are greater differences between the gas giants than there are between the terrestrial worlds.
A further problem is that the gas giants, Jupiter and Saturn in particular, have compositions that more closely resemble the Sun and stars than they do the terrestrial planets. In fact, Jupiter and Saturn have compositions that very closely mimic the composition of stars. The biggest difference in composition is that over time they have become more differentiated, that is the the heavier material has sunk to their cores. Jupiter likely has a composition very close to that of the Sun and stars. So, if composition is the basis of planetary claims, Pluto is off the list. But, then so is Jupiter and likely Saturn, and maybe Uranus and Neptune. By composition alone, Jupiter should be placed with the stars, not the planets. So, why isn’t it?
So far in the series, I’ve been addressing the lower boundary of the planetary classification problem. As a reader pointed out, there is also an issue at the upper end of the boundary. What is too small to be a planet? Is Pluto big enough to be a planet? How does an object have to be to be a planet? Is is possible to be too small to be a planet? These were the issues addressed in my last posting. But, there is another issue. How big can planets get?
Jupiter is the largest planetary body in the Solar System. Jupiter is pretty large. Close to 1000 Earths could fit inside Jupiter. Jupiter has a mass of nearly 320 Earths. Jupiter has more mass than the rest of the planets of the Solar System combined. That is huge. However, objects have been discovered orbiting other stars that are even larger. The bodies orbiting other stars that are Jupiter sized and larger are believed to be composed mostly of hydrogen and helium as is Jupiter, and as are stars. What makes them planets instead of stars?
This post is already getting long, so I won’t go into all the explanation for what stars are. However, suffice it to say that stars shine because they are hot, and they maintain their thermal energy through nuclear fusion in their cores. Stars fuse hydrogen into helium, or at least they spend the bulk of their lives doing that. But, some objects form that lack sufficient mass to initiate nuclear fusion. That is about 0.08 times the mass of the Sun, or about 80 times the mass of Jupiter. If a stellar-like object slightly smaller than this forms, it is unable to initiate the normal nuclear fusion that keeps stars active, and so it gradually cools off. This failed star is called a brown dwarf.
So, what makes Jupiter and the other extrasolar Jupiter-sized bodies planets instead of brown dwarfs? This is a matter open to debate. A great deal of effort has gone into defining the small end of the planet definition, but comparatively little attention has gone into the large end of the planetary classification. That is understandable, though, if you at the history of the discussion. After all, we had Pluto which was already a point of contention at the small end. With the discovery of other tiny bodies, the low end of the planet spectrum began to be questioned. The largest planet in the Solar System continued to be Jupiter. Extra solar planet were all only a few times Jupiter’s mass. The smallest brown dwarfs were only a little less than the stellar size cutoff, and none seemed less than about 50 or 60 times Jupiter’s mass. There was clear gap between the largest planet and the smallest brown dwarf. But, as with the gap between the smallest planet, Pluto, and the largest non-planet in the Solar System began to erode and then disappear, so has the gap between the largest planet and the smallest brown dwarf. There are now a few planets at nearly a dozen times the mass of Jupiter, and even some speculated to have masses higher than that . A few brown dwarfs have been found with masses that may be only two or three dozen times the mass of Jupiter. These in-between bodies raise the question as to what constitutes the largest size that a gas giant can be before it is a brown dwarf. After all, they do seem to have the same composition.
One possible difference suggested is in nuclear fusion. While brown dwarf stars are too small to initiate and maintain nuclear fusion at a level sufficient to offset their radiation of thermal energy into space, they probably do have some fusion. Tritium, an isotope of hydrogen that has a proton and two neutrons, is easier to fuse than the more common isotope of hydrogen. Deuterium and tritium also fuse more readily than just regular hydrogen (something that was useful to know in the construction of the first hydrogen bombs). So, these non-stellar bodies can actually undergo some nuclear fusion using these other processes. However, deuterium and tritium are rare forms of hydrogen, and even lithium is exceedingly rare in the universe compared with hydrogen. Thus, these other forms of fusion, while useful for artificial fusion devices such as thermonuclear weapons, are not going to occur in nature sufficiently to power a star. So, brown dwarfs may still have some fusion going on, but they do not have enough to compensate radiative heat loss, nor can they maintain it for long. But, even these other forms of fusion require extreme heat and pressure not likely found in bodies that are too small. Jupiter and Saturn are believed to be too small to sustain such fusion (though I did read one paper that suggested that a very tiny bit of fusion might be possible in Jupiter).
So, a proposed discriminator between planets and brown dwarfs is that planets are too small to have any fusion at all while brown dwarfs are big enough to initiate some limited nuclear fusion, and stars are big enough to initiate and sustain themselves with nuclear fusion.
But, this still permits the inclusion into the list of planets bodies whose composition matches that of stars. And, of course, that means that planets would have a wide range of compositions. Thus, unless we want to kick Jupiter and Saturn off of the list of planets because they more closely resemble stars than they do Earth, we cannot kick Pluto off of the list solely based on it having a composition more closely related to comets than to Earth.
If composition or structure determines planets, then there is no way to reconcile the list of planets commonly used. This tackles items 4 and 5 of my list of possible planetary considerations, and it still does not give us a unique planetary definition.
-Astroprof
Defining Planets (Part IV)
Published on Feb 9, 2009 at 10:29 pm.
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The very first definition for planet was simply to designate a celestial body that appeared to move in the sky as a planet. But, that was ultimately deemed insufficient once telescopes discovered more bodies. The first satellites discovered of Jupiter and Saturn were called planets. But, astronomers eventually decided to restrict the term planet to only those bodies that orbit the Sun. That differentiated planets and moons. But, that, too, was deemed insufficient. Very soon, some astronomers realized that the stars could be suns. If so, then they may have bodies orbiting them similar to those orbiting our Sun. Those should be planets, too.
But, as I said in an earlier post, when bodies were found to be orbiting the Sun between Mars and Jupiter, they were at first heralded as new planets. Eventually, though, they were dropped from the list like the moons of Jupiter and Saturn. The problem there lay with the shear number of the bodies and their tiny size compared with the other Solar System bodies. They did not seem to fit.
The reason that the asteroids were kicked off of the planet list was due to their tiny size and mass. So, perhaps size and mass would be good discriminators for planetary status. This was also in the minds of astronomers when they were kicking the satellites of Jupiter and Saturn off of the list of planets. Astronomers at the time did not know how big Jupiter and Saturn were, but their satellites were clearly tiny by comparison. The shear size of the gas giants was at the time not realized, though. Had astronomers realized that Jupiter was big enough to fit nearly 1000 Earths inside of it, they would have not been so quick to dismiss its larger moons for size. We now know that the larger of these moons are the size of the smaller terrestrial planets, and Titan and Ganymede are even larger than Mercury. But, what size and mass is big enough to be a planet? In the Nineteenth Century, there was clearly a substantial gap in size between the smallest planet and the largest asteroid. Then, Pluto was discovered, and then the other Kuiper Belt objects started to be found.
Pluto was at first believed to be much larger than it really is and to have much more mass than it really does, so it was placed on the list of planets. Over time, it was found to be much smaller and less massive than thought. But, it remained on the list of planets as the tiny cousin of the other planets because it was still quite a bit larger than the asteroids. But, other objects nearly identical to Pluto began to be found. So, it made no sense to call Pluto a planet and not the others. To be consistent, either all the things like Pluto are planets, or Pluto is not a planet. So, which is it? Are they all planets, or are none of them planets? What is the cutoff for determining the size and mass of a planet (if that is the criterion)?
Many people proposed a cutoff at Pluto’s size. Bigger is a planet, smaller is not. But, what about something virtually exactly like Pluto, only a tiny bit smaller? Why is that not a planet? Why use Pluto as the gauge, anyway? After all, what makes Pluto special is that it was discovered first. That’s it. Pluto is not much different from the rest of the similar bodies out there. But, suppose Quaoar had been discovered first. Would Quaoar then be the gauge? By that reasoning, if Eris had been found first, then Pluto would not be a planet because it is smaller? That makes no sense at all.
So, perhaps we should just pick a size, like a nice even number: say 2000 km diameter, or perhaps 1000 km diameter. My problem with that is that it is a totally arbitrary size. After all, there is nothing special in the universe about the definition of a kilometer. A kilometer is basically 1/40,000 of the circumference of the Earth (Not exactly, but that is close to the original definition.). So, a diameter of 1000 kilometers is simply a diameter of 1/40 of Earth’s circumference. It works out that way simply because of how we pick the definition of a kilometer. That makes it a pretty silly arbitrary standard to select just to have a nice round number.
A more rational way to pick the magic size of a planet is to look at a distribution of sizes of bodies orbiting the Sun in the Solar System. I produced a plot of this type back in 2005 when I was giving a presentation on why we needed a new definition of the term planet. There is a near continuous distribution of bodies from dust, through meteoroids, to asteroids and Kuiper Belt objects, to Eris. Pluto is near the top of that continuous distribution. Then, there is a jump to Mercury. So, you just draw a line through that gap. Things bigger are planets. Things smaller are not. This gives eight planets. That means Pluto, Eris, and the rest are not planets.
However, that definition is inherently flawed, too. There are likely other bodies in the Kuiper Belt the size of Pluto and larger. If one of these is found midway in the gap between Eris and Mercury, then it muddies the definition. We need a definition that stands the test of time, not one that is good for just now. Furthermore, we want a definition that is good for other stars, too. After all, it may well be that there are plenty of bodies orbiting other stars that are between Mercury and Eris in size. What are they? We need a definition that works for other star systems, too.
Of course, if all you are looking at is the size of a body, any definition that included Mercury as a planet would also include Ganymede and Titan. Most astronomers tend to balk at any definition that includes those bodies. Personally, I am not so sure that Ganymede and Titan should not be included with the rest, but I’ll get into that later.
Throughout this post, I have talked mainly about size in terms of physical dimensions. A similar argument could be made for mass as the delineator between planet status and non-planet status, and you get very similar results to what you get from looking at size, so I won’t belabor the point.
This covers issues with definitions 1, 2, and 3 from my last post. I’ll tackle more of them in my next post.
-Astroprof
Defining Planets (Part III)
Published on Feb 7, 2009 at 2:19 pm.
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Pluto was an oddity among the planets for much of the Twentieth Century. As I said in my last post, it just didn’t seem to fit with the other eight planets. There was debate for decades on whether or not Pluto should really have been on the list. In the end, most astronomers were content to simply talk. After all, Pluto was already on the list, and it was simpler to leave it on the list than to remove it. And, for some reason Pluto seems to have become one of the more popular planets — perhaps because it was discovered in modern times. But, as I said last time, once other objects very much like Pluto started to be discovered in the 1990s, the debate on Pluto’s status as a planet began to heat up. After all, how could you say that one object is a planet and another, nearly identical object, is not a planet? Some criteria for planetary status was needed. Though Pluto’s planetary status was beginning to seriously be called into question, there was no urgency in the matter, since Pluto remained a bit larger than any of the other bodies beyond Neptune. That changed early in the 2000s as bodies nearly the size of Pluto started to be discovered. Then, it became silly to call Pluto a planet while something 80% of Pluto’s size was not a planet. The real problem, though, came with 2003 UB313 was discovered. This body was larger than Pluto. So, if Pluto is a planet, it makes no sense to have something very nearly identical, and even larger, be a non-planet. One option open to astronomers was to declare all of these bodies sub-planets with Pluto remaining on the list of planets for historical and cultural reasons. That idea, though, flies in the face of the whole concept of taxonomy. It would be ridiculous to have only one of a number of nearly identical bodies classified differently than the rest. Whatever one of these bodies is, so are the rest. And, we should not be held to keeping Pluto on the list of planets simply because it was already on the list. After all, as I posted earlier, there have been several bodies that have been initially classified as planets only to be later removed from the list decades later when we learned more. Science should follow consistent rules, not bend to public pressure.
So, this leads to the central point of this series of posts. Just how do you go about defining the term planet? We need to pick a reasonable definition of planet, and then go back to the current list of planets and see which bodies actually meet the definition, not look at the list and create a definition to fit the list. The definition of planet needs to be well thought out. It needs to be something that is workable. The criteria for planetary status needs to be something that is as clear cut as possible. A definition that leaves a large number of bodies as “maybe planets” is not very good.
Most important of all (and, I think forgotten by most people involved) perhaps the term planet is obsolete and needs to be replaced altogether by new terms describing the bodies to be studies. After all, the term planet already was being used to encompass a surprising variety of bodies: small rocky planets, large gas-rich planets, and an icy body. In other sciences, such diverse descriptors would long ago have been assigned separate labels.
Why do we want to assign a label of “planet” to different bodies? Is there really a scientific reason for this? Or, is the label simply something so that textbook authors can know how to list the bodies? Do astronomers need to know the descriptor in order to study the objects? Do we need a label so that we can differentiate bodies? Just why do we need a label, anyway? Does labeling certain objects planets and other objects non-planets in any way help our understanding of these bodies? Does the label even help with students learning about these bodies? Or, does the label hinder our understanding and learning? Are we somehow limiting our studies to certain objects because they are planets and ignoring other, virtually identical objects, because they simply do not have that named label attached to them? We should not have a definition of planet that is designed solely for the sake of textbook authors’ ease of giving textbook chapter titles. The definition should not be so that school children have an easy mnemonic with which to remember the names and order of the planets. The definition should not be such that it is designed to increase or to limit the funding of research for certain objects (the presumption being that there may be more funding to study planets than to study things that are just like planets, but simply not on the list). And, most importantly, the decision of a definition should be made solely for scientific purposes. The designation “planet” should signify that the body with that designation is in some important way different from a similar body without that designation. These are the things that must go into defining the term planet.
So, shortly after the discovery of a body nearly identical to Pluto,astronomers got together to propose a formal definition of planet. This group failed to come to an agreement on the definition of planet. So, in 2005, the IAU formed a committee composed of a variety of people from the astronomical community to develop a solid definition of planet. They worked for about a year on a proposal and produced on in August of 2006. I blogged about the proposal at the time. When I first heard about it, I did not like it. The proposal fell short of the suggested requirements of a definition that I set forth in the previous paragraph. My thought at the time was, “They worked for a whole year on this, and this is the best that they could come up with?” The IAU general assembly apparently had a similar reaction to what I had. They rejected the proposal, and I think that they were right to do so. However, they then went and made matters worse with a new definition that many consider to be even worse than what the committee had come up with. They were in a hurry to come up with a new definition and did not give it the year’s worth of discussion that the committee did. As much as I disliked the committee’s proposal, it may have been better than what we eventually got. I firmly believe that the current official definition is flawed. Many other astronomers feel that it is flawed. And, I firmly believe that one day a new definition will have to be devised.
For one thing, there are bodies orbiting other stars besides the Sun. It makes sense to categorize these bodies as planets if they meet the same guidelines as planets for the Solar System. However, the definition that the IAU came up with after scant hours of discussion does not address the matter of planets around other stars. It also does not decide what to do with a body that had been a planet of a star and is ejected through gravitational interactions of some sort (with another planet or with a star). Yet, there seems to be indications that such bodies may exist.
So, how do we go about defining just what we mean by the term planet? First off, let me say that I am not a planetary astronomer, but I am a pretty good generalist in the field. So, I do believe that I have something to offer.
First of all, we need to know what makes a planet. There are several ways that planets can be defined:
1 ) Planets can be defined by orbit. This would differentiate planets from moons, at least.
2 ) Planets can be defined by mass. Big things are planets, small things are not.
3 ) Planets can be defined by size. Similar to number 2.
4 ) Planets can be defined by composition. What makes it up determines if it is a planet or not.
5 ) Planets can be defined by structure. Planets have some different physical characteristic than non-planets.
6 ) Planets can be defined by method of formation. Planets form slightly differently than non-planets.
7 ) Planets can be defined by circumstances of discovery. This would be a rather descriptive definition.
8 ) Planets can be defined by circumstances of observation. If you can see them, they are planets.
9 ) Planets can be defined by their effect on their surroundings. If they influence other bodies, they are planets.
10 ) Planets can be defined by a combination of these criteria. One characteristic might not be sufficient to create an unambiguous definition.
Unfortunately, none of these criteria gives rise to a perfect definition of planet, and I will go into that in my next installment of this series. I’ll also tackle each of these guidelines one by one and see how they fail to give a proper definition. Many yield ambiguities, or they exclude bodies from the list that most would believe should be kept on the list. Clearly, defining planet is tough.
So, stay tuned. I’ve got more to write about.
-Astroprof
Defining Planets (Part II)
Published on Feb 4, 2009 at 11:43 am.
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In my last post, I gave a sort of history of how we got to having 9 planets in the Solar System, according to science textbooks for most of the Twentieth Century. But, Pluto was a problem from the very beginning.
Already, by the beginning of the Twentieth Century, astronomers knew that there was a difference between the planets of the Inner Solar System versus the planets of the Outer Solar System. Mercury, Venus, Earth, and Mars were small worlds mostly made of rock, with a thin atmosphere in some cases. You might wonder at my calling Earth “small,” but it really is small when compared with the worlds of the Outer Solar System. Nearly 1000 Earths would fit inside Jupiter, and over 840 Earths would fit inside Saturn. Uranus and Neptune would each hold over 50 Earths.
The other big difference between the inner planets and the outer planets is the nature of their atmospheres. Mercury has so little as to be essentially a vacuum. Mars’ atmosphere is very scarce. Venus’ atmosphere is nearly 90 times denser than Earth’s, but still the atmosphere extends only hundreds of miles up and adds little to the apparent size of Venus. The inner planets have atmospheres of nitrogen, oxygen, and carbon dioxide. There area few other gases hanging around, but those make up the bulk of the atmospheres. The outer planets, though have atmospheres that are composed mostly of hydrogen and helium. In fact, there is so much hydrogen and helium that the atmospheres of these worlds compose a major percentage of their mass and apparent size. For Jupiter and Saturn, hydrogen and helium are the principal components of those worlds, and even for Uranus and Neptune hydrogen and helium are major components. Clearly something is different between these worlds. The outer planets, being so dominated by hydrogen and helium are often called “gas giants” to differentiate them from worlds that are mostly solid bodies with comparatively thin atmospheres.
When Pluto was discovered, the faulty mathematics that had predicted it had suggested that it was a massive body. The gas giants fit the bill, having much greater masses than the smaller rocky worlds. However, gas giants are large. If Pluto had been a gas giant, then it would have appeared as a small disk in telescopes when viewed at high magnification. Instead, it was just a dot. Eventually, astronomers had to accept that Pluto was much smaller than they had been expecting. More careful study showed that the mathematics predicting a massive body beyond Neptune were in error. So, Pluto didn’t have to be so large, after all. That didn’t cause anyone to remove Pluto from the list of planets. It just made astronomers realize that it was a much smaller planet then they had thought.
Over the years, the size of Pluto began shrinking in textbooks. Obviously, Pluto was not shrinking. Rather, the ability to measure its size was getting better, and Pluto was found to be much smaller than anyone had thought. As I said, when Pluto was first found, astronomers were expecting a gas giant. Clearly, Pluto didn’t fit the bill. And, when a more careful mathematical analysis showed no disturbance in Neptune’s orbit after all, that limited Pluto’s mass (and size) even more. At the time, astronomers knew of only four kinds of objects in the Solar System: small rocky planets (the terrestrial planets), large gas rich planets (the gas giants), tiny chunks of rocky material (asteroids), and tiny chunks of icy material (comets). If Pluto were as tiny as an asteroid or a comet, it would never have been seen. Comets at the time were not well understood, but Pluto didn’t look like any that we knew at the time. Asteroids were not known to exist so far from the Sun. It obviously wasn’t a gas giant. So, that left a rocky terrestrial planet in a strange orbit in the Solar System where no other terrestrial planets were known. Rocky surfaces reflect only so much light. So, in order to reflect as much light as it does, Pluto was thought to be about Mars sized (give or take a bit). I remember growing up I had a Solar System book that had cardboard cutouts of the planets to scale. I had them hanging up in my room. Pluto was about the size of Mars. Even at that time it was out of date, though, because astronomers had already realized that Pluto was much smaller than Mars. Word just hadn’t reached the makers of the book.
Pluto’s surface is covered in ice and frozen gases. In fact, ice is likely the major constituent of that world. In that, Pluto is rather more like the comets than had first been thought. That, too, doesn’t boot it from the list of planets, though. After all, if the asteroids are rocky, that makes Earth more like asteroids than it is like Pluto, Neptune, or Saturn. But, ice is far more reflective than rock. So, Pluto can be much smaller and still reflect as much light as it does. Soon, it became apparent that Pluto was quite a bit smaller even than Mercury. Then, in 1978, James Christy discovered that Pluto had a moon. That moon, Charon as it was eventually named, turned out to be the key to really discovering Pluto’s true size. As Charon orbits Pluto, it sometimes moves in front or behind Pluto. That happened for about 5 years between 1985 and 1990. Studies of these mutual occultations permitted astronomers to finally accurately measure the size and mass of Pluto. Pluto was found to be tiny (under 1200 km in diameter). You could fit over 150 Plutos inside of Earth. In fact, more than three Plutos would even fit inside the Earth’s Moon! Pluto has only about 0.2% of Earth’s mass! It is tiny, indeed! Had that been known when Pluto was discovered, there may have been some more debate about where it should be considered a planet or not. But, tiny as Pluto is, it was still much larger than the largest asteroid, and it was the only body of its size known. So, there was no compelling reason to boot it from the list of planets.
Pluto’s orbit, though, is also a bit odd. All of the rest of the planets have orbits that are almost in a plane. The rest of the planet deviate no more than about 7 degrees from that plane. Pluto’s orbit, however, is inclined 17 degrees. That puts it well outside the range of the rest of the planets. What’s more, Pluto’s orbit is far more elliptical than other planets. It has an eccentricity (a measure of how far from a circle an orbit is) of more than 0.25. For comparison, Earth’s orbital eccentricity is 0.017. But, orbital eccentricity itself is not enough to kick it off the list. Mercury also has a very elliptical orbit, with an eccentricity of 0.21, and it is the one with the 7 degree inclination orbit. One could argue that if orbital eccentricity and inclination alone disqualify a planet, then Mercury is also not a planet. Incidentally, I actually have heard that argument given by those so vehemently opposed to Pluto’s planetary status that they are willing to sacrifice Mercury from the list just to get rid of Pluto!
Pluto’s orbit has another oddity. It has an orbit that is elliptical enough that it crosses that of Neptune. The two never run into each other, though, because the orbits are inclined so that when Pluto is at the same distance as Neptune from the Sun, it is not at the same level as Neptune’s orbit, so they always miss. But, still, if Pluto came close enough to Neptune, then the vastly large planet’s gravity should hurl Pluto off into the cosmos. That, too, won’t happen because Pluto’s orbit has a resonance with Neptune’s that keeps the two from ever doing that. This is much the same concept as Cruithne and Earth’s orbital relationship, or Earth’s co-orbital asteroids. Now, that is something that people often latch onto as a reason to say that Pluto is not a planet. It is small enough that its orbit can be dominated by a larger planet. This, too, has long been a reason to debate Pluto’s planetary standing.
However, despite Pluto’s tiny size and odd orbit, all of the talk of de-listing it from the list of nine planets was just that: talk. It was something that astronomers would talk about, but mostly out of curiosity. Pluto was still the only thing like it out there, so it was an oddity. When I taught my planetary astronomy class, we had nine planets: four terrestrial ones, four gas giants, and Pluto. It did not conform. It was the odd one. It did not fit with the other bodies. It might should have not gotten onto the list, but it was there. Astronomers are sometimes a pretty conservative lot. So, once on the list, it stays unless there is a really compelling reason to remove it.
Then, new discoveries at the end of the Twentieth Century brought the whole matter to a head. Other small icy bodies were found beyond Neptune. The astronomer Gerald Kuiper had proposed that some comets come from objects orbiting the Sun in a giant belt beyond the farthest planet. These icy bodies, when their orbits are disturbed enough, dive in close to the Sun and act as the comets that we have known from antiquity. All of the comets were tiny. But, astronomers proposed that some of these things would be much larger than others, just as many asteroids in the asteroid belt are much larger than the average near Earth crossing ones. Finally, these large objects started to be found. At first, they were still much smaller than Pluto. Then, some were found that were close to Pluto’s size. This triggered a debate about where the cutoff should be for being big enough to be a planet.
Then, in 2003, the object that came to be known as Eris was discovered. It was larger than Pluto. It also was of the same basic composition as Pluto. It had a similar orbit. If Pluto was a planet, then clearly this nearly identical but larger body was also a planet. Now, there was really serious debate. What to do with Pluto now that there are more things like Pluto.
At first, you’d think that Pluto having been kept on the list because it was the only thing like it in the Solar System would mean that when more things like Pluto were found that Pluto would be instantly booted. Indeed, that is the argument that many have for Pluto not being a planet. But, ironically, that there are more things like Pluto out there also has others arguing that this is a compelling reason for Pluto and these other bodies to be planets. Now, Pluto is no longer an anomaly. There are several planets like Pluto. Clearly these are two polar opposite opinions. Which is right? Or, are neither position right? So, while we have had a long standing water-cooler debate on whether Pluto should really be on the list of planets or not, the discovery of other bodies like Pluto meant that there needed to be serious discussion on just what the definition of planet should be.
And, I will continue with this in my next post.
-Astroprof
Image courtesy NASA, HST
Defining Planets (Part I)
Published on Feb 2, 2009 at 4:48 pm.
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In 2006, the International Astronomical Union voted on a new definition of the word planet. I wrote about the matter at the time, but I now feel that I should revisit that discussion. Last week I wrote about an asteroid sharing Earth’s orbit and that this asteroid raised issues with the IAU definition of planet. I got a number of responses to that posting. One of the most talked about consequences of the IAU’s new planet definition is that Pluto, long on the list of planets, was removed from the list. One of my commenters who wants Pluto to remain a planet, Laurel, even has a blog about the status of Pluto. Recently, even Flying Singer, who has not weighed in until now, has decided that the definition of planet should be amended. His favored definition also includes Pluto. There are a lot of opinions and emotions involved in whether Pluto should be on the list. It is not as simply, though, as whether there are 8 or 9 planets, nor whether Pluto is on or off the list. Whatever decision applies to Pluto applies to all of the other bodies just like Pluto. And the matter did not start with the 2006 IAU meeting. I had already written about this over half a year before the decision, and I even gave a public presentation in January of 2006 about the controversy as to whether Pluto should be a planet or not.
Interestingly, most of the discussion about the new definition has centered on whether or not Pluto should be a planet, and what definition of planet should be used that would allow Pluto to remain on the list. Really, that is not how science should be done. we should first decide what makes a planet and then decide if Pluto is on the list. If it is, then fine. If not, then that should be fine, too. Personally, I think that the IAU definition is seriously flawed and needs to be revisited. I’ll be writing more about this for the next several posts. First of all, though, I want to talk about the history of how all this came about. All too often, the pro-Pluto crowd and the anti-Pluto crowd have squared off and hurled insults at one another without stopping to look at each other’s positions. That helps no one. As it turns out, there are very valid reasons for having a definition of planet that excludes Pluto. But, there are also very valid reasons for having a definition of planet that includes Pluto. But, as I said, creating a definition to either include or exclude Pluto is wrong. We need to put Pluto aside. Next, without regard for Pluto, create a definition of planet. Then, see if Pluto fits the definition.
This is a very complex issue, and anyone who argue that it is NOT complex is not being realistic, in my opinion. So, I cannot simply cover the topic in one post (no matter how long). I am going to string this out for the better part of this week. First, we need to talk about how we got into a situation where the definition of a planet was even an issue. Next, we want to talk about what sort of definitions could exist. As it turns out, there are several possible ways to define planet. All have something to say for them, and all of them lack in some area.
Oh, and astronomers are not the only ones who reclassify things now and then. Recently I heard from one of our biologists that there is a proposal to reclassify the species and genus of drosophila. For those that don’t know, that is the iconic fruit fly on which so much research is done. So, where are all the people who are screaming about Pluto? Why are they not concerned that drosophila is possibly being changed, too?
If you are not aware of the discussions that had been going on for the prior two decades (or more), then you might wonder why the IAU even decided to bother with redefining the term planet. Well, actually, as it turns out, they weren’t redefining planet; rather, they were defining planet. There never had been an official definition of planet. The term just happened, and astronomers generally agreed informally on what they meant when talking about planets.
Growing up, I learned the planets of the Solar System, like all the other kids in school. We had to learn nine of them. I’ve always been an avid reader of science fiction, and of course space adventurers were always visiting other planets. In the mid 1990s, I was teaching astronomy when word came out of the discovery of planets around other stars. Some years prior, planets had been found around a pulsar. We all knew what a planet was, or at least we thought we did. So, why did they have to come up with a new definition?
The problem is that, as I said earlier, there never really was a definition of planet. The word and concept the concept of planet comes to us from ancient times. People looked up at the sky and saw stars. Night after night, they saw the same stars in the same orientation to one another. The human mind tries to find patterns in what we see, so we see patterns in the random assortment of stars. These became the genesis of the constellations. People also looked up saw the Sun, the Moon, and stars. Clearly, the Sun moved. It was sometimes high in the sky, and sometimes not. The Moon, too, moved. Every night, people could see that it was a little farther to the east in the sky. But, while most stars seemed to always stay put, there were five star-like objects that appeared to move around. These wandering stars were special. It is from the Greek word for “wanderer” that we get the word “planet.” So, a planet was a wandering celestial body. By that definition, even the Sun and Moon were planets.
In ancient times, most people believed that the Earth was fixed (after all we don’t feel it moving, to we?) and that the Sun, Moon, and stars went around the Earth. Eventually, however, the heliocentric model began to gain support with the work of Copernicus, Galileo, and Kepler. It became apparent that Earth went around the Sun like the planets. That made Earth a planet, and the Sun not. Galileo’s discovery of four large satellites of Jupiter led to their being called planets, too. The Solar System, for a while had 11 planets, only five of them orbited other planets rather than the Sun. Soon, satellites of Saturn were discovered, and more satellites of Jupiter were found. Eventually, astronomers decided that planets orbited the Sun and moons orbited planets.
But, it is not so simple. In 1781, William Herschel discovered Uranus. This body was the first discovery of a new body whose designation as being a planet has stood the test of time (so far!). But, then astronomers found other bodies orbiting the Sun between Mars and Jupiter: Ceres, Pallas, Vesta, and Juno. These were called planets since they clearly orbit the Sun and not another planet. Quickly the list of planets soared to over a dozen. I think that I read somewhere a few years ago that at one time textbooks listed about 17 planets. But, these new bodies between Mars and Jupiter clearly were much smaller than the rest of the bodies labeled planets. Furthermore, there were a lot of objects between Mars and Jupiter. At the time that these bodies were first being discovered, many astronomers believed that a mathematical formula might exist that showed a pattern for the spacing of planetary orbits. This formula predicted a planet between Mars and Jupiter, but just one planet. So, these bodies could not fit the bill. Some astronomers speculated that perhaps there had been a planet there that had somehow been destroyed. That idea turns out not to be correct. Eventually, the shear number of bodies between Mars and Jupiter, together with their extreme difference in size compared with the other planets, suggested that they should have their own designation rather than called them planets. Informally, they became known as asteroids. It should be noted, though, that for decades professional astronomers have called asteroids minor planets, and the body of astronomers who keep tabs on them are called the Minor Planet Center.
Along the way, Neptune was found. Uranus was discovered by accident. Ceres was discovered by an accident of sorts, too. Astronomers were looking for a non-existent body predicted by a faulty equation. They just got lucky that Ceres was in the area that they were looking. But, soon more asteroids were discovered. Today, we know that the asteroid belt is full of these bodies, and more are being found all the time. But, Neptune was different. While a faulty trick of numbers was responsible for astronomers looking for a planet between Mars and Jupiter, Neptune was predicted to exist from the understanding of orbits and gravity. A slight wobble in Uranus’ orbit could be best explained by the presence of another body. That body turned out to be Neptune. Clearly Neptune is a body very much like Uranus. So, if Uranus is a planet, then so is Neptune.
Astronomers were not so quick to give up on the numerology of the Titius-Bode Law (not really a physical law). So, they began to look for something beyond Neptune. To complicate matters, there were a number of errors in reporting Neptune’s position. Actually, the errors were more in how the position was reported, rather than the position itself, and how the standard changed over time. If the conversions were not made properly, then it appeared that perhaps Neptune, too, had an unexplained wobble in its orbit. Several big names in astronomy at the beginning of the Twentieth Century were among those championing the idea of another planet beyond Neptune. So, a search was on for the missing planet. During one of the searches, Clyde Tombaugh discovered a tiny dot. That became known as Pluto. At first, it was heralded as the missing planet. Later, though, Pluto was found to be far too small to have explained the disturbance in Neptune’s orbit. Further analysis of the orbit showed that the disturbance was not there, anyway. So, Tombaugh got lucky in finding Pluto, much as astronomers got lucky in finding Ceres and the other bodies in the asteroid belt.
When it was first found, Pluto was assumed to be at least as large as the smaller of the terrestrial planets. Over time, though, it was found to be much smaller than had been first thought. But, that is not the sole reason for it being booted off the list of planets. For years, I taught my astronomy students that there were nine planets: four terrestrial planets, four gas giants, and Pluto. But, all of that changed. And that is the topic of my next posting.
Oh, and a very good reference for those interested in knowing the history of all of this can read David Weintraub’s excellent book Is Pluto a Planet? It is very informative, and the author explains why Pluto could or could not be considered a planet. Weintraub makes a case that Pluto is a planet. But, there are perhaps some better arguments that could be made for Pluto’s planetary status than the author gives.
Stay tuned.
-Astroprof
Image courtesy NASA, Cassini
