Defining Planets (Part VI)
Published on Feb 19, 2009 at 11:27 pm.
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Filed under planets.
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
Astronomy Link List on February 20, 2009 at 3:58 am: 1
This article has been added to the Astronomy Link List.
Boxorox on February 27, 2009 at 7:27 am: 2
Wonderful! Astroprof, you have certainly covered many aspects of planetary qualifying standards well in this series.
My feelings on the matter are that we are focussed too heavily on trying to cobble together a composite single definition for planet. Perhaps the task can be both simplified and simulataneously made more complicated by allowing the term “Planet” to be an umbrella label for any planet-like body that is mostly spherical and consider that any such body is of the PLANET CLASS, then consider next which TYPE of planet each might be. Obviously, we already know of planet types which are terrestrial, gas giants, etc. which address their characteristics based loosely on composition and surface natures. As we look to the exosolar systems, we are definitely finding that the typing will expand greatly. Planet, as a general, inclusive description should be an easy label that we can apply to a body just by looking at the body itself and a “quick” survey of the environment in which it exists. Beyond the classification, typing of the planets must require some degree of investigation. Typing by compositional character may be the wrong route to take. Considering size, shape and orbit may be more useful. Taking our own solar system as a home-based analog, we only have 8 to 20 examples to draw from, but looking to the wider field, as you mentioned above, we can increase the population to nearly 400 and as we get better evidence of their true natures, then we’ll likely have enough sense of what planets are to establish something that resembles a universal definition of ‘Planet.’ And isn’t that what we ultimately seek to attain? What good would it be to determine what planets are just by looking at our own neighborhood, if we ignore the far richer field out beyond!
Link list – 20th February 2009 | Astronomy Link List on April 6, 2009 at 10:05 am: 3
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