Defining Planets (Part V)
Published on Feb 14, 2009 at 4:43 pm.
6 Comments.
Filed under planets.
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
Laurel Kornfeld on February 14, 2009 at 7:08 pm: 1
Thank you for providing a thorough, detailed analysis of the issues surrounding planet definition and the reasons why the boundaries between one type of object and another are not as clear cut as most people think. I recommend you send this entire series to the IAU Planetary Sciences Division for consideration, as it is far superior to anything they have produced. The real universe does not come in neat little categories–things exist along a spectrum, i.e., planets range from tiny dwarf planets to sub brown dwarfs larger than Jupiter. And it’s okay to recognize the complications and acknowledge that we may not know enough to definitively classify some of these objects as belonging to one category and not another.
Mang on February 14, 2009 at 8:58 pm: 2
Great series. Keep these coming!
It occured to me that under different circumstances the presence or absence of an atmosphere might have been suggested as a criteria. Of course it has all kinds of problems with bodies inside/outside the frost line and with atmospheric composition, and moons. And it would leave Pluto inside and outside the club at different points in its orbit. I’m not sugggesting for a minute that this is a good criteria, just that it makes about about as much sense as some of the criteria that have been used/considered in the past.
Mang on February 14, 2009 at 9:21 pm: 3
There is another consideration in this debate - the audience using the language being debated.
Any specialized field has specific terminology. Beyond this there is the public or the educated public. These groups are not going to fully share the same terminology and it isn’t desireable that they do. It’s therefore a goal to have the terminology not conflict or contradict in serious ways. This allows the public to be imprecise but still generally in the same direction. It also allows the specialist to pursue high degrees of prcecision. It also frankly heads off aspects of the kind of debate we’ve seen around Pluto.
In the mind of the public: stars shine in the visible spectrum; planets are mostly round bodies orbiting stars; moons orbit planets; asteroids are generally thought of as irregular; comets grow tails and have highly eccentric orbits. Dwarf planets would still be planets. These general ideas work in other star systems as well.
Your exploration of the boundary between gas giant and planet is well taken. I think people can easily see the Brown Dwarf as failed star explanation. Perhaps there is a boundary between a planet and a failed planet? This would suggest some things that planets would need to accomplish. I wonder if this is where you are heading?
Boxorox on February 17, 2009 at 9:42 am: 4
This has been a great series. I see the discussion about distinguishing between the various types of planets, dwarf planets, brown dwarfs (almost-stars) and stars as similar to mountain climbers who argue about when does a hill become a mountain. What are the valid criteria to class some as one and the rest as another sort of object. Along the lines of hills and mountain, there are also volcanoes to consider. Volcanoes comes in all sorts of shapes and sizes but are generally regarded as mountains just because it has been the custom to do so. It’s not very scientific to class them this way, but everybody tends to understand the method.
The importance of establishing a clear definition of planet, in my view, is so that we can more easily understand what we see as we gather more evidence of planetary systems orbiting other stars. What we have not seen yet, but should certainly know to exist, are systems of bodies in which a brown dwarf is the primary member orbited by planets or planet-like bodies. We may someday be able to sense them due to gravitational effects, but when we eventually stumble upon such a group of objects, it will completely open the debate again unless we come to grips with what really qualifies as a planet.
Objects in orbit of stars, which must surely number in the thousand if not millions and more in our galaxy, undoubtedly span the size range from the smallest to the largest. There will be no specific deviding line between the various classes unless we specify physical laws which can govern these classes. Sustainable nuclear fusion is one such characteristic which clearly defines what a star is. For bodies of lesser mass, physical attributes recognizable to all may be less easily defined, but a universally acceptable convention of standards should surely be possible to outline. This discussion and others like it serve to make these distinctions possible.
I hope the IAU is open to input on these issues.
Astroprof on February 17, 2009 at 10:18 am: 5
Thank you, everyone, for reading my ramblings. Perhaps when I get to the end of the series there will be some more things for people to think about in debating the matter. Just about any definition will have its supporters and detractors. Just about any definition will have both good and bad points. If we are going to define planet, we need to look at all the possible ways of doing it rather than quickly jumping on one.
Mang on February 18, 2009 at 4:56 pm: 6
Another dose confusion - Interesting question what if Pluto is basically a comet? Just one larger than we’ve seen and one that reached hydrostatic equilibrium. And then what are Eris and the others? What impact on the debate if any would that have?