Defining Planets (Part IX)
Published on Mar 3, 2009 at 4:39 pm.
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I’ve been mired in paperwork at the college, so I have been slow blogging. Here we are, though, in Part Nine of this series, and I still have some more to write about!
I spent all of February covering aspects of what may go into a definition of the term planet. If you are new to reading my blog, go back to the first posts of February 2009 to read the entire series. I wrote about the history of the term, the controversy that arose when more bodies started to be found in the Solar System, the objects that used to be on the list of planets but were removed, and then about the more recent controversy that finally led to the IAU’s decision of a formal definition of the term. I then talked a little about the problems with the definition that arose, and then I began writing about the possible criteria that a planet definition might encompass. In Part III of the series, I laid out several possible criteria. Since then, I have addressed to one degree or another the first 8 of the ten points. It took me a month to go through them all! So, you can easily see why the IAU committee took a year to address these issues, and you can can see why the IAU final decision, which took hours to come up with, has left many people unhappy.
Point 9 of my ten was to address whether or not a planet was something that affected its environment in a specific manner. After all, we have had a tough time finding a workable definition based upon the physical nature of the planet, its composition, its formation, or any other property about what a planet is. So, perhaps a planet is defined by what it does. We’ve already talked about the orbit. But, what else might a planet do that would set it apart from a non-planet?
Well, we can be very self centered and start off by saying that a planet might have characteristics that allow it to support life. After all Earth supports life, right? But, that gets into a thorny situation very fast. We are not sure if Mars can support life. There is a lot of argument going on over that. Venus is far too inhospitable at its surface, and Mercury is likely out. Uranus and Neptune are also out. While it has been speculated that some sort of microorganisms might exist in the clouds of Venus, Jupiter, and Saturn, that has so far been pure fanciful speculation, with not a shade of hard evidence that life really could exist there, much less that it does. The only body in the Solar System that we know for sure can support life is Earth, and limiting the list of planets to Earth is pretty silly. If we add Mars because it is close enough that we can imagine that some extremephile life could exist there, then that gives us only two planets. We could add Europa, a moon of Jupiter, but we don’t know for sure that it can support life, either. So, we want some other definition.
Another possible criterion might be that a planet shines only by reflected light. That is, it gives off no more energy than it gets from its star (the Sun, for our Solar System). But, that is not necessarily a good definition because the planets of the Solar System give off slightly more energy than they get from the Sun due to trapped internal heat escaping. For planets like Earth, that discrepancy is tiny. But, Jupiter is still generating heat from contraction and it actually generates more energy that it radiates into space than it gets from the Sun! Likewise, Saturn and Neptune radiate considerably more energy into space than they get from the Sun. Granted, this radiation is at non-visible wavelengths. These worlds reflect visible light rather than shining in their own right in visible wavelengths, but why should we restrict the definition of the term planet to just those wavelengths that we can see? After all, other beings from other worlds may see a different band of the electromagnetic spectrum than we Earthlings do. There are even species here on our own home world that sense different wavelengths of light than we do. So, this would not be a good criterion.
One of the ways that we are finding planets around other stars is through their gravitational effects on the star itself. As the planet orbits the star, the star wobbles. Might a planet be something that makes its star wobble? But, there we get into shades of wobble. The bigger the planet and the closer the planet is to the star, the bigger and more noticeable the wobble. The smaller the body, the smaller the wobble. Technically, any body with mass will have an gravitational effect, so that would include all of the asteroids and comets, too. Where do we draw the cutoff? Now, we are back to the problems associated with defining planets by size or mass, so we need something else.
Another way of finding extrasolar planets is to look for the effect that the planet has on the parent star’s light as it passes in front. Like the previous suggestion, this is attractive because these criteria would apply to a definition of extrasolar planets as well as to objects in our own Solar System (imagining the wobble of the Sun or the effect on the Sun’s light if seen from some distance away). The final IAU definition specifically refers to objects orbiting the Sun in its definition, which leaves out all purported planetary bodies around other stars. But, the effect on the light of a star can come from two physical processes. The gravity of the body can bend the light, and the physical body itself can simply get in the way and block light. Since gravity is present any time there is mass, the first idea will always play a part. Again, it will simply be a matter of degree. In the second case, any opaque body will do the same thing, again the size of the body will determine the degree to which light is blocked. Once again, we would wind up having to make an arbitrary cutoff. Personally, I would much rather have a clearcut definition rather than one with an arbitrary cutoff at some point. It seems silly to me to say that one body is a planet and another body, identical in every way save for a kilometer here or there or a few kilograms here or there, is not a planet. Philosophically, that bothers me. But, I will admit that there just might not be any better way of doing it. After all, geographers and geologists have run into this same issue in defining a hill and a mountain. A mountain is taller than 300 meters, and a hill is shorter than 300 meters. That is an arbitrary designation, and I don’t like them.
So, what else might a planet do? Well, one possible suggestion that was proffered has when all of the debate about Pluto was gathering steam was to say that a planet was something that has a moon. The reasoning was that if it was big enough for its gravity to hold onto a moon, then it must be a planet. That seemed to put Pluto firmly on the list. It also left Eris, the body then known as 2003 UB313, on the list. But, it also put the asteroid Ida on the list because the tiny moon Dactyl orbits Ida. Several other bodies were also added because they had moons. Ceres has no moons, so it was back off the list. Calling Ida a planet and Ceres not is silly because Ida is tiny by comparison, measuring only 56 kilometers long and about 24 kilometers wide (Ceres is about 950 kilometers across). Even sillier, it leaves Mercury and Venus off the list, as they also have no moons. Calling Earth a planet and Venus, nearly identical in size and mass, not a planet is downright stupid, in my opinion. So, that idea got nowhere.
When the IAU was working on its definition of planet, though, they came up with another new idea, and that was that a planet was a body that managed to clear its orbit. The idea is that a planet is a body that has grown to sufficient mass that it has gravitationally cleared the neighborhood of its orbit. Unfortunately, what is meant by this is a bit more technical than comes off from mass media reports, or even the summaries typically given about clearing the neighborhood of the orbit. In fact, I am not sure that the IAU definition even specifies exactly what is meant by clearing the neighborhood of a body’s orbit. So, let me address this issue next.
At first glance, it would appear that the “clearing the orbit” criterion would eliminate most of the bodies currently on the list of planets. After all, Earth shares its orbit with a handful of asteroids. In fact, that is what got me started on this whole defining the planet series in the first place. In January, I wrote about the asteroid 2009 BD, which has nearly the same orbit as Earth. I followed up that posting with one about other Earth co-orbital asteroids. That raised the whole question about the definition of a planet as being a body that has cleared its orbit. Furthermore, Jupiter shares its orbit with a host of asteroids, many in quite stable orbits about points nearly 60 degrees in front and in back of Jupiter as it orbits the Sun. There are plenty of asteroids with very elliptical orbits that cross the orbits of the planets, too. In fact, practically every body in the Solar System would be taken off the list if nothing else in its orbit were permitted in order for it to be a planet! While clearing the orbit is often cited as a good reason to drop Pluto from the list, Pluto’s very existence also would question whether Neptune should be on the list, either. After all, Pluto crosses inside Neptune’s orbit.
Looking at the criterion closer, though, clearing the orbit refers to getting rid of objects of comparable size. But, even that is a bit ambiguous. What is comparable size? We are back to the arbitrary cutoff thing, again. So, lets look at this in a more detailed manner. The biggest problem here is that we are trying to come up with a precise scientific definition using imprecise language. So, let’s switch to the more precise language of mathematics. If we can come up with a mathematical formula that unambiguously arrives at a non-arbitrary value that has to be met in order for a body to become a planet, then perhaps we are in business. As long as we are hand-waving, we’ll never get a very good definition. But, does such an equation exist?
A couple of interesting possibilities exist. One simply computes the ratio of the mass of the planet candidate to the mass of everything else that shares its orbit. If the ratio is bigger than 1, then it is the end product of the accretion. If it is less than 1, then the possibility exists of things running into one another accreting into something larger. However, I don’t like this as a way of determining if the planet candidate has cleared its orbit. After all, what about the possibility of binary planets? In our own Solar System, the Earth-Moon system is very close to being a binary planet system. Pluto and Charon certainly are a binary pair of whatever they are. And, of course, we want a definition that would hold for extrasolar planets, too. What would you call an Earth sized body if it orbited at a Lagrange point in Jupiter’s orbit, 60 degrees in front or in back of that giant planet? Would it not be a planet simply because of Jupiter’s presence in that orbit? So, we need something better than simply comparing the mass of a body to the total mass of everything else in its orbital neighborhood.
So, can we compute anything else that might be useful in determining planet or non-planet status? Another possibility has been proposed by Steven Souter. He refers to work done by Alan Stern and Harold Levison. Stern and Levison extend the work of Ernst Opik to devise a formula to compute the likelihood that a small body will pass close enough in a single orbit to a larger body to experience a deflection sufficient to move it out of its orbit. They extend this to compute a term related to the likelihood that this will then happen within a Hubble time (13.8 billion years by today’s measure, though Stern and Levison used 12 billion years). I am not sure why they picked a Hubble time rather than some other time interval, but the equation works just fine for any time interval that you put into it, so you can also use a time comparable to the age of the Solar System is you’d like. The result is linear in time, so all that changing the time would do is to scale the term. The interesting thing here, though, is that if this term is >1 then the larger body manages to deflect the majority of bodies from its orbit. If the term is <1, though, then this has not occurred in the time specified. Soter proposes this term to be a planetary discriminator. He notes that the value of the discriminator is not only much less than 1 for Pluto, Eris, and Ceres, but it is also much greater than 1 for the eight bodies that are pretty much accepted as planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). The smallest of the discriminators is that of Mars, and even that is over 300,000 times larger than the value for Pluto. Among the bodies that he studied, there was no larger gap in values. So, he proposes that this significant gap would be a way to separate planets from non-planets.
At first glance, this looks like a good argument. It also is similar to a suggestion that I had made myself some years back as a way to separate out planets and non-planets. You just plot out the characteristics, and then you find the gap between the larger ones and the smaller ones. If you are lucky, the gap is very large and so you don’t have to draw an arbitrary line and then explain why to extremely similar bodies are planet and non-planet based on their falling just barely on opposite sides of that arbitrary cutoff. As I said, at first glance this looks very good. But, I can see that there may be problems if you look at it a bit more closely.
For one thing, the equation relies quite heavily on the physical properties of the orbits. That would mean that this term changes depending upon where the planetary candidate is located in the Solar System. This means that the farther a potential planet is from the Sun, the smaller its discriminator term will be. Thus, a body could be considered a planet if it is close to the Sun, but not if it is farther way. Does that make sense? Well, it might to some people, I suppose, but it doesn’t set well with me. If it is a planet, then it should be a planet no matter where you put it. Also, suppose there is a body whose mass and orbital period are extrasolar planets whose discriminators are very close to unity. We are again back to having arbitrarily drawn lines, particularly since, as I said, there is no real reason that the time factor has to be the Hubble time. And, of course, this does nothing to describe those bodies that are planet sized bodies ejected from orbits around stars by gravitational interactions. Shouldn’t these rouge planets also count?
I think that Soter’s planetary discriminator idea has a lot of merit, and I can see why using this to define the clearing of a body’s orbital neighborhood sounds like a very good criterion for planetary classification. And, in our own Solar System, such a system would work just fine. It clearly separates the larger bodies from the smaller ones. It provides a couple of other findings, though, that may raise some eyebrows. For one thing, if you put in put in the values for Earth’s Moon, you find that it, too, yields a discriminator factor greater than one. So, if Luna orbited the Sun at 1 AU instead of Earth, then it, too, would be a planet. That again raises the question of whether the Earth-Moon system should be a planet and a moon or a binary planet. Furthermore, while this system may work in the Solar System, we can easily imagine another planetary system where the values would not be so clearcut with such a large gap in the discriminator values between planets and non-planets. Thus, while this may be better than many of the other factors considered, it still needs a bit of work.
Incidentally, while Soter uses Stern and Levison’s work to establish his discriminator, they did not use this term in the same manner. Stern and Levision argue for hydrostatic equilibrium as the determination of planetary status, and they use this discriminator as a way to differentiate between what they call the uberplanets (the ones with value greater than 1) and the unterplanets (the ones with value less than 1). They call all bodies that are in hydrostatic equilibrium planetary bodies, including the planets, the largest moons, and free floating bodies between stars that are otherwise planet-like. I rather like this, and it mirrors what I like to teach in my classes. The largest moons are planet-sized, planet-structured, and planet-like. They would be planets, without question, if they orbited the Sun instead of Jupiter or Saturn.
So, this gives us something else to think about when we are trying to define planets. Soter’s planetary discriminator seems to have promise, but I think that it needs work.
-Astroprof
