Defining Planets (Part IV)
Published on Feb 9, 2009 at 10:29 pm.
10 Comments.
Filed under planets.
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
Astronomy Link List on February 10, 2009 at 3:46 am: 1
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Boxorox on February 10, 2009 at 7:17 am: 2
Astroprof - You are making a very well-reasoned approach to resolving this issue. I agree that whatever comes out to be the most widely accepted definition of ‘planet’ should be one that works well for objects orbiting other stars. It will be the planets orbiting those stars which will be our test cases for the definition we hopefully will come up with soon.
It seems that you have resolved that size alone is not a completely valid constraint for determining what is or is not a planet. Mass as well may be ruled out, at least at the low end of the scale. At the upper end, in the multi-Jupiter range, mass becomes critical, literally, as it will determine whether fusion takes place which would place the body into the star-like category.
So it seems rather clear to me that shape is a determining criterion. A planet should be spherical. But that specification elicits the question: How spherical? Most rotating planet-like bodies exhibit equatorial bulges, especially the gas giants and those with moons, so a tolerance will of necessity need to be set (1% axial disparity, 2% ?. . .).
In reference to your treatment of the asteroids, Ceres displays many qualifications for planethood. What prevents it from being accepted as a planet is that it is small, less than 1000 km, and is a member of a belt of similar though smaller objects. We may find out in the long-run, depending on how we resolve the issues with Kuiper Belt objects, that Ceres does indeed need to be re-inducted into the Planet Club. I am not presently ready to accept this, but it is potentially possible.
Laurel Kornfeld on February 10, 2009 at 6:31 pm: 3
Ceres should be considered a planet, not an asteroid because it is spherical. The threshold for an object being considered a planet should be the attainment of hydrostatic equilibrium, which yields a round or near round shape. The demotion of Ceres from planet status was wrong because 19th century astronomers did not know that unlike most of its neighbors in the asteroid belt, Ceres is spherical. Moons of planets that are round are geophysically planets too–they are secondary planets as their primary orbits are around other planets and their secondary orbits are around the sun. The term secondary planets was actually used to describe round moons of planets in the 19th century. Objects in hydrostatic equilibrium behave far more like planets than like asteroids. If we established that these smaller bodies are a subclass of planets known as dwarf planets, and that round moons are a subclass known as secondary planets, a lot of the controversy would be resolved because we would be incorporating both the what and the where of these objects.
Astroprof on February 10, 2009 at 8:08 pm: 4
Boxorox, thank you. I am trying to approach this in a reasonable manner and not to just rely on emotion. I was also going to address the other end of the spectrum, large masses, at the end of the series, but I may just address that sooner.
Laurel, you are getting to point 5 of my previous post. I am not there yet! Also, I was planning on leading up to the possibility that arbitrarily cutting off the larger moons from planetary status just because of their orbits is as unreasonable as kicking off the smaller planets just due to size.
Astroprof on defining planets, parts III and IV « AstroGeek on February 11, 2009 at 11:37 am: 5
[…] of posts on defining planets. I previously posted about parts I and II, here are parts III and IV. Possibly related posts: (automatically generated)Astroprof Defining PlanetsTom Snyder interviews […]
Boxorox on February 11, 2009 at 11:51 am: 6
This is getting good (better, it has been really good all along).
Ceres might well get that promotion some day, once we cut away the tradition of the past 8 decades and examine in more detail what is out there in the extended Solar System. I would be more ready to accept a Ceres planet than a Pluto planet. Pluto presents a problem to me because it violates Neptune’s “airspace.” It is one of my personal criteria that an object which in all other respects qualifies as a planet should be not considered a planet if it crosses the orbit path of its next inner neighboring planet. In view of Neptune’s much more circular orbit, we would not really say that it is Neptune which is transgressing against Pluto. I also think that Pluto’s inclination, well in excess of 10 degrees to the ecliptic, is a problem.
Back to Ceres, we perhaps should not hold Ceres to blame that it has not well cleared its orbit space of debris. It has succeeded to survive the travails of the past 4.5 billion years to be the king of its domain. Though I certainly don’t wish to attribute feeling to these bodies, I still wonder if we should award Ceres its due recognition and induct it into the Planet Club. -mrj
Work from home no fee on February 11, 2009 at 8:40 pm: 7
Great post, added to my markers
Mirage LasVegas on February 13, 2009 at 11:24 am: 8
Thanks for clarify this to me. This series of articles are a keeper!
Mark Snow on February 13, 2009 at 9:30 pm: 9
Perhaps I am missing something in the planet definition debate thing because I just think of solar system objects as different populations. The inner rockies, moons of rockies, the outer gas giants, moons of gas giants, belt asteroids, kuiper belt objects, oort cloud objects, dust to larger rocks not herded into the asteroid belt. I don’t see the need to specify beyond this type of general description. I would then expect to compare our populations and their distribution with those of other star “debris” fields. Hopefully I’m not too far afield here, as populations are what I describe to my high school students. I am going to admit to a bias here and say that I see planet defining to be a sort of self limiting exercise not leading to further insight. But, that is just my lay experience harrumphing.
Smiles.
P.S. Thank you for taking time to blog…very much appreciated.
Link list – 10th February 2009 | Astronomy Link List on April 6, 2009 at 10:02 am: 10
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