Planet or not, Pluto does not disappoint. Over the past few days, Pluto has become a tangible and wonderful place. The New Horizons spacecraft has sent back absolutely spectacular imagery with many surprises for us to digest. From mountains on Pluto, and the strange polygonal features on Tombaugh Regio (aka Pluto’s heart), this icy world has captured our imagination and reawakened a public interest in space exploration. Pluto is a world unlike any we have observed close up to date. Over the next few months, our understanding of the solar system will no doubt change, just as it has for each of the interplanetary missions that have ventured forth into new territory.
This recent flyby has reawakened the old debate over Pluto’s planet status. So this week I would like to go over the controversial IAU decision in 2006 that lead to the “demotion” of Pluto, and some arguments for and against that decision. My personal opinion is that the IAU decision is far to restrictive on the definition of a planet, and in addition it fails to adequately define what exactly a planet is. I will say however, that initially I agreed with the IAU decision when it was first made. However, in the years since, I’ve come to disagree with the IAU decision. With some careful thought, its not hard to see why the definition falls short. To start, I’ll list the IAU definition, in their own words:
IAU Definition of a Planet:
A “planet” is a celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighborhood around its orbit.
Many of the arguments made in favor of this definition revolve around the 1801 discovery of Ceres, which was originally classified as a planet. After the discovery of Ceres, many other bodies were discovered in what later became known as the Asteroid Belt, and it was decided that the large number of Planets being discovered should be reclassified as Asteroids. In a similar fashion, many other Pluto-sized objects have been discovered in the Kuiper Belt in recent years, and so rather than add perhaps dozens of other planets to the roster, we simply reclassify Pluto and these other objects as KBOs (Kuiper Belt Objects). Collectively, objects like Ceres and Pluto are now classified as Dwarf Planets.
In some sense, the above argument does make sense. In both cases of the Asteroid Belt and the Kuiper Belt, we are dealing with a system of objects which are distinct from the reset of the solar system. But the same could be said between the four inner terrestrial planets vs the four outer giant planets. These two groups are also distinct from one another, but will still refer to all eight of them as Planets.
Perhaps my least favorite argument in favor of the IAU decision is the argument that if we were to classify all of these objects as “Planets”, then it would be hard for school children to memorize all the planet names. You may laugh (and I certainly hope you do) but this argument has been made by some who I would not have expected to make such a nonsensical argument. After all, we don’t define other scientific subjects in terms of what school children can memorize. That would be a little like saying that we shouldn’t add any more elements to the Periodic Table just because it would be too difficult to memorize, or restricting the number of allowed mammals because we already have a lot of known mammals.
It is true, that if we were to call all of these objects “Planets”, then we would have dozens of planets. But I submit to you, the reader, that this is not a horrible thing. There are also a number of scientific reasons why the IAU definition is simply not a good one. So without further ado, lets tear the IAU planet definition to pieces.
I will start with criterion (b), “has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape”.
This is the one part of the definition that I agree with for the most part. We need some sort of physical characteristic that defines the object. Having enough mass for the object to pull itself into sphere is a defining characteristic, one which distinguishes it from other objects that are not as massive. However, this criterion might also include things like stars, so it lacks the necessary specificity to really classify a planetary object. Obviously stars should not be considered planets because they are yet another distinctly different class of objects, namely objects that undergo nuclear fusion at their core; planets do not do that.
Criterion (a): is in orbit around the Sun.
This one seems like a no brainer… at first. First off, it specifically says “the Sun”, which immediately leaves out planets around other stars. But as we know from the Kepler mission, most stars have planetary systems of their own. Now, let’s consider a binary star system, or a system with two stars orbiting each other. The galaxy is full of such systems. Both objects are considered stars, despite the fact that they orbit one another. Shouldn’t we also consider the possibility of binary Planets? Suppose that two round non-star objects of the same mass, both of which meet criterion (b) orbiting each other. If they are the same mass, then they would both orbit a center of mass in-between them, and that two-Planet system would then orbit their star. This is indeed a similar situation to the Pluto-Charon system. There you have two similarly sized objects orbiting a center of mass in-between them. Or, perhaps more poetically, one does not orbit the other, but they orbit each other, in a beautiful Keplerian dance.
Now here is the fun part. Let’s look at the rest of the solar system. Many of the moons in the solar system are quite large (and round). Ganymede, the largest moon of Jupiter and of the solar system, is actually larger than the planet Mercury! This is also true of Saturn’s moon Titan. So here we have objects classified as moons, which are larger than something else that is called a planet. And remember our Keplerian dance, which is also true of any two gravitationally interacting bodies. It can also be said that Jupiter and Ganymede orbit each other, as much as one can say that one orbits the other. In this cases though, the center of mass that both bodies orbit is within Jupiter, but the relationship still holds. In fact, this is one method for detecting extra-solar planets, by detecting the wobble of a star, which is in fact a result of the star’s orbital dance with its companion planet.
But wait! What about the Earth-Moon system? We could make the same argument. Perhaps the Earth and Moon are in fact a binary planet system, just as the Pluto-Charon system is.
I would submit to the reader that an object should be classified by it’s own physical characteristics, rather than where it happens to physically reside or which object it happens to orbit. Or, put more simply, a Planet is a Planet whether it is on its own, or orbiting another planetary body. In fact, the galaxy is likely filled with so called Rogue Planets that were flung from their host stars long ago and now exist without a star entirely. Are these objects not planets simply because they do not have a star? That’s a little like saying that the Hydrogen in H2O is different than than the Hydrogen in CH4 or H2 just because it resides in a different system of atoms.
And finally, criterion (c): has cleared the neighborhood around its orbit.
This one perhaps is the worst criterion of the three, as it is completely unrealistic for any object to meet this condition. This idea supposes that the object is massive enough to gobble up any debris (asteroids, dust, etc.) that might be somewhere in its “neighborhood”. However, none of the eight recognized planets can meet this criterion. Earth for example has an estimated 12892 so called Near Earth Objects (NEO) that orbit around the sun in Earth’s “neighborhood”. That doesn’t seem very “cleared out”. Further, many of the planets, such as Jupiter have so called Trojan Asteroids which share Jupiter’s orbit, trailing in front or behind Jupiter in what are known as Lagrangian Points in the Jupiter-Sun gravitational system. Thus even the largest of our planets has not “cleared out” its orbit. Further still, the farther out a planet is from its host star, the larger it would have to be to sufficiently “clear out” its orbit. So if you were to put Earth out in the Kuiper Belt, it would certainly not be considered a planet under the IAU definition.
I’m certainly not the first to make these arguments, but this week seemed like a good week to bring up some of these points again. We should have a scientifically relevant discussion about what a Planet is, and what criteria need to be met in order to be considered a planet. It is true that objects in the Kuiper Belt are indeed different than objects in the inner solar system. However, that should only mean that we have many different types of planets, and should not mean that we should take them off the list. We really have three types of planets:
1. The inner terrestrial planets. (Mercury, Venus, Earth and Mars. Perhaps Ceres should be here as well.) Defined by their rocky composition.
2. The outer giant planets. (Jupiter, Saturn, Uranus and Neptune) Defined by their size and gaseous composition.
3. The distant icy planets. (Pluto, Eris, Makemake and many others) Defined by their icy composition.
Notice how each of these groups is defined by its physical characteristics, rather than what they happen to orbit, or what their “neighborhood” looks like. This makes far more sense in my opinion than the IAU decision, and allows us to really start classifying planets as we find more in our own solar system and other star systems beyond.
Whatever we call Pluto, and the other objects out there, it is truly a fascinating world that is worthy of scientific study and our attention. After all:
“What’s in a name? that which we call a rose
By any other name would smell as sweet”