Definitions or traditions? The Pluto Paradox

If you’re old enough to be reading this blog, then when you were in school, there were nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus (giggle), Neptune, and Pluto.  Which means that in August 2006, you felt the sting of betrayal when the International Astronomical Union (IAU) decreed that our beloved little Pluto was no longer a planet.  I’m sure there are many groups out there still claiming that this is all a plot by the Common Core, the liberal media, President Obama, President Bush, the tea party, or the academic elite to ruin and confuse the education of our precious little cherubim one planet at a time.  There’s even at least one petition begging that Pluto’s planetary status be reinstated by the IAU (though if you sign it, you can’t read my blog anymore).

Petitions and nostalgia aside, the debate about Pluto’s planet-ness is complete – Pluto is not a planet.  Why not?  There are a few reasons, mostly to do with gravity.  Pluto has always been an outlier in the cadre of nine planets.  Really far away, and much, much, much smaller than the others.  But Pluto isn’t actually all that lonely.  It orbits as part of an area of space known as the Kuiper Belt.

Kuiper Belt orbit. Credit: Don Dixon, by way of Universe Today
Kuiper Belt orbit. Credit: Don Dixon, by way of Universe Today

The Kuiper Belt is similar to the asteroid belt int that it contains a lot of debris – condensed bodies of matter that orbit the Sun in a specified area of space – there are over 1000 known objects in the Kuiper Belt, and counting.  .  Up until 2003, Pluto was the largest known body in the Kuiper Belt, and therefore retained its tenuous grasp on planethood.

In 2003, however, astronomers discovered the existence of Eris, a Kuiper Belt object larger than Pluto.  This led to a quandary of definitions.  If we were to insist that Pluto remain a planet, then we would have to declare Eris to be a planet as well.  And any other objects of similar size in the Kuiper Belt, of which there could be many that we simply haven’t spotted yet.  If the IAU went that route, then we could jump from 9 to 100 planets, most of which would not fit the one of the important requirements of planethood, which is to be gravitationally dominate in a region of space.  So instead of creating that level of future confusion, astronomers created a new class of celestial body, dwarf planets, with Pluto and Eris as the first representatives.  1

And the official planet count dropped to 8.

Enter Planet X.  Planet X has long been theorized, mythologized, and subsequently scoffed at by the scientific community.  Konstantin Batygin and Mike Brown (coincidentally the man who discovered Eris and thus “Killed Pluto“) of the California Institute of Technology (Caltech) in Pasadena say that this time, things are different, because this time, “we’re right.”

Planet X is a mathematically theorized planet outside of the orbit of Neptune with an extremely long orbit, about 15,000 years (or if you’re a resident of Planet X, one year).    As we’ve seen in our discussion of gravitational waves, much in science that deals with extreme size or distance, large or small, is only mathematically theorized.  Black holes, the Higgs particle for a long time, much of quantum mechanics, are either currently or were until recently only mathematically theorized.  Planet X is similar in that no one has actually observed it directly.  Like observing black holes by observing the consumptive destruction, Brown and Katygin theorized planet X by observing the strange orbits of 6 objects that orbit past Neptune.  Essentially, their orbits bring them close to the same area of space with quite a bit of regularity, too regularly for coincidence to be a likely explanation.  There’s probably something large and gravitationally dominant out there, tugging on their orbits.  Something like a Planet.

Showing the Orbit of Planet X versus the known planets. Source: Caltech/R. Hurt (IPAC), by way of NPR
Showing the Orbit of Planet X versus the known planets. Source: Caltech/R. Hurt (IPAC), by way of NPR

So the fate of the Terran Family of Planets hangs in the balance once more.  If Planet X does exist, then we’re back up to 9, with the little Kuiper belt dwarf planets separating the old stalwarts from the new, lonely inclusion. Statistical certainty aside, changing the number of acknowledges planets in the solar system is a process, a process of definitions, as we saw already with Pluto.  Planet X will almost certainly not be added to school-curriculum until somebody actually gets a picture of it, which could take a very long time, given the amount of area needed to scan to find it.  But someday, we may jump back up to nine planets, just a different nine planets.

This saga of definitions intrigues me for what it says about science, and in particular how science changes.  It’s important to remember that Pluto wasn’t downgraded from planethood because science was wrong about Pluto, or because Pluto somehow ceased to exist, or became something other than what we thought it was.  Pluto is still the same size, Pluto is still in the same place, Pluto is still everything that we thought it was.  The fundamental data about Pluto never changed 2.  What changed was the context in which we understood the data about Pluto. What had to change was our definitions, the language that we use to describe it, based on new information as our knowledge of far-off things grew.  Likewise, Planet X does not change what we already know about the solar system – it just adds something new.  Obviously, there are scientific discoveries that fundamentally change everything – the structure of the atom, DNA, electricity, etc.  But there are other discoveries that simply require a reevaluation of our language, a shifting of definitions.  Tradition and nostalgia might cause us to instinctively rebel against these sorts of changes, because they seem on the surface to be unimportant.  Why couldn’t we call Pluto a planet, and Eris not a planet, especially in light of the fact that we’re well on our way to discovering a whole new planet!?  And I think the answer is that tradition and nostalgia can get in the way of accuracy and specificity.  Words have meanings because we agree that they have meanings, and sometimes it is important to change our agreed-upon meanings so that we can accurately represent the world as we understand it.  Even if that means saying goodbye to an old friend in the registry of planets.

Or saying hello to a new one.

  1. It is worth noting that many astronomers had stopped calling Pluto a planet years before the discovery of Eris.
  2. Of course I’m referring only to this latest downgrade to dwarf planet.  Over the years, much of our understanding of Pluto has changed, and with the New Horizons mission to study Pluto, our understanding of Pluto will undoubtedly change a great deal in the future.

3 thoughts on “Definitions or traditions? The Pluto Paradox”

  1. Jean, I laughed at your first line– many times, I have marveled at the “truth” that was then, and what we’re learning now thanks in large part to the Hubble and the Kepler space telescope. And thanks to you and you’re blog, we’re all aboard for the ride! I look forward to your upcoming features.

  2. Wow, Jean. Thanks so much for this. The extent of my knowledge about this planet discussion is simply: Pluto is no longer a planet. Needless to say, I learned a tremendous amount about this discussion from reading this post.

  3. Cool post – clearly very well researched. I appreciate how you move from the debate over Pluto’s planethood (which, evidently, is moot) to a larger point about the nature of our reactions to scientific progress. Funny how bent up people can get over something that will (most likely) never affect their day-to-day lives.

    Still, did the have to call the yet undiscovered celestial body Planet [Roman Numeral Ten]? That just muddies the waters…or spacewaves…or something.

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