The Last Planet?

Credit: Eliot Young (SwRI) et al., NASA

The last few years has seen an intense, sometimes emotional, and occasionally bizarre debate over what should and should not be considered a "planet".  It has been one of those debates that pits the seemingly obvious against the arcane.  But any way you take this, Pluto is (or was) the last planet.

What I believe we have here is a small scale version of the kind of knowledge shift that James Burke described in The Day the Universe Changed.  And while this is a smaller, it's also more significant than just the Day Our Solar System Changed.  Burke's commentary is insightful and points out how the state of our knowledge prejudices our thinking. As an example he cites a story told about the philosopher Ludwig Wittgenstein (found here):

Somebody apparently went up to [Wittgenstein] and remarked what a bunch of morons we in Europe must have been (800 years ago before Copernicus told us how the solar system works) to have looked up there and thought that what we were seeing was the Sun going around the Earth, when as any idiot knows the Earth goes round the Sun, and you don’t have to be Einstein to understand that.

To which Wittgenstein is said to have replied… “But I wonder what it would have looked like if the sun had been going around the earth.” The point being of course that it would have looked exactly the same.

So how does this make Pluto the last planet?

Unless you are a space or astronomy buff you can be forgiven for not knowing about the controversy over the International Astronomical Union (IAU)'s decision that Pluto isn't a planet.  Pluto is far from the first object to be kicked out of the planet club.  In fact, you may be surprised at some of the others that were kicked out before it.

It turns out that the term planet had no formal scientific definition before 2006.  Essentially, everyone was operating on a defacto understanding that had evolved and changed over thousands of years.  Planet meant wanderer and included the naked eye planets Mercury, Venus, Mars, Jupiter and Saturn, as well as the Moon and the Sun!  The Earth wasn't a planet because people beleived that everything revolved around it.

Telescopes made it possible to find more things in the sky that we couldn't see.  It also helped change our view of the Earth-centric system.  Once we realised that the Earth and everything else in the Solar system orbited the Sun then the planet club changed dramatically.  Both the Sun and Moon had to go and the Earth was a new member.  And if our moon wasn't a planet, neither could the moons of Jupiter discovered by Galileo.

As we began to see more wandering objects out there, the ranks of planets swelled.  Uranus was discovered in 1781.  The dwarf planet Ceres and large asteroids Pallas, Juno, and Vesta were discovered from 1801-1807 and joined the club until more asteroids were discovered begining with Astraea in 1845.  A year later Neptune was discovered after a hunt based on prediction and calculation.  Yet more calculations  suggested another planet (that became known as Planet X) existed beyond Neptune. 

The hunt for the mysterious Planet X began in 1900.  Pluto was photographed in 1915 but it was missed.  Finally, in 1930 Pluto became the last planet.  Almost half a century passed until scientists realized that the calculations that predicted Pluto were based on inaccurate data. The discovery of Pluto had been an accident.  Better observations also confirmed that that Pluto was both smaller than originaly thought and not alone.

The problem was that the accelerating pace of discoveries in astronomy threatened to challenge the understood definition of planet and the potential for future discoveries created further uncertainty.

So the IAU, the organization responsible for coordinating all things astronomical, decided they needed some consistency and thought a definition would be helpful.  After having upheld  Pluto's status as a planet in 1999, they investigated and then failed to come to a consensus on a definition of planet.  At the 2006 meeting the IAU was faced with confirming a tenth planet if things remained unchanged.  A last minute vote on a hasty resolution of an arguably flawed definition set the seeds for this unusual debate. Pluto was out.

The new definition removed Pluto from the list of planets and effectively barred entry for a number of recently discovered planet-like objects, specifically Eris (larger than Pluto), Sedna, Makemake, Orcus, Quaoar, Varuna, and Haumea.  While the new definition has some merit, it is unsatisfying, poorly explained, and biased to some degree to objects located closer to the Sun.  It also leaves out about 335 (at time of this writing) exoplanets that orbit other stars.  Worse Pluto is now formally defined as a "dwarf planet" (but not a planet) which is linguistically confusing to say the least.

The reaction to the new definition was anything but sedate.  Blogs abounded with opinion, facts, emotion, and misinformation.  There were even web sites set up to promote both sides of the debate.  And in bizarre reaction, the sates of New Mexico and Illinois jumped in with legislation to formally recognize Pluto as a planet.

I for one don't believe having planet preceded by an adjective is going to confuse the general public or force people to memorize larger lists of planet names.  To me a dwarf planet is still a planet.  No confusion.  But a dwarf planet not also being a planet is just odd.  The general public would be right to ask what planet the academics that thought that one up came from.

The specialists already have a veritable zoo of names for the things they are studying.  And these are not names that will resonate with the general public - nor should they.  While most people can probably figure out what a Trans Neptunian Object; is their eyes will propably glaze over with terms like Plutinos, Twotinos and Cubewanos.

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 desirable 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 precision.  In short, it allows different groups and audiences to use a degree of precision appropriate to their needs.

Frankly, I believe that the IAU overreached and erred in trying to define something that they never had control of in the first place! Indeed, the IAU should generalize or formally undefine “planet”, demote the lot, and instead provide more precise terms for scientific use. The term "planet" WILL continue to be used by the general public and educators as an introduction to the subject.

Far from being demoted, Pluto will be remembered for forcing this much needed rethink. Of course, Pluto would not be in this position without the (relatively) recent advances in the study of things orbiting stars including Eris, etc. and exoplanets.

Finally and regardless of the names there a lot of interesting and newly discovered things orbiting our Sun.  A sample of which a sample includes:

• Orcas a dwarf planet that has been described as the anti-Pluto with an almost identical orbit but on the opposite side of the solar system like some giant cosmic balance.  The yang to Pluto's ying.  It's newly discovered moon S/1 90482 (2005) is looking for a name.  (Personally I like Vanth as a kind of anti-Charon but there have been a lot of other good suggestions).  Update: Vanth it is! See Orcus Porcus.
• Haumea a football shaped dwarf planet.
• Drac with its "climbing the walls" orbit around our Sun that is not only nearly vertical but backwards.
  
• Charon and Pluto the first known binary (dwarf) planet.

Update: If you want to see where some of these fit in check out  Orbits: Intro to NASA's Orbit Simulator - solar system small body tour
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Over at AstroProf there is a great series of (longish) articles exploring the history and reasoning behind the current debate over what is and what is not a planet. 

• Defining Planets (Part I) - Introduction and Background to the Debate
  
• Defining Planets (Part II) - Our changing understanding of Pluto the outsider
  
• Defining Planets (Part III) - More Plutos, more criteria, and the search for a definition
  
• Defining Planets (Part IV) - Size matters, or does it?
  
• Defining Planets (Part V) - It's what is on the inside that counts?
  
• Defining Planets (Part VI) - Structure, Differentiation, and Hydrostatic Equilibrium?
• Defining Planets (Part VII) - How they're made?
  
• Defining Planets (Part VIII) - Wanderers, Visibility, and order of discovery:  Back to the basics?
• Defining Planets (Part IX) - Clearing their Orbit and ideas based on some heavy math?
  
• Defining Planets (Part X) - Conclusion

Additionally, I recommend the following articles:

• Ten Things You Don’t Know About Pluto | Bad Astronomy
• Episode 64: Pluto and the Icy Outer Solar System | Astronomy Cast
  
• Episode 1: Pluto's Planetary Identity Crisis | Astronomy Cast   
• 2012: No Planet X | Astroengine (and the rest of the No 2012 doomsday series)
• Update: Pluto's Big Hill to Climb a discussion about why characteristics of orbital dynamics (like clearing the neighbourhood) may not make for good classifications
  

Here's a recent post at Astroengine about a short film (and trailer) called Naming Pluto.

Some other articles and web sites that may be of interest covering this topic include:

• Update:  Pluto Could Still Be A Planet! (Who Cares?) | astroengine
• Update:  Is Pluto a planet after all? | New Scientist
  
• Ground Rules for Debating the Definition of Planet | Mike Brown's Planets
• Is Pluto a Planet? | The Planetary Society - Bill Nye 
• Pluto officially declared not a planet | The Planetary Society
• Why is Pluto Not a Planet | Universe Today
• Pluto's Out of the Planet Club | Universe Today
• Planet or Not, Pluto Beckons | The Planetary Society 
• Giving Pluto Another Chance | Music of the Spheres
• Why Pluto had to go | COSMOS magazine
• Sorry Pluto, Eris is Bigger | Universe Today
• New Mexico Declares Pluto a Planet | About.com
• Illinois restores Pluto's planetary status | The Register
  
• The Status of Pluto: a Clarification | IAU Press Release 1999
• The Great Planet Debate | Sky and Telescope
• Spirited Pluto Battle, But a Great Debate? | Discovery - Cosmic Ray
• The Pluto Revolt: Leading Astronomers Want the Plutoid to be Reinstated as a Planet | Universe Today
• My thoughts on the Great Planet Debate | The Planetary Society
• The IAU draft definition of "planet" and "plutons" | IAU News Release 2006"
• Result of the IAU Resolution votes | IAU News Release 2006
• FAQ on the new definition of planet | IAU
• Pluto | Nine Eight Planets
• Naming Names Around Pluto

 Some sites both for and against Pluto being a planet or not:

• The Demote Pluto site
• The Save Pluto site
  
• The Pluto is a Planet site 
• The Pluto Petition site 
• Laurel's Pluto Blog
  

Thanks to little David, one of our Beavers, who was a bit upset about Pluto's being demoted.