Astronomy textbooks all relate the story of the historic Great Debate in 1920. Back then astronomers Harlow Shapley and Herbert Curtis argued over the true scale of the universe at a public debate at the Smithsonian Museum of Natural History in Washington D.C.
Curtis said that the universe is composed of many galaxies like our own. Shapley argued that these so-called "spiral nebulae" were just nearby gas clouds, and that the universe was composed of only one big galaxy, the Milky Way.
Both astronomers were partly right and partly wrong in that each underestimated or overestimated the size of our galaxy. Deep space observations by Edwin Hubble a few years later decisively settled the question.
Fast-forward to 2008 and another Great Debate highly publicized in the media and on Internet blogs. Yesterday the fireworks flew in front of a popular audience 40 miles north of the Smithsonian Institution on the campus of a the Applied Physics Laboratory -- a center that is operating two interplanetary spacecraft, one enroute to much maligned Pluto.
The debate between Pluto Hugger Mark Sykes and Pluto Detractor Neil deGrasse Tyson was not of cosmological significant – like the size of the universe – but was simply over the definition of the word “planet.”
Unlike the scholarly 1920 discourse, this 90-minute long dialog was a spirited, energetic, rambunctious free for all. It was entertaining and even had a little playful shoving. All it needed was Jerry Springer walking onstage.
But with all due respect to the highly intelligent participants, this is not a Great Debate for posterity as the Cutis-Shapley faceoff was.
The irony is that unlike the 1920 debate, when the cosmic evidence on both sides of the discussion was fragmentary and partly faulty, we have a fire hose gusher of data on the physical nature of planets across the solar system. On top of that we’re discovering extrasolar planets at such a frantic rate – 307 at latest count – one has to check the latest tally on the Internet weekly.
Another irony is that at a time we’re estimating that the galaxy is filled with billions of certain classes of planets, we’re downsizing the solar system by demoting Pluto.
This makes the debate over “what is planet” seem frivolous, distracting and irrelevant to some planetary astronomers. All it has succeeded in doing is to cause consternation in our science education system that obsesses on getting factoids right and memorized by students.
On the plus side, at least Sykes and Tyson agreed that the International Astronomical Union made a terribly mistake in voting on a contemporary definition for “planet” in 2006. The IAU's historic role has been to refine technical terms in sub-categories, but never to revisit a broad definition in the public lexicon.
They also agree that the public and educators should not be preoccupied with the number count of planets or rote memorization of their names with silly mnemonics.
But Sykes and Tyson want to settle the “Pluto problem” by zooming off in opposite directions.
Sykes wants to keep things simple. He’s happy with saying that a planet is a star-orbiting object that is big enough to be round. At first pass this ups the solar system count to an unlucky 13 planets when we include Pluto and some of its icy cousins over 4 billion miles from the sun. And, it leaves the back door open to adding potentially dozens and dozens of other large Kuiper Belt Objects than have been or are yet to be discovered.
Tyson, who is backed into a corner by his unilateral demotion of Pluto in an American Museum of Natural History exhibit in 2000, turned Clintonesque by wanting to throw out the word “planet.” (Remember President Bill Clinton’s "It depends on what the meaning of the words 'is' is" he said during his 1998 grand jury testimony on the Monica Lewinsky affair.)
Tyson repeatedly emphasized, “we’re desperately in need of a new lexicon” for describing the varied worlds found whirling around our sun and stars across the galaxy. Tyson didn’t say what this new lexicon would be. He left that to the astronomy community who he scolded for not keeping up with current affairs and instead hanging onto the quaint ancient term “planet” (which originally meant “wanderer”).
Sykes tried to preserve the word planet by telling Tyson: “That’s why God made sub-categories,” i.e. Jovian, terrestrial and dwarf planets, among many more types.
Tyson insisted that the word planet has no scientific or educational value. The word alone “raises 20 questions” Tyson emphasized: How big is the world? What is its mass? What is its composition? What are its orbital parameters? What is its geology? etc. “How can you put Pluto in the same class as Jupiter?” he asked in astonishment. (Actually Earth and Pluto have and interesting kinship in that they both have major satellites born from a titanic collision.)
Tyson also warned that the term planet could give scientists and students tunnel vision in interpreting new space phenomena and missing the broader perspective. He favors categorizing solar system bodies as “families of objects.”
Neil breathlessly described the need for a big “matrix” where all planet properties could be presented to students. And, they could compare and collate planet characteristics to their heart’s content. They could do “data cuts,” and exoplanets could be added as “another line in the grid” Tyson said.
This began to sound to me suspiciously like some incarnation of the Periodic Table of the Elements, or worse, an Amtrak train schedule. My brain began to ache at the though of such a chart and how my students would grapple with it.
Now, just imagine if paleontologists had such a catfight over the word “dinosaur” every time a weird new fossil was unearthed. Imagine this argument: “Just the word ‘dinosaur’ raises 20 questions: What was its weight? Was it a predator or herbivore? Where did it live? In what geologic epoch did it live? Blah, blah, blah.”
Using the same mindset as Tyson’s one could argue that the word "dinosaur" is horribly simplistic. It collectively describes the dominant vertebrate animals that dominated the land for a huge stretch of time on Earth, 160 million years. Creatures tens of million of years apart in evolution and extending over a vast range of sizes all fit under the definition of a dinosaur.
Both scientists acknowledged that the whole planet debate has been politicized. The New Horizons Mission was publicized and funded as visiting the “last planet in the solar system.” The mission would not have gotten congressmen to open our wallets if it were sold as the “mission to the first Kuiper Belt Object.”
"There simply has never been a debate like this over a historical term," says renowned Harvard University astronomy historian Owen Gingerich. One other semantics battle he remembers was between Edwin Hubble and Harlow Shapley. Hubble first discovered galaxies beyond our Milky Way but refused to call them "galaxies." Why? because that was the term Shapley insisted on using. Hubble simply called them the "external nebulae." (The irony in that in 1988 Hubble's understudy, Alan Sandage, published the "Hubble Atlas of Galaxies." "Edwin Hubble must be turing in his grave," says Gingerich.)
For all its spirited entertainment and cathartic value for Pluto Huggers who have been sulking since the disastrous IAU vote, the debate was anemic in terms of substance. It didn’t nudge the astronomy world an inch closer to resolving the question what is and what is not a planet. Getting astronomers to converge on this dilemma is like herding cats, where the cats are trying to herd themselves, said one attending astronomer.
But the bottom line is that the term planet is a cultural concept. It will only change if the broad public changes the way they use it, Gingerich emphasizes. It cannot be changed by fiat and for all practical purposes is beyond the control of the IAU.
Photo Credit, top to bottom: American Physical Society; Ray Villard; Chesley Bonestell; Adler Planetarium; Rich Penney/Sandia
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