How I Killed Pluto and Why It Had It Coming (23 page)

But but but. But if they were on the up and up, why would they hide the fact that they had accessed the data? Why wouldn’t they mention it in the early days, when Ortiz and I were exchanging cordial e-mails? Sure, there was no official channel for mentioning that they had known about our database, but given that I opened a friendly back channel the day after Ortiz’s announcement, might he not have mentioned it to me then?

I went back and checked the e-mails recently. It is true that Ortiz never publicly denied having used the data, even in the early days. He just never answered the question. All of the denial came from his German friend, who, I still believe, had been equally duped. I wonder if he secretly suspected that something was amiss or if he was simply as trusting and naïve as I used to be.

On that Friday morning in late July, I made the instant decision to announce to the media that Xena was the tenth planet. I had been swayed in part by Diane’s arguments and in part by the urgings of the media relations person to whom I spoke that morning. But even though I was a bit blindsided that morning, all spring I had been trying hard to understand how we should define the word
planet
.

I asked an old college friend with a Ph.D. in philosophy: What does a word mean when you say it?

“Words mean what people think they mean” was his smoothly philosophical reply. “So when you say ‘planet’ it means what you are thinking when you say it.”

I probably should have known better than to ask him. I remembered that in college he had told me that he woke up every morning surprised that reality was still reality.

Still, maybe there was something to it. Maybe words do just mean what we think they mean.

Perhaps it is wrong for astronomers to attempt to redefine a word when people already know what it means when they say it. Perhaps the job of astronomers is, instead, to
discover
the definition of the word
planet
as people use it. After all, the word
planet
has been around much longer than, well, our understanding of planets.

So what do people mean when they say the word
planet
? That spring, well before anyone knew that the world was about to be handed a tenth planet, I started asking everyone I saw. The answers were diverse and, more often than not, scientifically misguided: large rocky bodies in the solar system (well, no, there are gas giants), things with moons (not Mercury or Venus!), things that are big enough to see with your eye (Uranus, Neptune, and Pluto are out), things that pull the earth around in its orbit (that’s just the sun). But when I then asked people to name the planets, everyone had exactly the same answer, starting with Mercury and ending with Pluto. People who felt themselves quite up to date and informed would then explain that maybe Pluto shouldn’t be called a planet, but they certainly knew that it currently was one.

So, again, I ask: What do people mean when they say the word
planet
? They mean a slew of unscientific clutter. And then they mean nine specific objects in the solar system.

I always pressed people further: How would you know if something new was a planet? The answer was always the same: If it was as big as the other planets. Or, as I interpreted it, according to my unscientific springtime poll, everything the size of Pluto and larger that orbits around the sun is a planet.

Isn’t that the real definition, then? Shouldn’t astronomers leave the word alone if it already has a meaning?

I remained torn. If Pluto was a planet, why were the many things just a little smaller than Pluto not considered planets? It
made no scientific sense at all. Why draw such an arbitrary line right around the size of Pluto? Isn’t the job of scientists to guide the public’s understanding of nature rather than acquiesce to unscientific views?

In addition to everything else happening that spring, while Xena and Santa and Easterbunny were just being found and studied, and Lilah—still known as Petunia—was growing and beginning to kick inside Diane’s stomach, I was teaching introductory geology at Caltech for the first time. I’m not a geologist. I’ve never taken a single class in geology. If you gave me a handful of different types of rocks, chances are I could identify only a small number of them. I still get confused by the meanings of
strike
and
dip
.

Luckily, most of my students didn’t realize this.

I was pretty good at teaching that class, actually. The class was the equivalent of what is called a “rocks for jocks” class at many other universities, meaning that it is intended for people who won’t end up majoring in geology. Caltech, though, is not known for its jocks. All of the kids in the class who don’t major in geology are majoring instead in physics or biology or mathematics or engineering. I affectionately referred to the class as “earth science for eggheads.”

But why was I teaching a class about which I knew nothing? One reason only: I had begged. As an astronomer who studies planets, I have ended up at Caltech not in an astronomy department but in a planetary science department. And the planetary science department is tacked on to the side of the geology department. The people I see walking around the halls and coming to my classes tend to be geologists. After having been at Caltech for almost a decade, I thought it might be time to actually learn some geology. And what better way to learn than by teaching it myself?

I had intended to spend most of the winter preparing for the class; instead, I spent it working on the newly discovered Santa and Xena. As the first class came around in April, I was barely on track with the teaching. And then we discovered Easterbunny that week.

Still, I stayed about two weeks ahead of the class, learning the material as I went along. Over the course of the term, I said only one thing that I now know to be blatantly wrong. (To anyone who took my Ge 1 class in 2005, I apologize. The mineral peridotite does
not
change into spinel as it is compressed by high pressure; its
crystal structure
collapses to one that is identical to that of spinel, but the chemical compositions of the two minerals are totally different.)

Teaching earth science to eggheads has so far been the highlight of my teaching life. The earth is a spectacular laboratory that you can get to by simply walking out the back door. The eggheads and I took trips to the local arroyo to understand debris flows in the Los Angeles mountains; we walked one mile south of Caltech to our local thrust fault; we took a bus up the east side of the Sierra Nevada, stopping to see ancient volcanic flows, now-dry Ice Age lakes, and a 50-million-year-old mountain range now buried almost to its top in debris. All the while, I tried to pull the students out of the mind-set that is all too easy to get into in the middle of a hard first year at college: Give me the information; tell me what I need to know, what’s on the exam. In earth science for eggheads the message was instead: Look around you! What is happening here? Why?

Because my head was so immersed in the geological world that spring, it is perhaps not surprising that I started looking to the earth sciences for examples of the ways in which scientists were confronted with words that had previous meanings. Geologists, in fact, have had a more difficult time than astronomers
on this issue. While planets are up in the sky and don’t form part of most people’s everyday experiences, daily life is filled with geology. People see mountains, rivers, lakes, oceans. Or should they really be called hills, streams, ponds, and seas? When is something a mountain instead of a hill? A river instead of a stream? A lake or a pond? An ocean or a sea?

Geologists have never attempted to define these things. The words simply mean what people think they mean when they say them.

I grew up on a little rise in northern Alabama called Weatherly Mountain. As a child I assumed that the word
mountain
had some sort of meaning. When we took our first family trip west and encountered the Rocky Mountains rising six thousand feet from their base, I was stunned. Our three-hundred-foot-high mountain looked to be a molehill in comparison. But still, Weatherly Mountain will always be Weatherly Mountain.

The best geological equivalent to the word
planet
is the word
continent
. What does the word
continent
mean? As far as I can tell, the definition is something like: big coherent chunk of land. How big? The only answer I could ever find was “big enough.” Australia is big enough. Greenland is not.

I began to quiz people about continents as much as I quizzed them about planets. I heard all sorts of interesting theories about how the word
continent
was defined, including a few from people who knew a little geology. I was told, emphatically: A continent is any island on its own continental plate. Greenland doesn’t qualify because it is on the same plate as the rest of North America and thus is not separate. I pointed out that continents have been around much longer than the plate tectonic theories of the 1970s. And I pointed out that by the “scientific” definition, we should really count the south island of New Zealand as a separate continent.

So how do we really define continents? Simply by tradition. The seven continents are the seven continents because that’s what people mean when they say the word
continent
.

But even that is not entirely true. Apparently some people mean different things. As I quizzed more and more people, I learned that, for example, many Europeans do not consider Australia to be a continent. Argentinians consider North and South America a single continent (the Panama Canal is not enough of a break for them, I guess). And rational people in many places believe that Europe is considered a separate continent only because, well, that’s where the people who defined the continents in the first place all came from.

Can it really be that the most important classification scheme for our understanding of landforms has no scientific basis whatsoever? Shouldn’t geologists get to work defining their terms more carefully?

And yet, when geologists talk about continental crust or continental shelves they know exactly what they are referring to. They never use the word
continent
by itself unless it is just to refer to one of those landmasses that we agree are called continents.

For the public, having a handful of continents whose names everyone can remember (even when everyone doesn’t always agree) is an important way to organize our understanding of the world around us. It is too difficult to make sense of the hundreds of countries on the earth without an organizing principle. The continents are a way to bring the vastness of the earth down to a human scale.

And so with planets. The planets are our way of organizing the universe beyond the earth. In fact, they are the grandest organizational scheme that most people know. Ask someone to describe what is around them and they’ll describe their neighborhood.
Press further and they might talk about their town and region. If you keep pressing, perhaps they will mention their country, next their continent (
that word again!
), and finally the world. But if you don’t give up there and you ask for more, you will ultimately be led down the descriptive path of the solar system. You’ll be told about the planets. And after the planets? What next? More often than not, you will be left with blank stares.

When people describe their neighborhoods, they don’t care about the scientific meaning of the words they’re using; they care about recognizable landmarks to specify the points and the boundaries of their lives. The planets are these landmarks. That is what people mean when they say the word
planet
.

Is the word
planet
, then, specific or descriptive? When people say the word
planet
, do they mean precise places—Mercury and Venus and Earth and the others—or do they mean those places and anywhere else like them?

I found history to be a useful guide. When Uranus was accidentally discovered, it was quickly accepted as a planet; Neptune, likewise. Even Pluto, whose status was at stake in all of this, was accepted into the club with only a little grumbling. Sure, it was thought to be much bigger when it was originally accepted, much more like the other planets, but with its acceptance the bar was accidentally lowered, and most people—except for me and a few other nitpicky astronomers—meant Pluto, too, when they
s
aid
planet
.

All of this planet-or-not-a-planet business would eventually be decided by the International Astronomical Union, which, by international agreement since 1919, has the right and the responsibility to make sure that everything in the sky is categorized, named, and filed in its right place. Before the IAU came along, the skies were filled with objects named by whatever system
the astronomers categorizing them chose. The reddish star in the upper right of the spectacular constellation Orion is known not just by its common name Betelgeuse, which in Arabic means “armpit of the giant,” but also by HD 39801, for its place in Henry Draper’s catalog from the 1920s, and by many more names, including PLX 1362, PPM 148643, and my favorite, 2MASS J05551028+0724255, in other catalogs. The IAU now has procedures and policies for what to do about almost every type of discovery in the sky. A new supernova explodes? It gets a year and a letter. Supernova 1987A was the closest and brightest in living memory, and those five characters can still provoke a glowy sigh in an astronomer of a certain age. The procedure is much more systematic, though the names are not quite as evocative as giants and armpits.

Only with the solar system does the IAU require history and poetry when it comes to naming things in the sky. By IAU decree, the moons of Jupiter are to be named after the consorts (voluntary or otherwise) of Zeus, craters on Mercury are to be named after poets and artists, and features on Saturn’s gigantic moon Titan (and I can only call them “features” because we have no idea what they really are) are named after mythological places in literature.