How I Killed Pluto and Why It Had It Coming - Mike Brown (2010)

Chapter 11. PLANET OR NOT

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 said 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.

The IAU was quick to act after the discovery of the first objects out beyond Neptune in the Kuiper belt. Things way out are named after creation gods in world mythology, though as the number of objects in the Kuiper belt grew faster than new creation gods, the rule began to be applied more and more loosely. Recently someone even got away with calling something in the Kuiper belt Borasisi, which is a god from a fictional story by Kurt Vonnegut.

For all of its preparedness for endless contingencies and countercontingencies, the IAU had never actually contemplated a question that was suddenly on everyone’s minds: What do you call something new that is bigger than Pluto? How do you recognize a new planet when you see it?

Like any good international organization, it knew what to do when faced with such emergencies: It needed to form a committee. Better yet, it already had one. At about the same time that Xena was discovered and astronomers (and everyone else) began to try to figure out how small the smallest planet could be, astronomers were also struggling with the question of how big the biggest planet could be. We had it easy in the solar system; it was unlikely that I would ever find anything bigger than Jupiter that needed to be categorized, but discoveries of things larger than Jupiter orbiting around distant stars were beginning to become routine. Some were about as massive as Jupiter or only a little more so. They were clearly planets. Some were only somewhat less massive than the sun. Those were clearly stars. Some were in between. What to do? The IAU formed a committee to decide. This same committee was now charged with figuring out the small end of planets, too.

When the reporters called to talk about the discovery of Xena, they wanted to know where it was, how we had found it, and how big it was. We didn’t yet know the size for sure but thought it could be as much as half again the size of Pluto. And then they asked: When is the IAU going to make a decision about planets?

“I hope they’ll decide before my daughter starts crawling,” I joked when Lilah was three weeks old.

When a bevy of new reporters called after the story about the Spanish snooping broke, they asked what was going to happen to the Spaniards, how the conflict was going to be resolved, and how this might change the way astronomers interacted and protected their data. I explained that many scientists worldwide suddenly realized that they, too, could be vulnerable to unintended snooping, and many were scrambling to find solutions. And then they asked: When is the IAU going to make a decision about planets?

“I hope they’ll decide before my daughter learns to stand,” I joked a few months later, since Lilah was already crawling by that point.

When we discovered that Xena was not alone at the edge of the solar system, that it had a tiny moon going around it, reporters called again and wanted to know how the moon got there, what it looked like, and what we were planning to call it (Gabrielle, of course, after Xena’s spunky TV sidekick). And then they asked: When is the IAU going to make a decision about planets?

“I hope they’ll decide before my daughter says her first words,” I joked the next winter, since by then Lilah was already standing and taking increasingly assured strolls around the periphery of the room.

In the spring, when we finally were able to use the Hubble Space Telescope to figure out just how big Xena really was, reporters called again and wanted to know what Xena was made of, how it had gotten so big, and how much larger than Pluto it was (only 5 percent, it seemed, which was uncomfortably close to not even being bigger than Pluto at all, particularly when you include approximately 4 percent uncertainty in the measurement). And then they asked: When is the IAU going to make a decision about planets?

“I hope they’ll decide before my daughter goes to college and takes an astronomy class,” I finally reverted to joking when it became clear that no decision was going to be made anytime soon.

People kept asking me when the IAU was going to make a decision because they thought I should know. But I didn’t know anything. During this entire period, no one officially connected with the decision making—and I didn’t even know who that might be—ever once contacted me to ask a question or to tell me what was going on. I assumed that I was going to wake up one morning, open the Los Angeles Times, and see that I was suddenly the official discoverer of a planet. Or that there were only eight planets. Or that I had discovered many planets. Or that I had discovered the only thing in the solar system larger than a planet that wasn’t a planet.

In the face of this uncertainty, I figured it best to be prepared for all options. I called up the person in media relations at Caltech who had—months earlier now—pressed me to decide whether or not to call Xena a planet in that original press release. I told him that we needed to prepare another press release, this time for the IAU decision.

“Great!” he said. “What did they decide to do?”

“Well, actually, they haven’t decided anything yet.”

“But they’re deciding soon, right?”

“Well, actually, I have no way of knowing what they are doing. They might decide tomorrow, and they might decide a decade from now.”

“So …” He paused. “What are we going to say in a press release?”

I knew that I wanted to have the opportunity to tell the full scientific story to the public. I had missed that chance, I felt, back in the original hurried rush when we had to make the announcement right away. I wanted the beauty and subtlety and essential order of the solar system to be at the center of the discussion after a decision was finally made. I cared less what the IAU decided—within limits, of course—than that the science got explained correctly.

“We’re going to write four different press releases,” I explained.

Ten planets made sense if you wanted your planets to have more emotional resonance than scientific significance. We very quickly wrote that press release, hailing Xena as the tenth planet. It made me proud to think of my tenth planet, but even from early on I admit that it also made me feel a little fraudulent. The discovery of Uranus was a big deal, and that of Neptune was amazing. But Xena? Little Xena? The tenth planet? Still: I channeled my inner geologist. If it mattered emotionally, that was all that counted. I was ready.

Scientifically, I agreed much more strongly with our second press release, explaining why there were only eight planets. Eight planets made sense if you were a scientific historian and realized that 150 years ago people had already decided to divide the solar system’s objects into big planets and small asteroids and that Pluto—and now Xena, too—thoroughly fit into the category of the small objects. I liked to think of this one as what people would mean when they said the word planet if they really understood the solar system. We hailed the bravery of astronomers for taking a scientifically sound stance in the face of what would certainly be considerable opposition. Even though scientifically I agreed much more strongly with this press release, I was glad we wrote it quickly; it seemed quite clear to me that astronomers would never have the audacity to actually get rid of everyone’s favorite runt planet. Still, to be safe, I thought it best that we have a press release ready. Having the discoverer of the onetime tenth planet agree that it should not be a planet seemed like a powerful line of argument.

Our third press release was for the possibility that the IAU’s decision would be to simply keep nine planets. Keeping Pluto in the planet club but refusing to allow in bigger newcomers didn’t make sense at all. Yet it did seem like an option that might be on the table, since I had heard a few people say, “Why do we need to change anything when we have nine perfectly good planets?” Our press release said that nine planets was a pretty dumb decision.

The last press release considered a more extreme possibility: that the IAU would stretch the definition of the word planet so far that there would suddenly be two hundred planets. A small but extremely vocal group of astronomers had been pushing for a while to thoroughly transform the meaning of the word planet. Unlike the ten-planet approach, which was an attempt to understand what people meant when they said the word planet, or the eight-planet approach, which was an attempt to discern what people would mean if they had all of their facts straight, or even the nine-planet approach, which was to stick to literally what people meant when they said the word planet (the nine planets and nothing else!), the two-hundred-planet approach was an attempt to legislate an entirely new and never before anticipated meaning for the word planet. The word was to mean, essentially, “anything in orbit around the sun that is big enough to be round.”

Why round? It is not simply that astronomers are enamored of that particular shape (though, really, why wouldn’t they be?). It is that that particular shape tells us something. If you throw a boulder into space, it will retain whatever irregular shape it originally had. If you throw a hundred boulders into space together, they might stick to one another due to the tiny amount of gravitational pull that each boulder generates, but they could still have almost any shape you might imagine. But if you put enough boulders up into space together, a fantastic thing will happen: The cumulative gravitational pull of all those boulders will take over. The boulders will pull together and crush and smash one another until you can no longer discern what shapes they had to begin with; instead, they will form a beautiful, simple sphere. Finding something spherical in space indicates that you have found a place where gravity has taken over. I am pretty certain that at no time in the previous several-thousand-year history of the word planet did anybody say “planet” and really mean “things that are round due to self-gravity.” It was a new definition by simple fiat. And it would lead to something like two hundred new planets, most of them out in the Kuiper belt.

I explained all of this to the person writing the press releases.

“Why bother writing this one up? It sounds crazy. No one would decide this, would they?”

Well, yes. I actually thought this was the most likely decision that the International Astronomical Union would make—if it was ever going to make a decision at all.

“But why would astronomers do such a crazy thing?” he wanted to know.

Desperation was all I could answer. Desperation.

As radical as the new definition was, it was the only one on the table that both smelled scientific and also retained Pluto as a planet. I could imagine that it would be hard for a scientific committee to conclude that the definition didn’t need to have a strong scientific basis (the nine- or ten-planet approach). I could imagine that an astronomical committee would be loath to provoke what would certainly be a huge public outcry if it kicked out Pluto (the eight-planet approach). So although the two-hundred-planet approach was the most radical, it had the cover of appearing the most conservative. I could just see it passing.

I didn’t like the definition, but I could live with it. The good news, for me, was that if this new definition was announced, I would have discovered more planets than anyone else in human history. Not just Xena, Easterbunny, Santa, Sedna, and Quaoar, but dozens more. The bad news was that I couldn’t remember most of their names.

•   •   •

The first anniversary of the discovery of Xena came and went with no hint of what was happening at the IAU. But it was okay. I was busy. Chad and David and I, now joined by some of my students and outside colleagues, wrote scientific papers about the size of Xena, the discovery of Gabrielle (the moon of Xena), the discovery of a thoroughly unexpected second moon of Santa, and the slab of frozen methane covering the surface of Easterbunny; and we still had much, much more to do. There were press releases to develop, talks to give around the country, interviews on TV and radio. But when I think of this time period, I have a hard time remembering almost any of it. What I really remember is Lilah and the moon.

As with any overeducated first-time parents, we were fascinated with understanding Lilah and what she was thinking and doing and understanding. I began reading scientific books on early childhood development, not as a way of pushing Lilah along faster or making sure she was okay, but simply because it was, at the time, the single most fascinating thing I could imagine. I read studies of the development of facial recognition and motor skill control, but what I found the most interesting of all were studies of language development. It seemed so hard for me to imagine that this little baby, being carried around in a bundle in my arms, would someday be sitting in a chair next to me having a conversation.

Diane and I often joke about parents who think that everything their children do is exceptional. Intellectually, we always understood that Lilah would likely be good at some things, not as good at other things. Exceptional is a pretty high bar. But reading these books about early childhood and watching Lilah develop, I finally understood. She is exceptional, because early childhood development is about the most exceptional thing that takes place in the universe. Stars, planets, galaxies, quasars are all incredible and fascinating things, with behaviors and properties that we will be uncovering for years and years, but none of them is as thoroughly astounding as the development of thought, the development of language. Who would not believe that their child is exceptional? All children are, compared to the remainder of the silent universe around them.

Emily Schaller, my Ph.D. student, who was always willing to try to engage my obsessions, handed me a book one day on how to teach your baby a primitive type of sign language. The thought was that children are ready to communicate before they have the vocal motor skills for speech. But they can use their hands and arms and fingers to tell you about the world around them.

Lilah’s first sign was, not surprisingly, cat (rake two fingers across your face as if to draw whiskers). The two cats—originally Diane’s, now a joint venture—who lived with us barely tolerated the loud newcomer to the house. But they eventually got used to her and realized that she posed no harm, so they would lie close to take advantage of me or Diane being immobilized while holding a sleepy baby and having a free hand available for an ear scratch. Then Lilah learned to roll over. The cats scattered, to eventually return when they realized she was still mostly immobile. Then Lilah started to crawl, and that started the years of Lilah chasing after the cats, the cats slinking away, always out of reach. To Lilah, the cats must have been like the end of the rainbow: always in sight, always just out of reach, and gone when you get there. Her first efforts to communicate with the external world were targeted directly at them. They, sadly, never returned the favor.

After cat, Lilah next learned flower. Flowers (scrunch up nose as if sniffing) were everywhere, first only outside on plants, but soon she generalized to flowers on her clothes or her shoes, or in pictures in books and magazines. I wanted to hook up wires and do experiments and comparisons and studies to understand it all.

“You want to do what?” Diane would say.

But, really, who wouldn’t? In our own house the most extraordinary thing in the universe was taking place, and it was passing by unexamined, unstudied.

“There will be no Lilah experiments,” Diane declared.

I know, I know. I wouldn’t really do it. I didn’t really want to hook up those wires. Mostly I just wanted to hold Lilah tight as she made signs to the world around her, and I wanted to tell her: You are the most extraordinary thing in the universe.

We had recently bought a new house. For the first few years of our marriage and the first six months of Lilah’s life, we had lived in a diminutive Spanish-style bungalow in a typically densely packed part of suburban Pasadena that I had bought years earlier. I loved my little bungalow. It was the house where I first cooked dinner for Diane. When Diane had moved in, I had warned her: I love this place and never want to move.

But the house had almost no sky.

I biked home at night through lit streets, car headlights glaring everywhere. I thought back to the days of living in the cabin and walking the trail by the light of the moon or stars; I thought back even earlier to the days of living on a tiny sailboat in the San Francisco Bay and staring up at the whole sky before finally closing the hatch for the night. From my bungalow, I could sit in the hot tub in the backyard and look up and see slivers of sky. Sometimes I could see the Big Dipper, sometimes Cassiopeia. But in my tiny night-sky universe, I never once saw a planet.

When Diane suggested that we probably needed to move to a bigger house to fit our now-expanded family, I reluctantly agreed. Maybe it was time. I grudgingly went to look at a few places with her. Nothing felt as perfect as our happy little bungalow. Then one day, with no expectations, we stumbled onto a house perched on top of a massive one-hundred-thousand-year-old landslide. Almost nobody knew it was a landslide, but I had made my geology students write about it nearly a year earlier. How could I not have fallen in love with the house? We bought it three days later and moved in the following month.

Living on a landslide has its advantages. We have a steep canyon in our backyard, because canyons are easily formed in rubble. We have landscaping boulders of every conceivable size and composition, and if we ever have the need for more, we just dig a foot underground to see what else the landslide brought down. The landslide makes a minor wildlife corridor, so we have an abundance of birds, an occasional bobcat, and even once a black bear.

For me, though, the best benefit of perching on the tip of the tongue of a landslide with the mountains rising to the north of you is that you have an uninterrupted view to the south. And if you go outside at night and look to the south, you get to see the spectacular constellations. You get to see Orion and Taurus and Scorpius. You get to see the blue of Sirius, the red of Betelgeuse. And best of all, you get to see the planets.

Lilah and I have spent the years since we’ve moved to our new house tracking Jupiter and Saturn across the sky, watching Venus set into the Pacific Ocean, seeing how red Mars looks in comparison to the pale stars. But more than anything else, we’ve watched the moon.

When we first moved into the new house, Lilah was still learning new word signs. One of her favorite combinations translated something like this: There is a light; turn it on (hand held overhead, first balled momentarily, fingers suddenly flying open to show you what to do). If you complied, she would even say “thank you” (finger tapping heart).

One spring night, the nine-month-old Lilah and I sat outside wrapped in blankets staring up at the moon, which was nearing full. A rollicking storm had been through for the past few days, revealing to us all of the places where our new house flooded. But the rain had stopped, and between the thick black clouds covering about half the sky we could see the brightest stars and the bright full moon, which had found itself in a large hole in the clouds and was shining down on the still wet and now sparkling nighttime landscape. I told Lilah about night and the moon and the rain. We heard coyotes across the canyon, and I told her about them, too (and about why the kitties were now going to be solely indoor kitties).

And then the moon ducked behind one of the thick clouds, and everything got dark.

Lilah looked around, looked up to where the moon used to be, and looked at me. Then she held her fist up in the air and flung her fingers open. She looked at me expectantly.

The cloud passed. The moon came back out and once again brightened the landscape.

Lilah smiled at me and tapped her heart.

•   •   •

I have an extremely vivid memory of the day that summer, a few weeks before her first birthday, when Lilah really learned to walk. She had previously taken a few halting steps before tumbling, or she had scooted along while holding a wall, but one day she instantly went from easily manageable (she would not have gone too far if I had looked away for sixty seconds) to fast, unpredictable, and apt to disappear in seconds. A day earlier, while trying to throw a friend in the swimming pool, I had broken my ankle, and I was now in a cast and on crutches. The fact that I had to take a few steps from the kitchen counter to the refrigerator was something I now mourned. But the worst thing was that Lilah was suddenly up and running just as I was slow on my crutches. The only way I could keep up was to crawl. So, that day, Lilah and I traded. She now walked. I now crawled. I had many theories about the precise symbolism of that transition, and all of the theories were ominous.

Lilah made up a sign-language symbol for me walking on crutches (two hands held in front of her, pointer fingers moving up and down), which I count as the moment when she first learned to mock me. I was, perhaps unsurprisingly, enamored of the mocking.

Six days later, still on crutches, I headed to Italy to give a talk at an international conference on the Kuiper belt. We talked about the formation of the Kuiper belt, the surfaces and atmospheres of the objects there, and what they might be made of, but the question of what to call them never came up. But at night, when we went to little cafés (the closest of which was precisely 1,032 crutch-steps away, which felt to me like the distance from Earth to Sedna) to drink prosecco and watch the World Cup soccer games, everyone wanted to speculate about Pluto and Xena and planets. I tried out my arguments about ten planets and about continents. The scientists balked. They didn’t like the idea that the definition of planet would include no science.

“So you think everything round should be a planet? You think that there should be two hundred planets?” I asked, assuming that that was going to be the obvious response.

“Of course not!” they responded. Wasn’t it obvious that there were only eight planets?

I thought the other astronomers were being naïve. It’s easy to sit inside the scientific bubble and make pronouncements, but they were forgetting how much of an impact this decision was going to have on the outside world. No one was going to let Pluto be killed, were they? But still, it was interesting. Within the field of people who studied the Kuiper belt for a living—people who had devoted their careers to the outer solar system and its many, many denizens—it was almost not worth having the conversation. Of course Xena was not a planet. And Pluto likewise. Hadn’t we settled that question 150 years ago when the asteroids became asteroids?

Naïve, I thought. I remembered back to the days when I used to think the exact same thing. Wouldn’t it be nice to just think about science and not worry about its impact on culture? Wouldn’t it be nice to be able to just say the thing that makes the most sense?

A week after I got home, my phone rang, and out of the blue I was told by a member of a previously unknown IAU committee (the third planet committee? The fifth committee? I couldn’t keep track) that Xena was to be a planet.

He couldn’t reveal the details of the decision on the definition of the word planet, but he wanted to prepare me for the onslaught of publicity that would surely follow. As I was the only living discoverer of a planet, he thought it best that I stay humble.

Humble? I thought, and chuckled to myself. My one-year-old daughter had recently learned to mock me in a sign language she had made up herself.

While he had not meant to reveal details to me, he already had. The “only living discoverer” could mean only one thing. If the IAU was going to pick the two-hundred-planet definition, there would have been perhaps a dozen living planet discoverers. If there was only one, it was clear that the IAU had decided on the ten-planet definition that I had come to terms with myself. Xena was to top off an elite list.

“Do you think the rest of the astronomers will go along with this?” I asked.

I was quickly assured that they would. “I’ve had a lot of conversations in the past few days. This is going to sail through the vote process.”

•   •   •

I went home that night and told Diane. We opened a bottle of champagne and drank to the amazing fact that I had discovered a planet. A planet. I had discovered a planet! After all of this time, Xena was officially going to be a planet, and I was officially going to be the only person alive who had found a planet.

Just then, Lilah walked around the corner from where she had been playing, saw the crutches under my arms, and immediately stuck out her hands and waggled her pointer fingers.

Okay, so I was slow, and I still had to crawl to be as fast as my one-year-old daughter. But I had found a planet. No one could take that away from me.