by Shane L. Larson
NB: I originally wrote this post to outline my TEDx NorthwesternU talk in 2014. Watch the video here. Please enjoy both!
As an astronomer, I get to talk to people about all kinds of things. Some people want to talk about black holes, or about asteroids killing the dinosaurs, or about life on other worlds. But the one subject everyone always wants to talk about is Pluto!That’s because people have this uneasy feeling that Pluto has been treated unfairly by scientists. We have this queasy feeling down in the pit of our stomachs because as children we were told Pluto was a planet, and now scientists have flip-flopped and are telling us it isn’t! And we feel bad for Pluto!
The truth is astronomers are engaged in a very serious (and good-natured) debate about what it means to be a planet. It’s the kind of debate scientists have every day about all kinds of things that don’t catch the attention of the public or garner headlines. The difference is people seem to care about Pluto!
The reason you and I are even talking about this is because in 2006 the International Astronomical Union reclassified Pluto as a “dwarf planet.” To use more common vernacular, Pluto was DEMOTED. Now that’s a word that carries a certain amount of emotional baggage with it! That visceral response we have to describing Pluto’s demise was captured by the American Dialect Society who made their 2006 word of the year,
plutoed (PLUE-toed): to demote or devalue something, as happened to the former planet Pluto...
The changing of Pluto’s status clearly struck a chord with people. But why?
Suppose I show you a picture of a “Pluto protest.” This image doesn’t phase any of you! That’s because in the back of everyone’s mind, there is a little voice telling you which side of the street you’d be standing on and what your sign would say! I’m right there with you. I’ve picked my side of the street, and I know exactly what my sign would say!So the world has divided itself into two camps — the Pluto Apologists, and the Demoters (they sound like rock-n-roll band names, don’t they?). Pluto inspires deep emotions in everyone, whether they are scientists or members of the public. Even today, nearly 8 years after the reclassification, discussion of Pluto sparks vehement debate, but the debate is nuanced and subtle, even among the cognoscenti. In one corner, there are people who are avid Pluto-philes and just would like Pluto to be back in the club. In another corner, there are people who think Pluto is clearly just a small rock, like many other small rocks, and not classifiable by the word “planet.” And in another corner there is a third group of people who think we really don’t know what the hell we mean when we say “planet,” and that our understanding of the Cosmos and what it means to be a “planet” must constantly evolve. This is the group I stand with.
The story of Pluto’s demotion and the ensuing arguments and exasperations about its status are an excellent backdrop to understand how science works and responds to ever changing knowledge about the Cosmos. The fact that people exhibit deep emotions about the entire affair is simply a very human dimension to the story, a manifestation of the fact that we long to be deeply connected to the Cosmos, that we want our perceptions and thoughts about the Universe to matter. That emotional connection is a foothold for us to explore this tale.
For kids, and short people like me, Pluto always had a certain allure because it was the smallest planet — the runt of the Solar system. I’ll freely admit that my love affair with Pluto started at a very young age, when I was even shorter than I am now! I got started looking at the sky because my Mom was a birder. She had an old beat up spotting scope that she would use to watch birds at our house, but at night I would grab it and look at the sky.
I saw the Moon and all the big planets — Mars and Jupiter and Saturn — but I always wanted to see Pluto. But it was impossible; that little spotting scope, for all the wonders it showed me, was just too small. It took many years, but eventually a 70-year old friend of mine named Hazel showed me Pluto through her 20” telescope. I got to see Pluto with my own eyes, and it only validated all that long held wonder and ardor I held for this small world. One thing led to another and eventually I built my own big telescope and now I show other people Pluto! My wife figures there are worse things I could be doing with my life.
But not all of us have had the chance to see Pluto with our own eyes. Even so, people all over the world love Pluto just the same. Part of the reason is that people recognize the story of Pluto’s discovery as a kind of modern fairy tale that could have happened to any of us.Pluto was discovered by a farmboy from Kansas named Clyde Tombaugh, who couldn’t go to college because hail had destroyed his family’s crops. But he loved astronomy, and in his early 20’s he built a telescope, and started sketching Mars and Jupiter from his family’s farm in Kansas. In 1929 he sent some of those sketches to Lowell Observatory in Flagstaff, Arizona, and they were so impressed they hired him to come run one of their new photographic telescopes. The job was only supposed to only last for three months, but he ended up working at Lowell for more than 14 years.
During his first year he was taking pictures of the night sky looking for Planet X, a proposed new world somewhere out in the dark beyond Neptune. Very soon after he arrived, on the nights of January 23 and January 29, 1930, he captured two images that would change the world, though he didn’t know it at the time. Astronomers lead rugged lives — we’re in the observatory all the time, we stay up late, we sleep very little, and sometimes we don’t get to our data right away. It took Clyde almost a month to go back to those images, but on February 18, he was looking at the images on a blink comparator, a machine that rapidly flashes back and forth between two astrophotos while you are looking at them. Stars stay put because they appear in both images, but things that move become very obvious.That night Clyde saw the tell-tale dot jumping back and forth across the center of these images and knew he had found a new world. The discovery was announced on March 13, 1930 and made headlines around the world. At the time, the Lowell Observatory had the right to name the new planet and they were flooded by suggestions. I’m sure if Stephen Colbert had been alive then, the Colbert Nation would have petitioned to name it after him. But in the end, the name Pluto was suggested by an 11-year old English girl named Venetia Burney. She was very interested in classical mythology, and suggested the name Pluto to her grandfather (Falconer Madan) who was a former librarian at Oxford. The name was passed through his professional colleagues until it arrived at Lowell, and on March 24, 1930 every employee at Lowell Observatory voted by secret ballot on the name for Tombaugh’s new world. “Pluto” received every single vote, and the name was fixed!
The name was almost instantly embraced in our popular culture. Walt Disney is famously rumored to have named Mickey Mouse’s dog companion Pluto after the planet, and in 1941 Glenn Seaborg continued a tradition of naming new elements after planets when he named a newly discovered chemical element “plutonium.”
Today, it’s almost exactly 84 years since the discovery of Pluto. What do we know about it? And why did it take 76 years for us to start arguing about whether it is a planet or not?
We can think of the Solar System in zones. Down near the Sun, the worlds are small and rocky. In this zone, we call the planets “terrestrial,” the domain of Mercury, Venus, Earth and Mars. A bit farther out, the planets get large, notable for their vast gaseous atmospheres and lack of solid surfaces. In this zone, we call the planets “jovian,” — Jupiter, Saturn, Uranus and Neptune. Beyond Neptune is the Third Zone. This is out where Pluto lives, and all the worlds out here are small, made up mostly of rock and ice, and are on weird orbits that don’t always line up with the solar system’s inner two zones. These worldlets are the detritus, the flotsam and jetsam, left over from the formation of the Solar System.
We’ve always known that Pluto lived out here in this Third Zone, and that it was a bit weird. It lives on a strange orbit that is highly tilted and sometimes is closer to the Sun than Neptune. In addition, it is smaller than other planets — it’s smaller even than the Earth’s Moon. One of the arguments that’s made for Pluto’s reclassification is that it is more like worlds in the Third Zone, than is it is like the terrestrial or Jovian worlds. Those of us who object to Pluto’s reclassification don’t disagree with this. What we don’t like, is the definition that is being used to define “planet.”
Here is the current definition:
- A planet must orbit the Sun
- A planet must have enough gravity to be round
- A planet must have cleared its orbit
Pluto fails only on this last point — it lives in a part of the solar system where there are lots of things floating around and there just hasn’t been enough time in all the 4.5 billion year history of the solar system to knock things out of the way. So why should this definition bother anyone?
The difficulty with this definition is the first and the last points — they are dynamical qualities that depend on the interaction of the object in question with other things, not on the physical properties of the planet itself. The definition was created this way because we all knew Pluto needed reclassified, but we didn’t know enough about Pluto itself to know how to do it any other way. We made up this definition, but now we have to use it for everything. As a way of defining the world, it represents a very narrow and provincial view of the Cosmos.
So why does a definition like this matter? Can it really cause us trouble in the future? Of course! You see, all of us organize our thoughts about the world by sorting. We do it every day — we decide what goes in our garden sheds, we decide where to put groceries in our kitchens, and we decide how to make different piles on our desks.
Scientists do exactly the same thing. We take things that we can see around us — rocks, flowers, slime molds, stars, galaxies — and we organize them based on what they look like. Everything you see gets sorted into a bucket, which is really useful when we’re learning about something for the first time — we quickly sort things based on how they look. We call this TAXONOMY, and this is the game astronomers are engaged in right now when we talk about planets.In today’s day and age, the rules for planetary taxonomy are being changed by technology. Telescopes are getting bigger, cameras are going digital and getting more sensitive, and roboticized telescopes are controlling those telescopes and cameras 24 hours a day, continuously searching the sky for things that go bump in the night. The result is a veritable bonanza of new objects being discovered, many in the dark beyond Neptune, in the Third Zone with Pluto, all of which have to get sorted into our planetary taxonomy.
Technology is also pressing us on other fronts. Today, after spending centuries speculating on the matter, we’ve discovered that there are indeed worlds around other stars (not just the Sun!). We know of almost 2000 planets orbiting other suns! As of the time of this writing (19 April 2014), there are 1783 “planets” in our catalogs (the air quotes are necessary because they aren’t planets — they don’t orbit the Sun!). We’re going to have to put all of them into sorting buckets — into our taxonomy — and it is going to challenge us to think about what we mean when we say the word planet.
Let me tell you about two of those many thousands of planets.One of the first “planets” we found outside the solar system is orbiting a dim, naked eye star in the constellation Pegasus, called 51 Pegasi, a star much like our Sun about 50 lightyears away from Earth. The planet is affectionately called “Bellerophon” after the Greek hero who tamed Pegasus, but astronomers call this planet “51 Pegasi b.” This planet is about half the mass of Jupiter — if it were here in our solar system we’d consider this a serious planet! But there is something odd about Bellerophon — it orbits 10 times closer to its parent star than Mecury does to the Sun! We call planets like this “hot Jupiters” or “roasters”, and we have no idea how they get to be so close to their parent stars! Here’s another “planet” we discovered only just last year. Astronomers call this world PSO J318.5-22. It is about six and half times the mass of Jupiter. If it were in our solar system, not only would it be “a serious planet,” it would be the most serious planet! It would be larger than any other world in our home system. So what’s special about this world? It’s what we call a ROGUE PLANET. It has no parent star it orbits. At some point early in its life, it was ejected from its home in some unimaginable gravitational battle. Now, it will drift forever between the stars.
BOTH of these worlds, and many like them, challenge our definition of planet. Looking around the Cosmos, we have found something new, some new worlds that we have to sort into our buckets, like the hot Jupiters and the rogue planets. Maybe it will make us think about Pluto once again too.There is one last part of Pluto’s story, still to come, and it also involves technology. In the summer of 2015, for the first time in history, a spacecraft from planet Earth will visit Pluto. It’s called New Horizons, and was launched in 2006. When it arrives at Pluto, after almost a decade of flying through the dark of space, it will blaze through the Pluto system in a single pass, measuring everything it can, and snapping every picture it can get. That data, those pictures, are precious commodities that will be sent back to Earth on the faint whisper of a radio link, and will, without fail, once again make us ask some deep questions about Pluto. For the first time, we’ll see Pluto up close, and we’ll start up this whole planet debate one more time — after the champagne is done, of course. The great truth in this story is this: Pluto doesn’t care what we call it. It is, more or less, the same today as it was when Clyde Tombaugh discovered it. For that matter, it is more or less the same as it was it formed more than 4 billion years ago. The notion that things can be sorted into “planets” and “not planets” is a human construct, something we made up to try and organize our imperfect understanding of the Cosmos. The debate gives me and you and all our astronomer friends an opportunity to chat and have fun and take a serious look at how we view the Cosmos and our place in it.
But I’m willing to make a promise: Someday, we’re going to come back to this question of “what is a planet.” I don’t know if we’ll change Pluto’s status — I hope we do! — but what I do know is this. We WILL change our definition of planet.
It doesn’t mean we were wrong, it doesn’t mean we were dumb, it doesn’t mean we were ignorant of the facts. It just means that we are wiser than we once were. And that’s what the entire game of science is all about — to become wiser when faced with Nature’s awesome spectacle.
This post is derived from a talk I gave at TEDx NorthwesternU 2014. You can see the video of that talk (17 min) here: TEDx – Pluto’s Day of Reckoning
For those of you interested in this debate, there are many great resources out there that you can take a look at. First, Mike Brown from Caltech has an excellent talk online at the Keck Institute:
- How I killed Pluto & Why It Had It Coming, Mike Brown (Caltech) — 15 September 2011 (Video link)
There are also several books that I would recommend.