Pluto’s Day of Reckoning

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!

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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!

How many people feel about Pluto’s demotion. [Image by Mathias Pedersen, used with permission]

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!

(L) A “Pluto Protest” staged in Seattle [Image: Wikimedia Commons] (R) My own sign, expressing solidarity with Pluto; my daughter is more vehement in her support.

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.

(L) My mom’s spotting scope, that I first used to look at the sky. (C) My friend Hazel and her telescope at a star party, circa 2000. (R) My own big scope, named Cosmos Mariner.

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.

(L) Clyde Tombaugh and his homebuilt reflecting telescope. (R) The Lowell Observatory 13-inch astrograph, used to discover Pluto. [Images: Wikimedia Commons]

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.

The Pluto discovery images, blinked back and forth as they might appear in a blink comparator. Click to animate. [Animation: S. Larson, from Lowell Observatory archive images]

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.

Headline announcing Pluto’s discovery on 14 March 1930; the world had yet to be named. [Image: Chicago Tribune]

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.

Venetia Burney [Image: Wikimedia Commons]

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:

1. A planet must orbit the Sun
2. A planet must have enough gravity to be round
3. 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.

Pluto is just one of many worlds that have been discovered in the dark past Neptune. How do we classify these “trans-Neptunian objects”? [Image: Wikimedia Commons]

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.

Hot Jupiters are large, gas giant worlds that orbit far closer to their parent star than any worlds we have ever seen in our own solar system. [Image: ESA/NASA/STScI [M. Komesser] (STScI-PRC2008-41) ]

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!

Rogue planets drift along among the stars, without orbiting a parent star. [Image: NASA/JPL-Caltech (PIA14093)]

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.

New Horizons will fly past Pluto in July of 2015. This is the first spacecraft ever to visit Pluto and will return the first, up-close pictures of this far away world. [Image: NASA/JHU-APL]

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.

An artist’s impression of the surface of Pluto. [Image: European Southern Observatory (L. Calcada)]

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.

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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.

• * “How I killed Pluto & Why It Had It Coming”, Mike Brown (Amazon link)
•  “The Pluto Files”, Neil deGrasse Tyson (Amazon link)
• “The Case for Pluto”, Alan Boyle (Amazon link)

The Size of the Cosmos

by Shane L. Larson

As many of you know, I ascribe much of my aspirations in life as a scientist to being exposed to Cosmos at a very early age.  Within the first five minutes of the first episode, Carl said a very big thought: “The size and scale of the Cosmos are beyond ordinary human comprehension.”

As I have grown into my career in science, I have lost sight of this simple fact. I’ve learned to write big numbers. I’ve learned to convert between meters and kilometers and lightyears when needed. I’ve even learned to use “crazy relativist units” and measure distance, time, energy and mass all in meters (something that confounds my students, my parents, and many of my astronomer friends!). I’ve done this enough now that when I calculate numbers, I know if they sound right.  Two million lightyears to a galaxy in the Local Group? Sure that sounds fine.  750 Megaparsecs to a quasar? Sure, I’m down with that.  1.3 billion kilometers to Saturn?  Word.

Developing a sense for big (and small!) numbers and whether they “sound right” is an essential skill for scientists, and we spend inordinate amounts of time training ourselves and our students to be facile with them.  But that completely bypasses Carl’s point — these numbers are HUGE.  They encode how utterly small we are on the grand scale of the Cosmos!

One of my hobbies is walking Solar System Walks when I encounter them (here is a long list at Wikipedia; another list at Air & Space).  These scale models lay out the Solar System, marking the location of planets at the appropriate spatial scale to give you a sense of how large the Solar System is (forget the Universe itself).  My favorite is one in Anchorage, Alaska, known as the “Lightspeed Planet Walk”  — if you walk at normal speed, the time it takes you to reach each planet is the same time it would take light to make the journey you made.  That is awesome.  Start at Earth, and shine a laser pointer at Neptune the moment you start walking; you’ll reach Neptune at the same time your feeble green laser beam reaches the real planet Neptune!

The center of the Lightspeed Planet Walk in Anchorage, Alaska, with a scale model of the Sun.

Despite the large physical scale of these walking models, I still often feel like they don’t capture the immensity in a way that really shocks my brain. I’ve thought about this fact a lot, and suspect it is because when I’m walking the model, it feels quite ordinary.  As I’m meandering from Mars to Jupiter, I’m not really thinking about how far I’m walking. I’m distracted by my daughter prattling about why Pluto should still be a planet, and watching ducks eat algae, and avoiding speeding mountain bikers.

But a couple of weeks ago, one of my astronomy friends showed me something that blew my socks off.  It’s a very simple demonstration you can do right at home that captures how messed up my mental picture (and I’ll bet yours!) of the solar system is!  I think my mental pictures are messed up because we often show the family of the Sun all together, to better show the relative size of the planets, like the image below.

A typical representation of the Solar System, often used in books, online references, and mass media.

What this image fails to show, is the spacing between the worlds.  We’ve known the relative spacing of the planets for some time, the distances having been worked out using basic geometry together with clever observations (many of which can easily be done in your own backyard), and through application of the laws of physics (notably Kepler’s Laws of Motion, and Newton’s Universal Law of Gravitation).

Folding pattern to make a reasonably spaced representation of the planetary orbits in the Solar System on a long strip of paper.

Let me teach you the trick my friend showed me.  Get a long strip of paper (adding machine paper, or other strip paper works well), about 1 meter long.  On one end, write the word SUN and on the other end write PLUTO.  Now fold the strip in half, and unfold it again.  What object in the solar system lies halfway between the Sun and Pluto?  It is the planet Uranus; write this on the fold.  Now fold the end marked Pluto down to Uranus.  Label this as the location of the orbit of Neptune.  What does this show us?  There isn’t  much in the way of planets in the outer half of the solar system!

Now fold the end marked Sun down to Uranus.  On  this new fold write Saturn.  Fold the Sun down to  Saturn and label the new fold Jupiter.  Fold the  Sun to Jupiter and label the new fold  Asteroids.  At this point, about 93% of your strip is  between the asteroids and Pluto.  This is the part of the solar  system that is euphemistically called “The Outer Solar  System.”  Fully half of the known planets in the solar  system are still to be squeezed between the Asteroids and  the Sun!  Let’s do that next.

Fold the Sun to Asteroids, and label this fold  Mars.  The last part is two folds before labeling: fold the Sun to Mars, then fold the end over to Mars again.  The result is three folds.  Starting at the  Sun, label them Mercury, Venus and Earth.  The entire procedure creates a map with amazingly accurate spacing between the worlds (yes! I calculated the errors; I was curious!).

The results of all your folding endeavours!

Now stare at your model for a moment.  The solar system is a lot of empty space!  The places that are easiest to get to are close to Earth, but are still very far away.  The distance to the Moon is about the width of a pencil line, and it took Apollo astronauts 4 days to cross that gulf.  Mars is six to eight months away by rocket.  Look how close it is to Earth!  It took the Cassini spacecraft almost seven years to get to Saturn.   When the New Horizons spacecraft flies by Pluto in 2015, it  will be have been outbound for almost nine-and-a-half years!  The  solar system is a big place. And the Cosmos is far vaster.

I think what amazes me the most about this model is that places I normally think of as very far away are much closer to Earth than my brain normally thinks of them.  Consider Jupiter; it is in the Outer Solar System.  But on the map, it is only 1/8th the distance between the Sun and Pluto!  Wow.

“The size and scale of the Cosmos are beyond ordinary human comprehension.”  Perhaps; certainly outside the realm of our everyday experiences. But our ingenuity gives us ways to push our brains to try to understand, and clever demonstrations like this one give you ways to ponder and think.  So get out your scissors, and start folding.

(L) The full length of the Solar System model. (R) My own version of this model, shown next to a typical Earthling.

The Dwarf Planets: Water under the bridge?

THE DWARF PLANETS: WATER UNDER THE BRIDGE?

Shane L. Larson

ACNN (Another Crazy News Network)

 

In 2006, in a musty old hall in Prague, Czech Republic, a group of astronomers belonging to the International Astronomical Union voted on the official definition of “planet.” In one fell swoop, the former planet known as Pluto was demoted, sending millions of us who learned about the “nine planets” into paroxysms of confusion.

“This is how science works,” says Dr. Horatio Allan Tibbets, an astronomer at the Cloudy Mountain National Observatory in Utah.  “Our knowledge evolves, and we have to adapt the imperfect language that humans use to communicate with. The science hasn’t changed, Nature hasn’t changed, but how we understand Nature is changing. Our old ideas may have been perfectly good for explaining what we knew about the Cosmos; but as we see new things, we have to have new ideas that encompass the old ideas and explain the new ideas at the same time.  It’s a tough tightrope to walk.”

The “Pluto debate” is one example of the evolution of scientific understanding, Tibbets says. “We made this big decision to change our taxonomy, demoting Pluto from the lineup of planets.  But now, we’re learning some new things and it seems we should revisit this question again.”

Dr. Theo Partido of the Department of Physics and Astronomy at Greater Oklahoma Polytechnic University is a proponent of the Pluto demotion and not changing the current classification.  “The simple fact of the matter is that Pluto is different than the other planets.  It’s not like Earth; it’s not like Jupiter.  It’s been put in its proper place and it should be kept there.”

Tibbets is part of a growing number of voices in the scientific community who disagree with this.  Armed with new facts and new observational data, they are attempting to open a new dialogue that will once again change our notion of what it means to be a planet.  “The fundamental issue is that we don’t know enough about the Cosmos,” said Tibbets. “This definition of ‘planet’ has been made by looking at only one solar system — ours! We don’t know enough about what other systems might be like, and that can lead to problems.”

By the current definition, a planet is only a planet if it has cleared its orbit of other bodies, like dust and rocks and asteroids.  That is a serious flaw in the minds of many astronomers.  “Imagine a system that has a planet like Jupiter in an orbit like Pluto’s,” said Tibbets.  “In a scenario like this, Jupiter could not have cleared its orbit in the age of the solar system! It would not be considered a planet by the current definition, which is crazy! There are very few people who would argue there are cases where a world like Jupiter should not be considered a planet.”

Astronomers gathered this week at Creeping Ivy State University for a contentious meeting intended to find common ground between the two sides of this debate.  While there has been insistence from both sides that they are here to have a discussion, the debate has already become heated and contentious.  The rhetoric is vehement and the tension is palpable.

“The Plutoers are out of touch,” claims Partido.  “The sooner they accept that they are wrong, the sooner we can move on.”

Tibbets does not mince words about this philosophical battle. “They’re ideologues, and don’t behave rationally.  They call me a ‘Plutoer,’ like we’re still juveniles.  Should I turn around and call them ‘dwarfers?’  No. Someone has to be a grown up; I’m here to talk about science.”  Tibbets and a growing number of colleagues have been arguing to revisit the definition of planet in light of new information that is emerging from exoplanet studies, as well as tantalizing new data from our own solar system.  “We can’t ignore that; that’s not how science works.”

“What we’d really like to see is a dialing down of the rhetoric and less clinging to ideology for the sake of ideology,” said Tibbets.  “But that message doesn’t seem to be getting through.”

That much is evident when talking to people on the other side of the debate.  “The Plutoers seem to think there is something to argue about here, but there isn’t,” insists Partido. “We’re right, and they’re wrong. Plain and simple. It’s water under the bridge, and they should get over it.”

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NOTE: I’ve been writing long essays for this writescience experiment; I wanted to see what it would be like to be confined to a smaller space.  This is about 25-30 column inches in a newspaper, based on word count.  More than would normally be dedicated to a science story?