What We’re Missing in an Eight Planet Solar System
I agree with those who say Pluto should be classified as a planet. Please let me explain why.
The Purpose of Scientific Definitions
Why do scientists classify things and define terminology? To promote science. We think more clearly when we sort things into organized classes. We communicate more effectively when we agree on the meaning of words. When the International Astronomical Union (IAU) changed the definition of a planet so that dwarf planets like Pluto are not classified as planets, did that promote science in these ways?
No, I don’t believe so.
When we discuss solar system bodies we are already more specific than to say planet. We say terrestrial planet or gas giant or ice giant. Everyone agrees small bodies like Pluto and Ceres are a different category than these others, and dwarf planet or unterplanet or some other term would be appropriate. There was no disagreement about that. Calling Pluto a dwarf planet is perfectly fine. But to say “dwarf planets are not planets” – that’s what the fuss is about. Even the label dwarf planet is less specific than we find in normal scientific discourse. There are many types of dwarf planets: main belt, plutino, twotino, cubewano, scattered disk, etc. The new definition doesn’t add any precision. As far as I can see, there’s no situation where the new definition makes communication easier for scientists. The number of words required to explain research is not decreased and the clarity of our thinking is not improved.
Even worse that this, the definition would actually hurt scientific discourse in the realm of exoplanets (planets in other solar systems), except that exoplanet researchers can ignore it. If they were required to abide by the new definition, they would not be allowed to use the word planet and would be forced into more verbose constructions like “planets and/or dwarf planets,” because we don’t know whether individual exoplanets have met the IAU definition or not. We can’t see their neighborhoods well enough to know if they have cleared out their orbits, and there is much we don’t understand about the dynamics of these diverse solar systems. The only way to discuss exoplanets efficiently is to ignore the IAU definition, and as far as I can see, that’s pretty much what everybody does.
In other words, we know our own solar system too well for the definition to be useful, and we don’t know exoplanetary systems well enough for it to be useful. So for the doing of science, the IAU’s definition isn’t useful.
Scientific Definitions Impact Culture, Too
However, there’s another purpose for scientific definitions: to communicate science to the public, to help educators teach concepts consistently, and to have an impact on the culture. This is where the definition of planet matters most.
For the longest time in human history, the word planet was an observational term describing the lights in the sky that move around relative to the background stars. Therefore, the sun and Moon were classified as planets, but the Earth was not.
Galileo insisted that the Earth is actually a planet because it moves and goes around the sun. Astronomers could have kept the word planet as an observational term but instead they redefined it to line-up with the new paradigm. The sun and Moon were kicked out of the planet club and the Earth was made a member. The new meaning of the word impressed the heliocentric paradigm upon the consciousness of the world. That’s what redefining a word can do.
Another Paradigm Shift in Planetary Science
It’s no wonder astronomers opted to change the definition of planet following Copernicus and Galileo. Switching from geocentrism to heliocentrism was one of the biggest paradigm shifts in human history. But if you think about it, you will realize there has been another paradigm shift in planetary science in recent years. It’s perhaps not as radical, and it has not been in the news, but it reaches deep into the human psyche. The new paradigm shift is in our understanding that solar systems are dynamic, dangerous, and wonderfully messy places.
It used to be that the planets were considered an exquisite clock that keep cycling in their paths forever without change. This view developed along with geocentrism. Ancient peoples like the Mayans, the Mesopotamians and the Chinese painstakingly recorded the planets’ movements and noted their remarkable constancy, which set them apart from the transitory things of Earth. The planets were associated with the gods and were considered to rule over the Earth. Temples were built to align with their regular motions. Astrological systems were developed, including the names of the seven days of the week (see box, above). Eventually, astronomers conceived of crystal spheres on which these planets were fixed to explain the regularity of their movements. The orderliness of the heavens became one of the foundations of ancient philosophy around the world.
The essence of this view didn’t die when Galileo pushed us to accept heliocentrism. We came to see that this clock-like system is not centered on the Earth, but it was still a clock-like system. Kepler’s laws uncovered a supreme mathematical elegance underlying the heavenly motions. Newton’s laws of gravity and of motion predicted the planets to be in ever changeless paths. Laplace showed that they could have started by condensing out of a cloud, thus answering Newton that the planets had a beginning, but he also showed that they are stable forever so presumably they might have no end. Literature shows how culture continued viewing the planets as ordered and reigning. In Elizabethan times Shakespeare wrote (in Troilus and Cressida):
The heavens themselves, the planets, and this center
Observe degree, priority, and place,
Insisture, course, proportion, season, form,
Office, and custom, in all line of order.
This belief in a more elegant solar system showed up in science, too. While astronomers knew that a few rocks do fall from the sky, they didn’t believe the craters on the Moon were caused by impacting rocks, some as big as mountains. They thought the craters were of volcanic origin. Asteroids didn’t fit our expectations of a simple solar system, so we couldn’t see what was staring us in the face.
This belief was rooted in human experience from before the beginnings of civilization. Just like geocentrism, it grew out of our limited human perspective. Geocentrism came from our limited perspective in space. The orderliness of the planets came from our limited perspective in time. We simply don’t live long enough to see how disordered they are.
Science helps us get beyond these limitations. It shows that the Earth is not the center of the solar system and that planets do not behave like the precise gears of a clock. We have learned some pieces of physics that Newton and Laplace didn’t know, things that cause planets to become unstable and to move around. When we put the new physics into computer simulations, we discovered that even the giant gas planets have migrated, and this explains a lot of what we see in the other planets today. Jupiter and Saturn probably moved much closer to the sun and then back out again. This helps explain why Mars is so small and why the asteroid belt has certain features. Half a billion years after that, the giant planets were moving around again and causing havoc. According to some computer simulations, there’s a 50% chance that Neptune and Uranus switched places during that time. And we have learned that the interactions between planets is exponentially sensitive to little changes and are therefore chaotic, unpredictable beyond the tiny span of just several to a few hundred million years (their Lyapunov time in chaos theory). We see evidence of planet migration in exoplanetary systems, too: the Hot Jupiters that are very close to their stars almost certainly couldn’t have formed where we see them. And who knows what other dynamics we are about to discover?
Well, that instability happened a long time ago and the planets are stable now, right? Wrong. We think eventually Mercury will get kicked out of the solar system by the gravitational tugging of Jupiter when their perihelia become aligned. In the process, Mercury might collide with Venus, and even Mars and the Earth could be thrown around. Only the four giant planets seem to be in safe orbits. Additionally, the Moon is moving away from the Earth and eventually the two must be re-classified as a double planet system. (So the Moon will get finally back into the planet club! Now that’s perseverance.)
The new view of planetary science includes lots of very cool physics that have gone way beyond the simplistic clockwork solar system. For the thrill of discovery, this is a wonderful time to be alive!
The Best Indicators of the New Paradigm
Nevertheless, the best indicators of the new solar system dynamics are not found in the eight major planets and the Moon. They are written into the smaller worlds like Pluto. Pluto is in a 3-to-2 resonance orbit with Neptune, meaning that every time Neptune circles the sun three times, Pluto circles it twice. That makes Pluto’s orbit stable so that Neptune doesn’t fling it far away. It turns out that there is an entire host of small bodies in this same resonance, and they are now called plutinos. They must have been swept into this resonance during Neptune’s outward migration. Other dwarf planets were too far from the safety of a magic resonance, and Neptune flung them far away into the Oort Cloud. One day we will discover those lost worlds.
Many other bodies are in different types of resonance with Neptune, giving rise to some of the classes of dwarf planets mentioned above. Triton, the moon of Neptune that orbits the wrong way, was probably a dwarf planet like Pluto before it was captured by the ice giant. In the Main Asteroid Belt we see a dwarf planet (Ceres) that cannot absorb the surrounding material to grow into a larger planet because Jupiter keeps the material stirred up. We also see families of fragmented protoplanets (early worlds) moving away from their long-ago world-shattering collisions in telltale ways. And it’s not just the major planets that push around the small ones. The last great migration of the four giants, which determined the present form of our solar system, was apparently triggered by the gravitational influence of many smaller bodies. In a sense it was that great mass of lesser bodies that reigned, that told the giant planets where to go in the sky. Without the small worlds, our concept of a solar system is dreadfully incomplete. Other evidences are still being found among the various types of dwarf planets, and we are just starting to examine the Kuiper belts of nearby stars. The study of solar system dynamics is about to enter its Golden Age and we can expect wonders.
How Has This Been Kept a Secret?
This discovery of solar system dynamics is in the main stream of progress in planetary science, and yet the public knows almost nothing of it. They are engaged in the cool side-topics, like whether there was ever life on Mars and whether there might be a world like Earth around another star. Let’s not take anything away from those questions. It will be amazing if we find life outside the Earth! But the most publicly cool questions are not the day job of most planetary scientists. Other branches of science have kept the public engaged with their main body of progress. Why haven’t we?
At the time of Galileo, the definition of planet was changed to drive the new paradigm into public consciousness. Ironically, the more recent re-definition of a planet – the one that kicked Pluto out of the planet club – was not made to enforce the new paradigm, but was instead made to preserve the old one. Many of us wanted to keep the view that planets “reign in their orbits” and that there are just a few of these reigning bodies. This ensures that we have a solar system that looks clean instead of dynamically messy, and simple instead of complicated with hundreds of smaller planets. Pluto gave us an early hint of the new paradigm because its orbit was more eccentric and tilted than the major planets. Pluto was tolerated as an oddball as long as it was alone. The controversy happened when we started realizing that there are many additional planets just like Pluto. That was going to force us to change our notion of planets, as in the days of Galileo. Or to state it negatively, it threatened to mess up the inherited view of what a planet should be.
Right now there are probably 85 additional dwarf planets in the Kuiper Belt and Scattered Disk that we have already found, and who knows how many more in the Oort Cloud? And there are already hundreds of more possible dwarf planets in the Kuiper Belt except we can’t see their size well enough yet to know if they meet the requisite size limit to be round. If we admitted all these hundreds of new bodies into the planet club, then the view of planets as a few reigning aristocrats could not survive.
Many astronomers wanted to re-define planet so that the idea of aristocratic planets would survive, not because that concept advances scientific discourse (it doesn’t), but because it is the culturally expected norm. We said that it matches our “intuition” that planets should be a small number of dominant bodies. If we examine ourselves we’ll discover this is an intuition we inherited from the ancient world, not from science. Scientists can easily handle classifications containing millions of members. How many beetles are there in the world? (Even the number of species of beetles is huge.) How many types of rock are there? I once laughed out loud when I read a comment on an astronomer’s website objecting to this need for a small number of planets: “I don’t trust astronomers who can’t handle numbers in the thousands.” (Wasn’t it a planetary scientist who brought the word Billions into popular use?) We can also handle classifications containing members that are very different from one another. The eight major planets are truly different from the dwarf planets in some ways. Likewise, sperm whales are very different from mice, and yet they are both classified as mammals. Biologists determined that something other than size tells the essential reality of life. When we pick the defining feature of planets, we should insist — like Galileo — that it tells the essential reality of solar systems.
The truth is, we felt the need for a small number of dominant planets because it’s what humans always knew a solar system to be, ever since we looked up to see lights moving in the nighttime sky. We now know better than this, but we haven’t examined ourselves well enough to see this is the origin of our “intuition” about planets. We still think the intuition is somehow natural and therefore right. This response to our intuition is different than what scientists did at the time of Galileo. Back then, we gave up our intuition and radically embraced the new view. It took time and lots of fighting, but that was the outcome. We redefined the central word of planetary science in a way that communicated a scientific revolution. This time, we redefined our central word in a way that communicates business as usual. Faced with the specter of hundreds of new planets cluttering up the crystalline celestial spheres, hundreds of worlds too scattered and under the influence of their larger neighbors, we opted to purge the pantheon instead.
This is disappointing. We rejected the ancient world’s geocentrism. Couldn’t we reject its belief in orderly, reigning planets as well? In my opinion, this is why the public is not more engaged in the central questions of planetary science. We have hidden it from them. We created a vocabulary that focuses attention on the old familiar planets that seem least changeable. We pushed Pluto and the other paradigm-shattering worlds into a lower tier of importance. As a result, we made our branch of science more “intuitive” and a lot less revolutionary.
With Pluto as a Planet
With Pluto defined as a planet again, along with the hundreds of other dwarf planets, people would learn that a solar system’s essence is not the few bigger worlds. The bigger worlds and the smaller ones — the sperm whales and the mice — are equally products of dynamical evolution, and that is the essential characteristic of a solar system. Planetary science would become more fascinating for students and it would fire their enthusiasm to learn. Imagine teachers starting the lesson on solar systems by saying, “Our star has literally hundreds of planets, including the eight major ones, and many we still haven’t discovered.” Wouldn’t that be cool? And in higher grades the curriculum would explain the many classes of planets: hot Jupiter, terrestrial, main belt dwarf, gas giant, ice giant, plutino, cubewano, and so on, including many classes we have yet to discover. Soon, students will learn the classes of solar systems themselves based on the classes of planets they contain. What an amazing universe we live in! Don’t you want to study it? This opens up new visions of our dynamic existence and ties our lives into the strange things we see in exoplanetary systems. And it calls us to extend civilization beyond the Earth, to settle the wild environment of our solar system to make our existence more secure. Embracing this paradigm shift with our vocabulary would impact the education standards, the development of curricula, and the discussions in the classroom. It would reinforce the new paradigm in the culture. It would let our world know that planetary science is still the science of revolutions.