The Ten Planets

By Dan Willmore

What is the difference between an asteroid and a planet?

How many planets are there? Growing up, I was taught that there are nine. However, in October of 2002 astronomers at the California Institute of Technology announced that they had discovered a new world, which they have named 'Quaoar,' after a Native American god who 'sang the universe into being.' Then, in March of 2004, the same team discovered another new sphere which they named 'Sedna', after an Eskimo goddess of the sea. Both of these worlds are smaller than Pluto, previously considered the smallest planet, and both of these worlds are bigger than Ceres, which has long been considered the largest asteroid. Being stuck in between, astronomers had to wonder wether these new bodies should be called planets or asteroids. Then in January of 2005 astronomers at Mount Palomar in California discovered a new object that was even larger than Pluto, and this threw the question wide. Does our solar system contain nine planets or does it contain ten? Or should we say it contains eight planets or twelve? No answer can be reached without first understanding what we mean by the word 'planet.'

Early Views

The stars have always been with us. People could see watch the earth change, as floods moved rivers, forests rose and fell, and the seasons themselves transformed everything that people could reach; but above the earth and the clouds there were always stars, bright, hard, and completely untouchable. Ancient people watched the stars turn in the sky, and imagined that the white dots could be joined into lines that formed pictures, gigantic lions or heroes displayed against the night. A few stars, however, did not hold to their places in the grid. Instead, they slid across the sky from season to season and night to night. Usually, they went in a single direction, but on a few occasions they actually seemed to go backward. The ancient Greeks called these stars 'planes', which meant 'wanderers', and from this we get the word 'planets.'

As could be expected, the Greeks decided that these planets belonged to their gods, and they worked hard to make sure the choices made sense. Moving quickly close to the sun, the planet Mercury seemed to belong to the god Hermes, the messenger of the gods. Red and angry, the planet Mars was awarded to Ares, the god of war, and Venus, silvery and beautiful in the morning, was awarded to Aphrodite, the goddess of love. The ancients recognized six of the planets that we know, Mercury, Venus, Mars, Jupiter, Saturn, and Uranus, all bright points that 'wandered' through the sky, but they added two more which might surprise some modern readers. For ancient astronomers, the Moon and the Sun were both 'planets' as well, because they too wandered across the sky while the other stars seemed to stay still. Everyone assumed that the Earth itself was the center of the universe, and that the 'eight planets' simply turned around it.

As the centuries passed human imagination and human knowledge almost seemed to be competing against each other. The planets in the sky seemed to be pure and divine, so the Italian poet Dante Alighieri imagined that the souls of the dead might go to live on the planets, with the supernatural heaven being the same as the heaven above his head. In his 'Paradiso' Dante decided that the souls of Christian soldiers should meet his protagonist on Mars, while the souls of affectionate women should be found on Venus, and so forth -- a neat if unconvincing extension of the old Greek ideas.

After Copernicus

In time, of course, Copernicus came along, and Copernicus showed that the earth turned around the sun; and then Galileo used his telescope to discover rings around Saturn, and smaller spheres turning around Jupiter. Science realized that the sun did not 'wander' at all, at least within the solar system, while the Earth itself was clearly 'wandering' around the sun. Furthermore, the solar system itself was filled with other, smaller 'wanderers', from bodies turning around Mars and Jupiter to little bodies orbiting the sun between the orbits of Mars and Jupiter. What could they be called? Were they all 'planets?'

Of course, they were not. If the Earth orbited the sun, science decided that the Earth itself was actually a planet, so the new lineup of planets included Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus -- a total of seven, which satisfied many people who felt that seven was a magic number, and fit for the heavens. At the same time science realized that our own moon, sometimes called Luna, really does orbit the Earth after all, and this created a new concept -- moons, bodies which orbit planets. The two bodies which orbited Mars were called moons now, and given names, Deimos and Phobes, meaning 'Terror' and 'Fear', which seemed appropriate to a god of war, while the four largest moons of Jupiter were named after wives or lovers of Zues, the king of the gods.

Nonetheless, that did not finish the job. If the sun was a star, then the sun had a category, but telescopes kept finding more objects which 'wandered' against the backdrop of stars. What were they? Looking through telescopes, scientists could see that many of the new objects were round and made of solid material, like planets. However, scientists felt that the new objects were too small to be planets -- after all, planets were supposed to be big things, associated with powerful divinities. At the same time, the new objects did not turn around planets, so they could not be moons, and the very fact that they moved proved that they were not stars. What could they be? Some scientists decided to call them 'asteroids', a new word, derived from the Latin word for 'star', which is 'aster' and the Latin suffix 'oid', which means 'similar too.' Thus, the new objects were simply called 'starlikes,' -- a really inappropriate title, if you ask me, because stars are enormous and usually incandescent, which none of the new bodies were. Others choose to call the new objects 'planetoids,' which means 'planetlike,' and seems to fit a little better.

As the centuries passed telescopes proliferated, and astronomers proliferated with them. The ancient word 'astronomer' actually means 'star namer', and thousands of astronomers were able to find and name thousands of asteroids. The smallest of them are just a few hundred feet across -- 'pebbles in the sky' -- while several are hundreds of miles across, and the largest, Ceres, measures five hundred and ninety miles from side to side. The smallest of the traditional planets, however, is Mercury, which is three thousand miles in diameter, making it clearly an order or two larger than Ceres; and when in the eighteenth century astronomers discovered Neptune, another planet much larger than Earth, the categories seemed clear. Five hundred and ninety miles was the width of the largest asteroid, and three thousand miles was the width of the smallest planet; planets were different than asteroids, and asteroids could never be planets. The solar system was orderly and it made sense.

The Discovery Of Pluto

At the end of the nineteenth century scientists thought they understood the structure of the solar system well. In the middle was the sun, of course, and next to that were the four rocky worlds -- Mercury, Venus, Earth, and Mars. After Mars came the 'asteroid belt', a region of 'minor planets' which included everything from Ceres and Juno to bodies which were too small for telescopes to find, and which were assumed to become meteorites when they came close enough for the gravity field of planets to grab them. Astronomers knew that some of these asteroids wandered outside the asteroid belt, but calculations suggested that there should have been another planet after Mars, but before Jupiter. Scientists guessed that a fifth rocky planet had indeed existed once, in the early days of the solar system, but they figured that the enormous gravity of Jupiter had torn this planet into pieces, and the pieces had become the asteroids themselves. After the asteroid belt came the 'gas giants', enormous planets far bigger than Earth or the rocky planets, but cold and covered with gas and fluid thousands of miles deep. Jupiter by itself was bigger than all other planets put together, and after Jupiter came Saturn, Uranus, and finally Neptune.

And then came Percival Lowell. Lowell was a bright young man from a wealthy family in New England, and Lowell was able to build the famous Lowell Observatory in Arizona. Once there Lowell studied the planets carefully, and probably worked harder than his telescopes could repay. I am sure that many readers will recall how Lowell looked at Mars and saw lines which he thought were canals. Actually, they were not, but the canals added to the mythology of Mars and fit well in the novels of Ray Bradbury and Edgar Rice Burroughs. Then Lowell looked at Venus and he thought he saw canals there too, which baffled him, because he knew that Venus was covered by clouds and that Venus turned, while the lines he was seeing always seemed to be facing him. It would be a hundred years before doctors proved that Lowell was seeing only the blood vessels within his own eye, lit up by the light from the eyepiece of his own telescope. Throughout the world millions of people believed in the canals of Mars, but only Lowell believed that there were canals on Venus.

Lowell was an honest and hard working man who experienced a lifetime of frustration. Finally, Lowell had a third big idea. Looking at the orbits of Uranus and Neptune, the outermost gas giants, Lowell thought he saw them wobble, as if they were being pulled by the gravity of another gas giant, a huge planet beyond the orbit of Neptune. Discovering such a planet was worth doing for its own sake, but it would also be a heroic cap for Lowell's career. Lowell kept searching for this ninth planet until he died in 1916. Lowell did not find the planet himself, but after he died the people at his observatory kept looking for it; and finally in 1930 Clyde Tombaugh found another new sphere.

What was this new world? Was it a new planet? Lowell had sent his people to find another gas giant, something the size of Uranus or Neptune, and the new planet was clearly too small to affect the orbit of its immense neighbors. Did this mean that the new heavenly body was an asteroid? The biggest asteroid, Ceres, was five hundred and ninety miles across, while the new body was fourteen hundred miles in diameter. This meant that the new world was twice as wide as Ceres, but slightly less than half as wide as Mercury, which is three thousand miles across. Fourteen hundred miles put the new body in the middle, and left room for a judgement call. Lowell needed a planet, so he decided to call it one, and he named it Pluto.

Why did he choose that name, and not another? Some say that Pluto got its name for simple mythological reasons. The god Pluto was the son of the god Saturn and the brother of the gods Jupiter and Neptune, so the name fit the neighborhood, while Pluto himself was the god of the underworld, and the planet Pluto is so far from the sun that it is 'colder than death.' Others say that Pluto got its name because the first two letters of the word 'Pluto' are P and L, and those are the initials of Percival Lowell. Either way, the public embraced the new planet, so hundreds of millions of children can name the nine planets, but not one person in a hundred can tell you what Ceres is.

Pluto broke all of the rules.

No matter what you call it, Pluto broke all of the rules. In the solar system the eight planets and the asteroids had always turned on the elliptic, a flat plane of concentric rings which made the solar system look like one of those old fashioned RPM records, with the sun as the pivot. Pluto, however, was outside the elliptic, as if its orbit had been tilted, so that part of the time Pluto would be above the elliptic, and part of the time Pluto would be below it, turning around the sun but at a different angle. Secondly, Pluto would not even hold its position in the solar system consistently. The eight planets kept to their places, so that Uranus always orbited outside the orbit of Saturn, and Neptune orbited outside the orbit of Uranus, making circles like the rings of an onion; but Pluto wove in and out. Sometimes Pluto was closer to the Sun than Uranus, and sometimes Pluto was further away. Why? Some scientists conjectured that Pluto had once been a moon of Uranus, but that an asteroid had struck it out of its orbit and sent it sailing around the sun. At least Pluto does have a moon of its own, Charon, named after the god who ferried the souls of the dead to the underworld, which allows Pluto to resemble the other planets in another respect. Nonetheless, the old order was gone: once there had been four rocky worlds, and then four gas giants, but now there were four, and four, and then Pluto, which did not fit in any group. Ptolemy and the other ancients had written about the 'music of the spheres', but if there had ever been such a harmony, something had smacked it pretty hard from the get go.

The Orbit Of Pluto

The Kuiper Belt

By the middle of the twentieth century astronomers were willing to question the idea of order altogether. Why should there be concentric spheres, even if they were only elliptical orbits? Why should the solar system have a 'final' planet, or any border at all? Earth's atmosphere gets thinner and thinner the further up you go, so a person can suffocate ten miles up, but machinery can still detect traces of air hundreds of miles above the surface; why should not the solar system also get thinner and thinner as it stretches away from the sun?

Gerard Kuiper thought so. Kuiper was born in Holland but went to school in America and finally worked for the Yerkes observatory, where he discovered new moons turning around Neptune and Uranus. In 1951 Kuiper tried to make sense of Pluto. Science already knew about an asteroid belt between Mars and Jupiter; what if there was another belt just beyond Neptune, a region where chunks of ice and rock might still remain so far apart from each other that gravity had never pulled them together into a ball? These chunks could have floated in the outer dark since the beginning of the solar system, and if the gravity of Neptune or Uranus finally pulled one of these chunks out of its orbit, the object would fall toward the sun until it melted, forming a 'tail' of vapor like any other comet. If Kuiper was right, then there should be more worlds like Pluto out there, cold spheres holding a lot of rock but also a lot of ice, turning near the elliptic, but not always on it.

As you have probably guessed, that leads us to the new discoveries. The California Institute of Technology has assembled a project to find Kuiper belt objects, and this is how Caltech discovered two new spheres, Quaoar and Sedna. So far Quaoar seems to have a lot of ice on the surface, as Kuiper said that it should, but Sedna is very red, red like Mars, which suggests that Sedna contains more rock than ice. Nonetheless, both objects are in the part of the solar system where Kuiper said they should be, and both objects are bigger than any asteroid. While Ceres is only five hundred and ninety miles across, Quaoar is eight hundred miles across, and Sedna may be eleven hundred miles across, coming close to our old friend Pluto, which is only fourteen hundred miles across.

Chart showing the comparative sizes of Mercury, Pluto, Sedna, Quaoar, and Ceres
Chart showing the comparative sizes of Mercury, Pluto, Sedna, Quaoar, and Ceres

Again, what is the difference between an asteroid and a planet? Throughout the twentieth century there was a gap between the sizes of the largest asteroid and Pluto, but Quaoar and Sedna filled that gap. (The circles in the picture below are all done to scale, and they should illustrate the transition.) Did this mean that Quaoar and Sedna really were planets? If so, what is Ceres? If Quaoar and Sedna were asteroids after all, what is Pluto? At this point the biggest gap was between Pluto at fourteen hundred miles and Mercury at three thousand miles. Furthermore, the Kuiper belt is still vast and remote, and astronomers had to wonder if another sphere would be found that would fill the gap between Pluto and Mercury.

The Ball That Broke The System

In 2005 that gap was filled. Mike Brown and his team at Mount Palomar found another object which was three times farther from the sun that Pluto, but still bigger than Pluto. Brown would call that object 'Eris,' after the Greek god of discord.

Brown knew that Eris would cause a lot of controversy. In 2006 the International Astronomical Union met and discussed how to sort out the solar system. The IAU decided that every planet had to meet three standards. One, every planet should circle the sun. Of course, this was an easy standard to meet; all of the official planets and asteroids already did this. The second standard was that the object should be large enough that its own gravity should be able to squeeze it into a sphere. Only a handful of objects met this second test; these included the nine planets that had already been accepted as well as Sedna, Quaror, and Eris, as well as Makemake, a new object that was three-quarters as large as Pluto, and Ceres. Finally, each planet should be big enough to 'clear its orbit.' This meant that the planet should be large enough that its gravity could suck in all of the objects in its orbit, leaving it alone at that distance from the sun.

This last definition was the most controversial. The International Astronomical Union observed that other objects crossed the orbit of Eris and Pluto. So this third standard meant that Eris would not be considered a planet, and now Pluto would not be considered a planet either. After more than half a century as a planet, Pluto would not be considered a 'dwarf planet.' Now there would only be eight planets, just as in the nineteenth century.

Most American astronomers disagreed. All of the new objects, from Pluto to Eris, has been discovered by Amnericans in America; and if all of the objects from Pluto to Eris were removed from the list of planets, all of the remaining planets would have been discovered and named by Europeans, and none of them would have been discovered and named by Americans. At the same time, the American astromers argued that the new standards were not consistent. After all, even some of the largest planets do not 'clear their orbits.' Even today there are asteroids which cross the orbits of Mars and Jupiter, and Pluto still crosses the orbit of Neptune. Nobody would ever say that Jupiter and Neptune were not planets; so where was the logic of excluding Pluto and Eris?

Nobody would ever say that Jupiter and Neptune were not planets; so where was the logic of excluding Pluto and Eris?

Finally the International Astronomical Union offered a series of compromises. The IAU suggested that the outer spheres could be called 'plutoids.' A Plutoid would be an object that was big enough to be a sphere, and to be outside the orbit of Neptune. Some astronomers think that the Kuiper belt may contain as many as seventy more 'plutoids.'

I don't know what to say about that. To be honest, the more I think about it, the more I find myself questioning the entire idea of scientific categories. Some scientific terms are closely rooted in nature, so that the element carbon will always have six protons, and the element oxygen will always have eight protons, and there will never be a grey area where carbon is 'oxygenish' or oxygen is 'carbonish' -- instead, carbon in all of its isotopes will always be carbon, and oxygen will always be oxygen, no matter how you combine it with other elements. Other scientific terms, however, are nothing more than labels that we invent only to help us sort out natural bodies which do not themselves have any sort of innate order. The International Astronomical Union has choosen a set of distinctions which have classified the solar system in a manner which satisfies some astronomers but not all of them. Right now it seems likely that the controversy will continue as new objects are discovered.

Copyright 2009 Dan Willmore

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