Pluto is a World
Perhaps people should start thinking about Pluto as a world, rather than a planet. I think that’s what most people really mean anyhow. Our solar system is full of worlds—beautiful and strange places that Captain Kirk might want to beam down to.
To the ancients a planet was a star in the sky that moved. There were five of these in all: Mercury, Venus, Mars, Jupiter and Saturn. They looked just like any other star, but they wandered around the sky as the days, weeks, and months passed. That alone was enough to make them special.
When, in the early 1600’s, Galileo turned his telescope on the sky, he discovered a sixth planet: the one beneath his feet. Galileo’s observations showed that the earth is not the center of the universe, but just another planet orbiting the sun.
In 1783 the astronomer William Herschel discovered Uranus. Although too faint to be seen by the eye alone, he knew immediately that it was no mere star, because it appeared as a small disk in his telescope. Sure enough, as the nights passed, it too wandered. This was the seventh planet.
In 1801, the Italian priest, mathematician and astronomer, Giuseppe Piazzi, discovered the eighth planet: Ceres. Yes, Ceres. Ceres was heralded as a major discovery, for it was found right where astronomers expected it to be, in the big gap between Mars and Jupiter. A year later another similar planet was discovered, named Pallas. Then came Juno, Vesta, and Astraea. If you had studied astronomy in the early 19th century you would have been taught of the 11 planets in our solar system. What an exciting time it must have been, with all these newly discovered planets. But there seemed something amiss with these new planets. Astronomers had expected one big planet in the gap between Mars and Jupiter, but they had found five. Based on how faint they were, these new planets also seemed to be quite small. There was some debate as to whether they were planets at all.
It all came to a head in 1846 when Neptune was discovered. Neptune was another major planet that was found way out beyond Uranus. In 1847, after nearly 50 years of being called planets, Ceres and its compadres were redesignated Asteroids. As many had come to realize, these asteroids were more akin to planet fragments than true planets. So once again there were only eight planets, and as the years passed many more asteroids were discovered. They number in the hundreds of thousands today.
Another fifty years passed. School children became used to the eight planets of the solar system. But astronomers wondered if there were yet more major planets to be discovered. They searched ever deeper into the outer reaches of the solar system, and as the 20th century dawned, Percival Lowell, a man who was flamboyant and wealthy, began a quest to discover what he called, “Planet X.” It took some thirty years, but eventually an astronomer named Clyde Tombaugh, who was employed to pursue Lowell’s dream, found a new wandering star. This was hailed as the first great new planet discovered in the 20th century. Pluto/Planet-X was expected to have a mass of ten Earths, and it was far beyond the region of the asteroids, so few questioned that it was a planet. But this turned out to be hubris. The calculations that had predicted planet X had been in error, and Pluto had been found quite by accident. By 1940, estimates put it at about the same mass of the earth. By the 1970’s, estimates of its mass had shrunk to 1/10 that of the earth. Pluto was shrinking, if not in reality, at least in the estimation of those studying it. By 1980 it was a mere 2/1000th the mass of the earth. Pluto also has a weird orbit that brings it within the orbit of Neptune for part of the time, and is tilted by a whopping 17o. People started to realize that, like Ceres, Pluto may not be a proper planet, but there was no compelling reason to change its status.
At the dawn of the 21st century, an astronomer named Mike Brown realized that there were likely more objects like Pluto out there, and much like his predecessors, he set out to find one. He and his team eventually discovered many such bodies, including Quaoar, Sedna, Orcus, Haumea, Makemake, and Eris. Eris is big—about the same size as Pluto, but more massive. By 2006, astronomers realized that the Ceres saga was playing out all over again. Rather than call all of these new Pluto-like objects planets, it was decided by the International Astronomical Union to remove Pluto as a planet in much the same way that Ceres had been. In addition, a new class of objects was created, called Dwarf Planets. These objects include Pluto, Ceres, Eris, and a growing list of others in the outer solar system.
Once again, there were eight planets.
If you look at the question of what constitutes a planet from an historical perspective, a case could be made for only the original five wanderers. Wandering star is, in fact, what the word planet means. Perhaps it all went wrong with Galileo, when people realized that they were living on a planet. After all, the earth is not a star that wanders in the sky. Or perhaps it went wrong when Uranus and Neptune were called planets. Even though they are both large and massive, they are too faint to be seen wandering with the eye alone. It certainly went terribly wrong with planet Ceres, which would have led us to hundreds of thousands of planets and counting. Alas, Pluto has suffered a similar fate.
Something profound has happened since people first followed the paths in the sky of planets—these wandering stars. We have come to know them as places--places you could visit, with mountains and valleys. Science fiction has depicted them as places where you can land your ship and walk around. In fact these are all worlds to be explored. These worlds include not only Pluto, but Ceres, and Vesta, our Moon, and many of the moons of Jupiter, Saturn, Uranus and Neptune. Even Pluto’s moon, Charon, is a world waiting to be walked upon. All of these worlds are so much more than wandering stars in the sky; they are places for us to explore.
Perhaps we should start thinking about them as worlds rather than planets, and we can skip the scientific definition. After all, we all know a world when we see one.