Why Pluto is no longer considered as a planet
By Steve Humphrey
My friend Tom Miller recently asked me to explain why Pluto is no longer regarded as a planet. Generations of grade school students were taught that there were nine planets orbiting the Sun, and we learned different mnemonics to help us memorize their order. For example, “My Very Elderly Mother Just Sat Up, Noticing Pastry.” (I made that up.)
When the first humans looked up into the night sky, they saw a collection of bright dots, now recognized as stars. These stars maintain their positions relative to one another, which allows us to see the same constellations (e.g., the Big Dipper) the ancients did thousands of years ago. These early astronomers also saw, over days or weeks, some stars move against the background of the fixed stars. These were called “planetes” or “wanderers” in ancient Greek. Some of these wanderers regularly returned over the course of a year, while others came and went. The latter was probably comets. Occasionally, a bright star would appear, blaze for a few weeks, then disappear. These were known to the Chinese as “guest stars,” now known to be novae or supernovae, stars that explode with unimaginable energy.
Galileo was one of the first to train a telescope on the heavens. By then, five planets had been identified through naked eye astronomy: Mercury, Venus, Mars, Jupiter and Saturn. The Copernican revolution was still in its infancy, so it took a while longer to recognize that Earth was also a planet. In fact, Galileo was sentenced to house arrest by the Church for supporting the view that the Sun was at the center of the Universe and not the Earth.
As telescopes got bigger and more powerful, astronomers were able to see further and further, and more “planets” were spotted. Uranus was discovered next in 1781 by William Herschel, and then Neptune was discovered, not by astronomers but by a French mathematician, Urbain Le Verrier. He noticed that the orbit of Uranus contained an anomaly. That is, its orbit was inconsistent with Newton’s theory of gravitation and suggested it might be explained by the gravitational influence of a more distant object. He calculated where such an object had to be in order to affect the orbit of Uranus and told the astronomers to look there. They scoffed but did it and, sure enough, found Neptune.
A similar anomaly was seen in the orbit of Mercury, so, flushed with his success, Le Verrier postulated a new planet, which he named Vulcan, residing inside the orbit of Mercury. It would always be very close to the Sun, so it makes sense that we would never see it since it would be obscured by the Sun’s bright light. Unfortunately, it was never found, and the explanation for the “anomalous precession of the perihelion in the orbit of Mercury” would have to wait until Einstein’s theory of gravity.
Pluto was discovered in the traditional way, in 1930, by Clyde Tombaugh, by observing its motion against the fixed stars, and generations of students thereafter were taught about the nine planets revolving around the Sun.
But not everything that orbits the Sun or transits the solar system is a planet. For example, the asteroid belt between Mars and Jupiter consists of a great many rocky bodies ranging in size from tiny to fairly large. The largest, Ceres, was discovered in 1801. But how do we distinguish between genuine planets and asteroids and other objects? There has to be some kind of criterion, though it will always be somewhat arbitrary. In 2006 the International Astronomical Union decided upon just such a set of criteria. The three criteria for being a planet are: it must be in orbit around the Sun, it must be massive enough to be rounded by its own gravity, and it must have cleared the neighborhood of its orbit. Pluto fails this last criterion, as does Ceres and the rest of the asteroids, and these are now regarded as dwarf planets.
Our best theory of the formation of the Solar System is that a cloud of gas formed and became gravitationally bound with net angular momentum. This cloud gradually flattened out into a pancake shape with a bulge at the center. The bulge condensed under gravitational influences and became the Sun, and the planets formed by accretion in the disc revolving around it and clearing their orbits of remaining debris. But Pluto’s orbit suggests that it didn’t form in this way. Pluto’s orbit is highly eccentric, moving above and below the plane of the ecliptic, and it even sometimes travels inside the orbit of Neptune.
Further, it lives in a cloud of icy bodies called the Kuiper Belt. As more and more Kuiper Belt objects were found, it seemed unreasonable to insist that Pluto remain a planet. If Pluto is a planet, so are the other such objects, and the population of planets grows dramatically. The mnemonic would be hopelessly large.
So, Tom, I hope this answers your question, and I hope you enjoyed our little lesson in planetary astronomy. If any of my readers have questions about science or philosophy, please let me know, and I’ll do my best to satisfy your curiosity.
Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics.
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