Why Pluto is No Longer a Planet

Source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

If names are not correct, language will not be in accordance with the truth of things.”


Where were you on August 24, 2006? On that day, the International Astronomical Union (IAU) announced that Pluto was no longer a planet. The IAU demoted Pluto to the status of a dwarf planet after over 75 years as our ninth and newest planet. The announcement surprised and shocked me and many people. And what is a dwarf planet, anyway?

A young astronomer, Clyde Tombaugh, discovered Pluto in 1930 at the Lowell Observatory in Arizona. The Observatory had hired Tombaugh to look for a new planet beyond Neptune. Tombaugh pored over photos of the night sky taken by the state-of-the-art telescope, funded by one of the wealthy and prominent Lowells of Massachusetts. This was the first planet discovered by an American. Ancient people knew the five planets visible to the unaided eye: Mercury, Venus, Mars, Jupiter, and Saturn. They recorded many observations. The word “planet” meaning “wanderer” comes from the Greek. These planets seemed to move or wander in the sky, unlike the stars, which seemed fixed to them. Therefore, the ancient astronomers classified both the moon and the sun as planets.

Europeans discovered the next two planets, Uranus and Neptune. When Tombaugh announced a new planet, American astronomers rejoiced. The Observatory asked the public to submit possible names for the new planet and an eleven-year-old schoolgirl from Britain, Venetia Burney, won five pounds sterling for her idea. Fascinated by Greek mythology, she suggested the name Pluto, after the god of the underworld, since she thought the faraway planet would be cold and dark. She was right. Pluto is 3.7 billion miles from the sun on average and nearly a billion miles from its closest solar system neighbor, Neptune. The surface temperature is about minus 400 degrees Fahrenheit and during its day, it gets less than one-thousandth the light from the sun that the earth gets. Some have described it as like a dimly lit room.

A few years later, an aspiring young cartoonist, Walt Disney, a space enthusiast, named a new character Pluto. Disney’s happy-go-lucky dog captured the imaginations and the hearts of people much more than the stern and forbidding Greek god.

From the early days, astronomers realized Pluto differed greatly from other planets. The eight planets all orbit in the same plane, when looked at from the side. The orbit of Pluto, however, tilts at a 17-degree angle, about the incline of San Francisco’s steepest streets. Also, the orbit of Pluto crosses the orbit of Neptune for 20 earth-years out of its 248 earth-year orbit of the sun. So sometimes Pluto was the eighth closed planet instead of the ninth. In addition, Pluto is tiny compared to the other planets. It is only two-thirds the size of our moon. Then the real trouble with Pluto’s status began with the discoveries of many more of these large orbiting bodies in the solar system.

Before the fateful 26th convention of the IAU, astronomers defined planets very generally as large spherical bodies orbiting the sun. Much earlier, astronomers reclassified the sun as a star and not a planet, and that distinction was clear. As people discovered other moons around planets, astronomers decided a planet needed to move independently around the sun in its own orbit, not orbiting a planet. Several of the larger moons around Saturn and Jupiter were bigger than Pluto. With the new discoveries on the edges of the solar system, there were a dozen or more potential planets and the high probability of many more being discovered. How far did astronomers want to stretch the definition of a planet? Would we want to include dozens or even hundreds of planets in our scheme of the solar system? How would we decide on the standards for being a planet? The IAU acted.

The IAU planet definition committee decided on two criteria pretty quickly. First, a planet needed to be an object independently orbiting a star, like our sun. Second, it needed to have enough material, enough mass, for gravity to pull it into a sphere. These two criteria ruled out a lot of the potential candidates but still left a great many, including Pluto. There was much debate within the astronomy community about what a third criterion should be or even if there should be a third criterion. Some people proposed the rule that Pluto and the other eight existing planets were planets and nothing else was. But that purely arbitrary rule lacked the rigor of science and would not stand the test of time. Finally, the committee agreed, though not unanimously, that a planet should have enough gravitational mass to “sweep out” its orbit. 

That means that a planet needs to be big enough to pull in smaller space objects, such as meteors and asteroids, and gobble them up or else sling them away into space. For example, when earth pulls a meteor into its gravitational field, the earth is sweeping out its orbit. It is this third criteria that did in Pluto. Small and large chunks of rocky material strew its orbit and Pluto does not have enough gravity either to pull them in or push them out. Finally, in a very controversial vote, the IAU ruled Pluto out of the planetary club. They created a new category of dwarf planet for Pluto and the other larger space objects that met the first two tests but failed the third.

Thus, Pluto became a dwarf planet. Disappointed and angry people, including some professional astronomers, protested the decision. Pluto’s supporters carrying signs marched in the streets, children wrote letters, and late-night talk show hosts had a field day. Someone coined the word “plutoed” to mean an unfair demotion or snub.

But categories in science are always under review. No one protested or cared very much when biologists created a new domain for Archaea to accommodate extremophiles, creatures living and thriving at extreme temperature, pH level, salinity, and even radiation level. The public does not seem disturbed that physicists will need to revise the Standard Model of fundamental particles to account for the neutrino. Scientific knowledge always changes as we learn more and discover more. After all, we now have exoplanets—planets orbiting stars many light years away. As we go deeper into space, today’s knowledge will surely change again.

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