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Strange Planets 3
Is Pluto a Planet?
In 2006, after years of internal debate, 4% of the members of the International Astronomical Union (IAU)—those meeting in Prague—voted to no longer call Pluto a planet. Instead, they said Pluto is a transneptunian object (i).
The IAU had no jurisdiction to change the definition of “planet” for the rest of the world. It is fine for an organization to tell others what it considers a word to mean, but common usage is the basis for definitions. Our language is filled with scientific words whose meanings have changed based on new discoveries and broader understandings. Few meanings have changed based on an organization’s vote.
Since Pluto’s discovery 76 years earlier, Pluto has been a thorn in the side of astronomers trying to explain how planets evolve, because so many characteristics of Pluto do not fit into evolutionary scenarios. No longer calling Pluto a planet (even though it is spherical, has three known moons, and orbits the Sun in the right direction) may reduce those man-made problems, but now calls attention to the more difficult question of how a thousand transneptunian objects evolved.
In 1930, after astronomers had been searching for a suspected ninth planet for 25 years, a tenacious farm boy from Kansas, Clyde W. Tombaugh (1906–1997), discovered Pluto. He later became one of my favorite professors. Going to his backyard to use his handmade 9-inch telescope was memorable. Professor Tombaugh was a warm, unpretentious man with the biggest smile you have ever seen. However, in class, he sometimes became irate at astronomers who made pronouncements but seldom touched a telescope.
Classification can be a useful tool, but at other times it leads to endless arguments, because the world (or, in this case, the solar system) is usually more complicated than theories imply. We can call Pluto anything we wish, but tens of thousands of books and hundreds of millions of students have called Pluto a planet.
What is a planet? Its original meaning was “wandering star.” I will always associate Pluto with Clyde Tombaugh and the worldwide excitement of finally discovering the ninth planet. For historical reasons, if nothing else, I suspect that millions of others will continue to call Pluto a planet as well as a transneptunian object.
Semantics aside, the scientific question remains: how could Pluto evolve?
i. Far more astronomers and planetary scientists quickly signed a petition opposing the IAU’s vote. They said:
“We, as planetary scientists and astronomers, do not agree with the IAU’s definition of a planet, nor will we use it.”
Jenny Hogan, “Pluto: The Backlash Begins,” Nature, Vol. 442, 31 August 2006, pp. 965.
A transneptunian object is a body that orbits the Sun—usually beyond the orbit of the planet Neptune, about 30 astronomical units, or 2.8 billion miles, from the Sun.
Contributing to the IAU’s decision to remove Pluto’s status as a planet was its small size (two-thirds the diameter of our moon) and the discovery, beginning in 1992, of what are now more than a thousand transneptunian objects, at least two of which are larger than Pluto. All are much farther from the Sun than Pluto.
A simple fix for the IAU would have been to define transneptunian objects as those bodies that always orbit the Sun beyond the orbit of Neptune. (Pluto’s orbit sometimes comes inside that of Neptune.) Also, an honest acknowledgement that all planets are unique would have clarified matters. Even the many planets that have been discovered outside the solar system are completely different from those inside the solar system. Evolutionary process will not explain them all. [See [Have Planets Been Discovered Outside the Solar System? ]
[From “In the Beginning” by Walt Brown ]
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