You've probably never seen Pluto. It's possible that the only way you remember its name is by reciting the mnemonic, "My very excellent mother just sent us nine pizzas." But by now, everyone has heard that Pluto, the smallest and outermost planet, has been demoted to "dwarf planet" by the International Astronomical Union.
The change, while regretted by some, did not come as a surprise to the astronomy community. Pluto has always been an oddball--tinier than Earth's moon, not solid like the inner planets, not huge like the outer gas giants, and with an orbit that is tilted about 17 degrees from the ecliptic. It is not visible with the naked eye, and was not discovered until the 20th century.
Percival Lowell, a gifted mathematician who floated the "Mars has canals" theory, believed that there were more eccentricities in the orbit of Uranus than could be explained by Neptune, discovered in 1846. He thought that there must be another planet beyond Neptune. He spent the last years of his life in search of this "Planet X" from his observatory in Arizona.
Meanwhile, a Kansas farm boy, Clyde Tombaugh, observed the night sky through a 9-inch telescope he built using farm machinery and automobile parts. As a young adult, he found that farming was not to his liking, and he sent some of his astronomical sketches to the Lowell Observatory, hoping for some advice. Instead, he was offered a job. In 1929, he began a detailed photographic search of the ecliptic. Less than a year later, on February 18, 1930, Tombaugh noticed an object shifting as he compared photographic plates. He told the observatory director, "I have found your Planet X."
It soon became apparent that this new object was not Planet X after all. It was far too small to have a gravitational effect on Uranus or Neptune. It is now believed that Pluto is a Kuiper Belt object, a ball of ice that is, essentially, a comet without a tail.
More recent advances in astronomy showed that there were other largish objects orbiting the sun. At least three of them--the asteroid Ceres, Pluto's moon Charron, and an unnamed object nicknamed Xena by its discoverer--are large enough to challenge for the title of planet. Recognizing the inevitability of such challenges, the IAU settled the matter last week, stripping Pluto of the title it has held for 76 years.
Pluto's legacy has not ended, however. Clyde Tombaugh had a long career in teaching and research, and remained an avid stargazer long after his retirement. (When the Smithsonian asked him to donate his original 9-inch telescope, he famously replied, "I told them I was still using it.") On January 17, 2000, the ninth anniversary of Tombaugh's death at age 90, NASA launched the New Horizons probe to explore Pluto and the Kuiper Belt. On board New Horizons is a container with some of Clyde Tombaugh's ashes.
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