Published in The Patriot Ledger April 13, 2013:
My Very Excellent Mother Just Served Us Nine Pizzas. That was never literally true in my case, but when I was young, that mnemonic helped us remember the sequence of the planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. What do we do now that Pluto is no longer officially a planet? Does my mother just serve us Nachos, or perhaps Nothing?
Some date the downfall of Pluto to 2000, when New York’s redesigned Hayden Planetarium featured models of the four inner or terrestrial planets and the four outer “gas giant” planets, but omitted Pluto. Visiting children, who had been taught the nine planets in school, often asked, “Where’s Pluto?” That soon led to a front-page New York Times article headed, “Pluto’s Not a Planet? Only in New York.”
The planetarium was soon deluged with hate mail from angry schoolchildren, many of who loved Pluto in part because they associated it with Disney’s loveable cartoon canine of that name (although the planet was actually named after the Greek god of the underworld, far less loveable). “Pluto is my faveret planet,” wrote one third-grader, “You are going to have to take all of the books away and change them. Pluto is a planet!!!”
Pluto is small, with a diameter of only about 1500 miles, less than half the diameter of Mercury and even smaller than our moon and several other moons of the solar system. Many of the third graders suspected that the downgrading of Pluto was simply a case of size discrimination, a common bias with which they were personally familiar.
The term “planet” is derived from the Greek word for “wanderer,” since the planets move around the sky among the fixed stars. The first six planets were known to the ancients, and are visible to the naked eye, the Earth by looking down, the others by looking up. Uranus, the seventh planet, was first observed by telescope by William Herschel in 1781, Neptune, the eighth, by Johann Galle in 1846. Irregularities in the motion of Neptune suggested the possibility of another planet farther from the sun, and after years of search for this “planet X,” Pluto was finally discovered by Clyde Tombaugh in 1930.
In January 2006, NASA launched the New Horizons spacecraft to explore Pluto and its moons. Only seven months later, the International Astronomical Union met in Prague and voted to approve the first formal definition of a planet, a definition that officially demoted Pluto, supporting the stance taken six years earlier by the Hayden Planetarium.
To be a planet, the IAU ruled, an object must (1) orbit the sun, (2) be massive enough for gravity to make it spherical, and (3) have cleared the neighborhood around its orbit. It was the third rule that killed Pluto. By 2006, hundreds of other objects had been found orbiting the sun out in Pluto’s neighborhood. Most are smaller than Pluto, but one discovered in 2005, called Eris, is more massive than Pluto. Rather than call Eris the tenth planet, the IAU classifies it and Pluto as dwarf planets and trans-Neptunian objects, i.e., objects beyond the orbit of Neptune.
Meanwhile, the robotic spacecraft New Horizons continues its multi-billion-mile journey to the cold outer reaches of the solar system, and is expected to reach poor little Pluto in July 2015. To me, and to all those former third-graders, Pluto’s still the ninth planet. And I still prefer nine pizzas to nachos or nothing.