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Q. Why isn’t Pluto a planet anymore? When did that happen?
A. In “Mission to Pluto: The First Visit to an Ice Dwarf and the Kuiper Belt,” Mary Kay Carson writes: “In August 2006, the International Astronomical Union voted to create a new category of space objects called dwarf planets. The solar system went from having nine planets to having eight, plus three dwarf planets.
“Pluto was a declared a dwarf planet along with Ceres, the biggest asteroid, and a newly discovered large Kuiper Belt object called Eris.” Eris’s first, informal name, was Xena, as in “Xena: Warrior Princess,” a television series.
Carson lists the IAU’s three requirements for planet status. “A planet must: orbit the sun; be round or nearly round, because its gravity has pulled it into that shape; and be big enough and have enough gravity to ‘clear the neighborhood’ around its orbit.”
She explains that last phrase: “It might help to think of planets as the schoolyard bullies of the solar system. In order to clear the neighborhood, a planet has to be big enough, and have enough gravity, to get rid of any celestial objects in its way.
“A large planet might clear its orbit by using its gravity to pull other, smaller, objects toward it and destroy them the way asteroids are destroyed when they hit Earth. Or a planet might clear its orbit by attracting smaller objects toward it, then turning them into moons that remain in orbit around the planet. Sometimes a planet will simply push a smaller body into a completely different orbit and get rid of it that way. “Dwarf planets have the
“Dwarf planets have the following characteristics. They must: orbit the Sun, be round, and not be a moon or satellite of any other planet. Pluto is a dwarf planet.”
Let’s detour to another dwarf planet, Ceres, for a moment. If you were in school before 1930, you’d have been taught that Ceres, not Pluto, was the ninth planet.
In his captivating “Pluto Files: The Rise and Fall of America’s Favorite Planet:” Neil DeGrasse Tyson writes: “On January 1, 1801, the Italian astronomer, Giuseppe Piazzi, discovered the planet Ceres happily and silently orbiting the Sun between Mars and Jupiter. The suspiciously large gap between Mars and Jupiter had finally been filled.
“But astronomers rapidly determined that Ceres was much, much smaller than any other planet. And then came another discovery — Pallas, another tiny planet in the same orbital zone.”
Tyson points out that controversies over the categorization of celestial bodies is nothing new. William Herschel, who discovered Uranus in 1781, wrote in an 1802 letter: “You know already that we have two newly discovered celestial bodies. Now by what I shall tell you of them it appears to me much more poor in language to call them planets than if we were to call a razor a knife, a cleaver a hatchet, etc.
“They certainly move around the sun, so do comets. It is true they move in ellipses; so, we know, do some comets also…Now as we already have Planets, Comets, Satellites, pray help me to another dignified name as soon as possible.”
Tyson continues, “By 1807, three more of these diminutive planets had been discovered: Pallas, Juno and Vesta. By 1851, 11 more had been logged, and the solar system’s planet count reached 18.
“By 1853, it was clear that a new class of objects had been identified: the asteroids. These bodies occupied a new swath of real estate in the solar system: the asteroid belt.
“Ceres was discovered first because it’s the brightest and largest of its class. At twice the mass of all other asteroids combined, of which there are hundreds of thousands known, Ceres swiftly went from being the smallest in the class of planet to being the largest in the class of asteroid.”
In 1846, English and French mathematicians — John C. Adams and Urbain Le Verrier — working independently, predicted the position of a planet using mathmatics. A German astronomer named Johann G. Galle found Neptune, the 8th planet, based on their calculations.
When and where was Pluto found? In America, specifically the Lowell Observatory in Flagstaff, Arizona. Tyson writes, “At about four in the afternoon on February 18, 1930, 24-year-old Clyde W. Tombaugh, a farm boy and amateur astronomer from Illinois, discovered what would shortly be named for the Roman god of the underworld.
“The name Pluto was first suggested over breakfast on Friday, March 14, 1930, by Venetia Burney, an 11-yearold schoolgirl in Oxford, England, after her grandfather had read her the news story that Lowell Observatory had discovered a new planet.
“With the name Pluto not yet taken she blurted out to her grandfather, ‘Why not call it Pluto?’ knowing that Pluto is, after all, the god of the dead and underworld, the realm of darkness. And what else, if not darkness, prevails 4 billion miles from the Sun?
“Venetia’s grandfather, Falconer Madan, was a retired librarian from the Bodleian Library of Oxford University who happened to be friends with many astronomers. Madan suggested the name to Herbert Hall Turner, Oxford professor and former astronomer royal. Turner promptly cabled the name to fellow astronomers at the Lowell Observatory.”
