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Pluto and its three companions.

Pluto (center), now classified as a dwarf planet, has three smaller companions: Charon, Nix and Hydra.

A planet by any other name

In speaking about the plight of Pluto, astrophysicist Terry Jones offered a glimpse into the workings of modern astronomy

By Deane Morrison

Sept. 19, 2006

NASA's New Horizons spacecraft lifted off from Cape Canaveral in January on the first mission to explore the ninth planet. When the spacecraft arrives in September 2014, however, it will find itself not at the ninth planet but at a dwarf planet because its target, Pluto, has been demoted. Too bad, because things had been looking up for Pluto, what with the discovery of two new moons last year. But, as University astronomy professor Terry Jones explained to an enthusiastic audience last week, even 75 years of affection must give way when new knowledge forces new ways of thinking. Speaking in Nolte Center on the Twin Cities campus, Jones opened a window on how astronomers work, revealing the human and technical frailties that sometimes, oddly enough, lead to progress. The story of Pluto is the story of how astronomers in the modern era have discovered that the solar system isn't a neat, organized little merry-go-round. Rather, the sun sits at the hub of an eclectic system that operates like a roller derby, with orbiting bodies crashing into one another (like the collision thought to have formed Earth's moon), perturbing each other's orbits (as we shall see with Uranus and Neptune) and throwing smaller bodies for a loop (as when planets hurl comets into the outer reaches of the solar system). For much of human history, only six planets were recognized: Mercury, Venus, Earth, Mars, Jupiter and Saturn. But astronomy took a giant leap in 1781, when the great English astronomer William Herschel discovered Uranus. The big bluish planet had been seen before, but not recognized as a planet, Jones said.

How round is the Earth?

According to the new definition, a planet must be round. But how about Earth, with all its mountains and seven-mile-deep oceanic trenches? Says University astrophysicist Terry Jones: "If you licked a billiard ball and blew it up as big as Earth, [the wet coating] would be the oceans. If you played pool with the billiard ball and blew it up, the [nicks from the cue stick] would be the trenches in Earth's oceans." It seems our planet is round indeed.

All was fine until about 1821, when it became clear that the orbit of Uranus wasn't following the predictions of Newton's law of gravity. Astronomers figured there had to be an eighth planet whose gravity was causing Uranus's orbital irregularities, and, sure enough, in 1846 German astronomer Johann Gottfried Galle found Neptune. It was no accident. "They knew where to look," said Jones. It wasn't long before anomalies showed up in the orbit of Neptune, too, and the hunt was on for a ninth planet. Enter the central figure in Pluto's history, Illinois-born Kansas farm boy Clyde Tombaugh. Tombaugh built his own backyard telescope. "He built his own telescope," said Jones, "and he sent his sketches of Jupiter and Saturn to Lowell Observatory [in Flagstaff, Ariz.]." Tombaugh's talent as an observer was recognized, and soon he was at work at the observatory taking pictures of the sky in search of the elusive ninth planet. The search was simple in principle. Tombaugh made photographic plates of big pieces of the sky, taking two pictures of each piece at an interval of several days. Then he used a machine called a "blink comparator" to compare the plates for each piece. The stars lined up exactly, but because planets move against the background of stars, planets appeared in one position in the first plate and at a different position in the second. And so Tombaugh found a faint little spot that moved. It was named Pluto, based on a suggestion from Venetia Burney, then an 11-year-old English schoolgirl, who thought Pluto's dark, cold realm recalled the domain of the Greek god of the underworld. Thanks to his discovery, Tombaugh won a scholarship to the University of Kansas and became a respected astronomer. He died in 1997, unaware that his status as the only American to discover a planet would be short-lived. "The New Horizons spacecraft is carrying some of his ashes on the way to Pluto, and we're talking about taking away its planetary status," Jones commented. The first warning sign appeared in 1940, 10 years after Pluto's discovery. It was found, said Jones, that the anomalies in Neptune's orbit didn't exist. In reality, the mass of Neptune had been mismeasured, and the orbit was correct for a planet of Neptune's actual size. Another sign appeared in March 2001, when the Hayden Planetarium in New York removed Pluto from its display of major planets. Then the dam really broke. In November 2003 came the discovery of Sedna, a round body slightly smaller than Pluto that orbits far beyond Neptune. Pluto's orbit brings it as close as 2.8 billion miles from the sun (crossing inside the orbit of Neptune) and as far as 4.6 billion miles, while Sedna orbits between about 7 billion miles and 90 billion miles from the sun. In January 2005, Eris, a body bigger than Pluto, was found orbiting between about 3.5 billion and 9 billion miles from the sun. And what about Ceres, the largest asteroid, orbiting between Mars and Jupiter? Pluto, Ceres, Sedna and Eris are all round bodies orbiting the sun. Why shouldn't they all be planets? And what if many more are found beyond Neptune? But Pluto has other problems. Unlike the other planets, its orbit is markedly inclined from the plane of the solar system, and it was the only planet whose orbit crossed inside that of another planet. A tiny ball of ice and rock, it seemed less a planet and more just a big member of the Kuiper Belt, a band of icy worlds circling the sun beyond Neptune. The International Astronomical Union (IAU), whose job includes naming celestial objects, wrestled with the definition of "planet." In September the IAU adopted a definition: A planet is a body that orbits a star, not another planet; whose mass is big enough for gravity to form it into a round shape; and that sweeps up debris from its orbit. Pluto failed on the third count and was relegated, along with Ceres, Sedna and Eris, to the new designation of "dwarf planet." Pluto will be the model for a new class of objects, as yet unnamed. "How we name something has a lot to do with how we study it," said Jones. "Could you, as NASA, go to Congress and say, 'We want to go to Pluto, the prototype of a new class of objects that may be called plutinos?'" No, Jones made clear, it's much better to ask for funding to explore a planet. In the audience was physics professor Robert Pepin, who has made a career of studying how the solar system formed. He said the thing he found most amusing about the whole episode was how the estimates of Pluto's size have changed over the years. "Tombaugh guessed how big Pluto was," said Pepin. "As more people estimated its size, it got smaller. If you plot Pluto's size versus the year of the estimate, you get an almost straight line that would go to zero this year."