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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."