Jewitt and Luu have now discovered sixty objects in what has come to be known
as the Kuiper Belt, named after the astronomer Gerard Kuiper, who suggested the
existence of such a belt in the 1950s. About a third of them are in Pluto-like
orbits, and all of them appear to be, like Pluto, amalgams of ice and rock. As
a result, few astronomers now question that Pluto should be regarded as a
member of the Kuiper Belt. But does that mean it shouldn't be considered a
planet? After all, though Pluto's diameter of approximately 1,400 miles makes it
tiny for a planet, it is huge for a Kuiper Belt object; the next largest known
member is only about 300 miles across.
Actually, Marsden points out, astronomers have had to ask themselves this sort
of question before, and under circumstances that could be seen as clearly
establishing a precedent for Pluto. When the small, rocky body later named
Ceres was discovered between the orbits of Mars and Jupiter, in 1801, it was
proclaimed a planet. A year later a second rocky body was found in a similar
orbit; several other discoveries along the same lines soon followed. Even
though the approximately 600-mile-wide Ceres is nearly twice the size of the
next largest asteroid, it was evident that it was merely the largest member of
what we now call the asteroid belt. In 1802 Ceres's planethood was summarily
revoked.
Marsden sees little ambiguity in the situation: Pluto should follow Ceres's
trail into nonplanethood. "If you are going to call Pluto a planet, there is no
reason why you cannot call Ceres a planet," he says. And no one, he points out,
is campaigning to have Ceres's planethood restored.
It would help if "planet" had a formal definition against which Pluto could be
measured, but none exists. Astronomy got by quite nicely for thousands of years
on a we-know-one-when-we-see-one basis. But now that questions about Pluto are
forcing the issue, many astronomers find themselves gravitating toward one or
the other of two proposed definitions. The first is "a non-moon, sun-orbiting
body large enough to have gravitationally 'swept out' almost everything else
near its orbit." Among the nine planets Pluto alone fails this test, and it
does so spectacularly, owing to the Kuiper Belt. The second is "a non-moon,
sun-orbiting body large enough to have gravitationally pulled itself into a
roughly spherical shape." Pluto passes this test—but so do Ceres, a half dozen
or so other asteroids, and possibly some other members of the Kuiper Belt.
"It's very difficult to come up with a physically meaningful definition under
which we'd have nine planets," says Hal Levison, an astronomer at the Southwest
Research Institute, in Boulder, Colorado.
Does it matter what we call Pluto? Marsden insists that it does. By calling Pluto a planet, he says, astronomers perpetuate a distorted and thoroughly
outdated image of a solar system that neatly ends in a ninth planet, rather
than trailing off beyond Neptune into a far-reaching and richly populated field
of objects. "It gives a misleading impression to the public and particularly to
schoolchildren," he says. "We ought to be explaining that there are four giant
planets, four terrestrial planets, two belts of minor bodies, and scattered
interesting material."