assiduous campaigning by its discoverers at Arizona's Lowell Observatory, distant
Pluto was hastily declared a planet in 1930, and ever since then schoolchildren
have been taught with great conviction that there are nine large bodies, or "major
planets," revolving around our Sun. Yet if it were discovered today, Pluto
would certainly not make the grade in the majors. For starters, it's too small-less
than half the size of Mercury and smaller than our own Moon-and it has no gravitational
influence on the orbits of the other eight planets. Brutal as it might seem, Pluto
has been demoted, though since the early nineties scientists have been haggling
over what diminutive name it deserves: asteroid, minor body, dwarf planet, or
perhaps the ultimate slap, planetesima...
distant, ice-covered world is no longer a true planet, according to a new definition
of the term voted on by scientists August 24, 2006. "Whoa! Pluto's dead,"
said astronomer Mike Brown, of the California Institute of Technology in Pasadena,
as he watched a Webcast of the vote. "There are finally, officially, eight
planets in the solar system."
a move that will force textbooks to be rewritten, Pluto will now be dubbed a dwarf
planet. But it's no longer part of an exclusive club, since there are more than
40 of these dwarfs, including the large asteroid Ceres and 2003 UB313, nicknamed
Xenaa distant object slightly larger than Pluto discovered by Brown last
know of 44" dwarf planets so far, Brown said. "We will find hundreds.
It's a very huge category."
clear majority of researchers voted for the new definition at a meeting of the
International Astronomical Union (IAU) in Prague, in the Czech Republic. The IAU
decides the official names of all celestial bodies. The tough decision comes after
a multiyear search for a scientific definition of the word "planet."
The term never had an official meaning before.
Is a Planet Today?
to the new definition, a full-fledged planet is an object that orbits the sun
and is large enough to have become round due to the force of its own gravity.
In addition, a planet has to dominate the neighborhood around its orbit. Pluto
has been demoted because it does not dominate its neighborhood. Charon, its large
"moon," is only about half the size of Pluto, while all the true planets
are far larger than their moons.
addition, bodies that dominate their neighborhoods, "sweep up" asteroids,
comets, and other debris, clearing a path along their orbits. By contrast, Pluto's
orbit is somewhat untidy. The new definition also establishes a third class of
objects that orbit the sun"solar system bodies," which would apply
to many asteroids, comets, and moons.
new definition of "planet" retains the sense that a true planet is something
going to be hard to find a new planet," Brown said. "You'd have to find
something the size of Mars. Finding a new planet will really mean something."
for now the vote is drawing some opposition. Planetary scientist Andy Cheng said
the definition is ambiguous, because it hasn't answered the question "how
round is round?"
will be an issue in the future," Cheng said. "Dozens of objects are
going to be straddling this line. The new definition is not going to help us with
still continue to maintain that Pluto is a planet," he said.
proposed definition was meant to apply to all planets in the universe. But, faced
with the difficulty of arriving at a consensus on universe-spanning criteria,
the IAU committee narrowed the definition to apply only to our solar system.
Conn Henry is an astrophysicist at Johns Hopkins University in Baltimore, Maryland.
He says he never considered whether Pluto should be a planet until a few years
ago. But when the planetarium at New York City's American Museum of Natural History
removed Pluto from the ranks of the planets, it got him thinking.
tiny thing in this oddball orbita planet? Give me a break!" Henry said.
think that, when the dust settles, people will recognize that there really are
just eight planets."