INTRODUCTION
Until 2006, there were nine planets, ranging from
Mercury out to Pluto, which usually lies beyond
Neptune. In August 2006, international astronomers,
after much argument, eventually voted that Pluto was
to be reclassified as a “dwarf planet.” Ignoring
conventional usage, they also voted that a dwarf planet
isnota planet. For decades following its discovery in
1930, Pluto was thought to be roughly Earth-sized.
Since the late 1970s, when Pluto’s moon Charon was
discovered, it has been realized that Pluto has just
1/500th the mass of Earth.
Two other astronomical developments forced
reconsideration of the definition of “planet.” In 1995,
the first planet orbiting another star was detected. At
least 200 extrasolar planets have since been found, and
discoveries continue. Despite search biases favoring
large planets close to the stars they orbit, most
planetary systems differ greatly from the solar system.
Many have large, Jupiter-sized planets moving very
close to their stars in highly elongated orbits, unlike
the more distant, nearly circular orbits of Jupiter and
Saturn.
In 1992, a small body was discovered beyond Pluto.
Since then, more than 1,000 of these so-called Kuiper
Belt Objects (KBOs) have been found, some quite large.
In 2005, an object now named Eris was found, which is
somewhat larger than Pluto. Is it the “tenth planet”?
Other nearly Pluto-sized KBOs have been sighted, and
astronomers expect that others may exist that are even
larger than Eris.
The International Astronomical Union (IAU),
founded in 1919, has about 9,000 members from 85
countries. Traditionally the IAU has defined official
names for asteroids, craters on planets, and so on.
Although the IAU had never previously defined the
word “planet,” it decided to do so. After failure of its
19-member panel to reach consensus, the IAU
appointed a new group of seven astronomers and
scholars to define “planet.” During a meeting in Paris
in June 2006, the group agreed that a planet must be
large enough to be approximately spherical because its
gravity overcomes its material strength and crushes
any large departures from sphericity. That definition,
with some controversial addenda, was announced at
the beginning of the IAU’s General Assembly (held
every three years) in Prague. The definition would
admit such diverse bodies as Eris, the largest asteroid
(Ceres), and Pluto’s moon (Charon) into planethood,
but not our own moon, Titan, or other Mercury-sized
moons. Soon there could be dozens of such “planets.”
Many astronomers strongly objected. Physicists who
study how bodies orbit each other argued that
planethood should be defined by dynamical
properties; after all, the original definition of planets as
“wandering stars” was based on their motions. The
planet-definition committee’s recommendations were
subsequently rejected. Now, a planet must not only be
largely spherical, but it also must be massive enough
for its gravity to have cleared its orbital neighborhood
of smaller bodies, a theoretical concept. Objects large
enough to be round but deemed too small to have
cleared their zones are now to be called “dwarf
planets.” All other solar system bodies, except the sun
itself and moons of other bodies, are to be called “small
solar-system bodies” of one sort or another—those
asteroids, comets, KBOs, etc., that fail the roundness
test. The IAU decided not to deal with defining
extrasolar planets.
Pluto is no longer considered a planet by the IAU.
Aspects of the new definitions are opposed by many
planetary scientists, including planetary geologists,
planetary atmospheric scientists, astrobiologists,
cosmochemists, and others who study planets but were
not at the astronomers’ meeting in Prague. (The few
hundred who remained for the final votes were mostly
stellar and galactic astronomers, not planetary
astronomers.) Important though it is that things have
names and are grouped into categories, the continuing
controversy over “What is a planet?” reflects cultural
values; it is not science.
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NEW VIEWS OF THE SOLAR SYSTEM
This introduction was contributed by Dr. Clark R. Chapman, Senior Scientist, Department of Space
Studies, Southwest Research Institute, Boulder, Colorado, and author of Cosmic Catastrophesand
others. Dr. Chapman is a past President of Commission 15 of the International Astronomical Union
(which deals with physical properties of comets and asteroids) and a past Chair of the Division for
Planetary Sciences of the American Astronomical Society. Unless otherwise noted, all articles in this
book have been critically reviewed by Dr. Chapman.© 2007 Encyclopædia Britannica, Inc.