2 Encyclopedia of the Solar System
use of advanced computer codes and high-speed, dedicated
computers. Serendipity has also played an important role
in many new discoveries.
Along with this increased knowledge have come numer-
ous additional questions as we attempt to explain the com-
plexity and diversity that we observe on each newly encoun-
tered world. The increased spatial and spectral resolution
of the observations, along within situmeasurements of at-
mospheres, surface materials, andmagnetospheres, have
revealed that each body is unique, the result of a different
combination of physical, chemical, and dynamical processes
that formed and shaped it, as well as its different initial com-
position. Yet, at the same time, there are broad systematic
trends and similarities that are clues to the collective history
that the solar system has undergone.
We have now begun an exciting new age of discovery
with the detection of numerous planet-sized bodies around
nearby stars. Although the properties and placement of
these extra-solar planets appear to be very different from
those in our solar system, they are likely the prelude to the
discovery of other planetary systems that may more closely
resemble our own.
We may also be on the brink of discovering evidence
for life on other planets, in particular, Mars. There is an
ongoing debate as to whether biogenic materials have been
discovered in meteorites that were blasted off the surface
of Mars and have found their way to Earth. Although still
very controversial, this finding, if confirmed, would have
profound implications for the existence of life elsewhere in
the solar system and the galaxy.
The goal of this chapter is to provide the reader with an
introduction to the solar system. It seeks to provide a broad
overview of the solar system and its constituent parts, to
note the location of the solar system in the galaxy, and to
describe the local galactic environment. Detailed discus-
sions of each of the bodies that make up the solar system, as
well as the processes that have shaped those bodies and the
techniques for observing the planetary system are provided
in the following chapters of this Encyclopedia. The reader
is referred to those chapters for more detailed discussions
of each of the topics introduced.
Some brief notes about planetary nomenclature will
likely be useful. The names of the planets are all taken
from Greek and Roman mythology (with the exception of
Earth), as are the names of their satellites, with the ex-
ception of the Moon and the Uranian satellites, the latter
being named after Shakespearean characters. The Earth is
occasionally referred to as Terra, and the Moon as Luna,
each the Latin version of their names. The naming sys-
tem for planetary rings is different at each planet and
includes descriptive names of the structures (at Jupiter),
letters of the Roman alphabet (at Saturn), Greek let-
ters and Arabic numerals (at Uranus), and the names of
scientists associated with the discovery of Neptune (at
Neptune).
Asteroidswere initially named after Greek and Roman
goddesses. As their numbers have increased, asteroids have
been named after the family members of the discoverers,
after observatories, universities, cities, provinces, historical
figures, scientists, writers, artists, literary figures, and, in at
least one case, the astronomer’s cat. Initial discoveries of
asteroids are designated by the year of their discovery and
a letter/number code. Once the orbits of the asteroids are
firmly established, they are given official numbers in the as-
teroid catalog: over 136,500 asteroids have been numbered
(as of September 2006). The discoverer(s) of an asteroid are
given the privilege of suggesting its name, if done so within
10 years from when it was officially numbered.
Comets are generally named for their discoverers,
though in a few well-known cases such as comets Halley
and Encke, they are named for the individuals who first
computed their orbits and linked several apparitions. Be-
cause some astronomers have discovered more than one
short-period comet, a number is added at the end of the
name in order to differentiate them, though this system is
not applied to long-period comets. Comets are also desig-
nated by the year of their discovery and a letter code (a
recently abandoned system used lowercase Roman letters
and Roman numerals in place of the letter codes). The nam-
ing of newly discovered comets, asteroids, and satellites, as
well as surface features on solar system bodies, is overseen
by several working groups of the International Astronomical
Union (IAU).2. The Definition of A Planet
No formal definition of a planet existed until very recently.
Originally, the ancients recognized five planets that could be
seen with the naked eye, plus the Earth. Two morejovian
planets, Uranus and Neptune, were discovered telescopi-
cally in 1781 and 1846, respectively.
The largest asteroid, Ceres, was discovered in 1801 in
an orbit between Mars and Jupiter and was hailed as a new
planet because it fit into Bode’s law (see discussion later in
this chapter). However, it was soon recognized that Ceres
was much smaller than any of the known planets. As more
and more asteroids were discovered in similar orbits be-
tween Mars and Jupiter, it became evident that Ceres was
simply the largest body of a huge swarm of bodies between
Mars and Jupiter that we now call the Asteroid Belt. A new
term was coined, “minor planet,” to describe these bodies.
Searches for planets beyond Neptune continued and cul-
minated in the discovery of Pluto in 1930. As with Ceres, it
was soon recognized that Pluto was much smaller than any
of the neighboring jovian planets. Later, measurements of
Pluto’s diameter by stellar occultations showed that it was
also smaller than any of theterrestrial planets, in fact,
smaller even than the Earth’s Moon. As a result, Pluto’s
status as a planet was called into question.