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The solar system is the group of celestial bodies, including
the Earth, orbiting around and gravitationally bound by the
star known as the SUN, one of at least a hundred billion stars
in our galaxy.  The Sun's retinue includes nine planets, at
least 54 satellites (see SATELLITE), more than 1,000 observed
comets (see COMET), and thousands of lesser bodies known as
minor planets (asteroids) and meteoroids (see ASTEROID;  METEOR 
AND METEORITE).  All of these bodies are immersed in a tenuous
sea of fragile and rocky interplanetary dust particles, perhaps 
ejected from comets at the time of their passage through the
inner solar system or resulting from minor planet collisions.
The Sun is the only star known to be accompanied by such an
extensive planetary system.  A few nearby stars are now known
to be encircled by swarms of particles of undetermined size,
however (see PLANETS AND PLANETARY SYSTEMS), and evidence
indicates that a number of stars are accompanied by giant
planetlike objects (see BROWN DWARF).  Thus the possibility of
a universe filled with many solar systems remains strong,
though as yet unproved.
Since primitive times humanity has been aware that certain of
the stars in the sky are not fixed, but wander slowly across
the heavens.  The Greeks gave these moving stars the name
planets, or "wanderers." They were the first to predict with
accuracy the positions of the planets in the sky, and they
devised elaborate theoretical models in which the planets moved 
around combinations of circles that in turn circled the Earth.
The Greek mathematician Claudius Ptolemy systematized an
elaborate geocentric scheme of this kind in the 2d century AD,
which passed with minor changes through the Middle Ages and on
to the Polish astronomer Nicolaus Copernicus (see ASTRONOMY,
HISTORY OF).  In his work of 1543, Copernicus proposed that
planetary motions centered on the Sun rather than on the Earth, 
but he retained the description of planetary motions as being a 
series of superimposed circular motions, mathematically
equivalent to the Ptolemaic theory.  In the same year
Copernicus died.  During the 17th century a German
mathematician by the name of Johannes Kepler abandoned the
concept of circular motion in favor of an elliptical scheme, in 
which the motions of the planets describe a simple series of
ellipses in which the Sun is at one of the foci.  Basing his
work on the observations of Tycho Brahe, his former employer
and a renowned astronomer, Kepler found (1609, 1619) three
important empirical relationships, concerning the motion of the 
planetary bodies, now known as KEPLER'S LAWS.  Kepler's labors
laid the groundwork for Sir Isaac Newton's law of GRAVITATION
(1687), from which it became possible for astronomers to
predict with great accuracy the movements and positions of the
Only the planets Mercury, Venus, Mars, Jupiter, and Saturn were 
known to the ancients.  The English astronomer William Herschel 
accidentally discovered Uranus in 1781 as the result of
telescopic observations.  Discrepancies between the observed
positions of Uranus and those predicted led John Couch Adams
and Urbain Jean Joseph Leverrier to propose (1846) that another 
large planet was exerting a gravitational force on Uranus.  In
the same year the planet Neptune was found close to its
predicted position.  In the 20th century smaller residual
discrepancies in the apparent positions of Uranus and Neptune
led to predictions of the presence of still another planet, and 
in 1930, Clyde Tombaugh discovered the planet Pluto close to
one of the areas of prediction.  Pluto's mass, however, is so
small that the discovery is now considered to have been an
accident resulting from intense scrutiny of that part of the
sky to which the predictions had called attention.  Yet another 
planet may remain to be discovered.
Galileo was in 1609 the first to use the telescope for
astronomical purposes, and it has since become an essential
tool in planetary studies.  In the 19th century planetary
astronomy flourished, thanks to the construction of large
telescopes and their systematic use for planetary observations. 
Two new tools, the spectroscope and the photographic plate,
were also developed in the 19th century and gave rise to the
new science of astrophysics.  For the first time it became
possible to determine not only the orbits and masses of objects 
in the solar system, but also their temperatures, compositions, 
and structures (see ASTRONOMY AND ASTROPHYSICS).  During the
early years of the 20th century great advancements took place
in the understanding of the physics and chemistry of the
planets in the solar system, and during the middle years of the 
century important further advances were derived from RADIO
Although most astronomers gradually turned their attention away 
from the solar system to the study of stars and galaxies, the
launch (1957) of the first artificial satellite ushered in an
age that transformed solar-system studies.  During the 1960s,
1970s, and 1980s spacecraft accomplished flyby, orbiting, or
landing missions to many of the planets.  At the present time
the reconnaissance of the planets in the solar system has been
accomplished for Mercury through Neptune.  The U.S.  MARINER
spacecraft have provided a good model of the atmosphere of
Venus, and the Soviet VENERA spacecraft have returned pictures
from the surface of that planet.  Mariner and VIKING (U.S.)
spacecraft have extensively photographed Mars from orbit, and
the Viking landers have carried out important initial
measurements of surface properties.  U.S.  PIONEER and VOYAGER
probes have returned data and images from the outer planets
Jupiter, Saturn, Uranus, and Neptune.  The investigation of the 
Moon has progressed through the stages of flybys, orbiters, and  
landers both of the manned variety (U.S.  Apollo) and the
unmanned variety (U.S.  RANGER, SURVEYOR, and LUNAR ORBITER,
and Soviet LUNA).  The success achieved in bringing to the
Earth samples from several different lunar landing sites has
made possible a continuing series of laboratory investigations
and further intensive study of Earth's satellite (see SPACE
The Sun is the only star whose surface can be studied in detail 
from the Earth.  This surface presents a scene of churning,
turbulent activity, largely dominated by strong magnetic
fields.  Magnetic lines of force emerging from the solar
surface appear as sunspots.  Arches of the magnetic lines of
force extending across the surface give rise to bright, shining 
solar prominences.  Wave motions generated below the surface of 
the Sun flicker across the surface and mount into the
atmosphere.  Brilliant flares appear in the vicinity of
sunspots, generating bursts of ultraviolet and X-ray emissions
from the Sun and accelerating ions and electrons to create the
high-energy particles known as cosmic rays.

The upper levels of the Sun's atmosphere are of very low
density, but the solar activity heats the gases there to very
high temperatures.  Here the electrons are stripped from atoms
to form ions, and the two types of particles together form a
plasma.  The gravitational field of the Sun is unable to retain 
this superhot plasma, and it streams outward into space as the
solar wind.  Measurements of the properties of the solar wind
are routinely carried out by U.S.  spacecraft at many different 
locations within the solar system.

Most of the mass (99.86 percent) of the solar system is
concentrated in the Sun, which thus exerts the gravitational
force that holds the scattered members of the system together.
There is a remarkable degree of orderliness in the motions of
the members of the solar system under the influence of the
Sun's gravity.  With the exception of the comets, some of the
asteroids, and Pluto, the motions of the bodies in the solar
system are confined to approximately the same plane, called the 
plane of the ecliptic.  There is a striking similarity in the
way in which these bodies revolve and rotate.  The planets all
revolve around the Sun in the same direction, and the Sun
rotates in this direction as well.  With only two exceptions,
Venus and Uranus, the planets also rotate in this common
direction.  Many of the planets, particularly in the outer
solar system, are accompanied by swarms of satellites, and
again, with a few exceptions, these also tend to revolve in a
plane close to the plane of the ecliptic and with the same
sense of motion.  All of these tendencies can be summarized by
saying that the angular momentum vectors of the bodies in the
solar system are for the most part aligned.
The nine planets of the solar system may be divided into two
groups:  the inner, or terrestrial, planets, and the outer, or
Jovian, planets.  This division is based not only on distance
from the Sun, but also on the physical properties of the
The Inner Planets
The inner planets are all comparable in size, density, and
other characteristics to the Earth and so are generally
referred to as the terrestrial, or Earth-like, planets.
Included are Mercury, Venus, Earth, and Mars.
The Earth is the largest of the terrestrial planets.  By far
the most massive constituents of the Earth are the iron core
and the rocky mantle and crust.  The water in the oceans and
the gases in the air form only a thin veneer of volatile
materials surrounding the rock of the planet proper.  The Sun
provides the heat and light that make the Earth habitable for
life as we know it.  The oceans and atmosphere of the Earth
absorb and redistribute the heat in a complex fashion.  Various 
types of geological evidence indicate that the Earth has passed 
through ice ages in the past, but it is not known whether some
unknown variability in the Sun, the great complexity of the
Earth's atmospheric weather system, or some other factor has
been responsible for these (see also MILANKOVITCH THEORY).  The 
early years of the Earth were apparently rather violent, as no
geological record is preserved of the first half-billion years
of its existence.
The Earth-Moon system is often referred to as a "double planet" 
system, because the Moon is more nearly comparable in size to
the Earth than the other satellites are to their primaries
(except for Pluto and its moon).  The Earth's MOON is 81 times
less in mass than the Earth but only 4 times less in mass than
the planet Mercury.  It is one of a group of the six largest
satellites in the solar system that have approximately
comparable mass, and the only such large one in the inner solar 
system.  Compared to the mass of its primary, the Earth, the
Moon is abnormally massive.  The return of samples from several 
lunar sites during the Apollo program, and the establishment of  
stations to measure seismic activity and other physical
quantities at these sites, has provided more knowledge about
the Moon than currently exists for any other body in the solar
system except the Earth.  If the Moon has a central iron core,
it is unexpectedly small, compared to that of the Earth, and of 
surprisingly little mass;  the bulk of the Moon is mantle and
crust that has had an extensive history of melting and chemical 
differentiation.  The Moon contains no atmosphere, and its
surface is heavily cratered.  Its topmost soil is a very
fine-grained substance with little chips of rock sprinkled
throughout.  This is called the lunar regolith.  The Moon is
heavily depleted in the more volatile elements and compounds as 
compared to the Earth.
The next inner planet toward the Sun is VENUS, long considered
a mystery planet because it is shrouded in clouds that hide the 
details of its underlying surface.  Venus is nearly as large
and as massive as the Earth, contains relatively little water,
and has nothing resembling the oceans of the Earth.  Instead,
carbon dioxide in an amount comparable to that in the carbonate 
rocks of the Earth fills the Venusian atmosphere, producing a
pressure at the surface about 100 times higher than that at the 
surface of the Earth and a temperature far too high to support
life of any kind as we know it.  Venus has a slow retrograde
rotation, so that it rotates in a direction opposite to that of 
most of the other objects in the solar system.
The next planet outward from the Earth away from the Sun is
MARS, which is only about one-tenth of the mass of the Earth.
Its tenuous atmosphere is composed principally of carbon
dioxide, with a pressure at the surface more than 100 times
smaller (0.7 percent) than that at the surface of the Earth.
The surface of Mars can be considered to be roughly divided
into two hemispheres, one a surface of ancient, heavily
cratered terrain and the other a geologically younger terrain
having a much lower density of cratering.  Mars has long been
suspected to be a possible abode for other forms of life within 
the solar system, and apparent seasonal differences in its
appearance were attributed to the presence of life.
Experiments performed by the Viking spacecraft landers,
however, found no evidence for the presence of Martian life
forms, however, and it has been found that the Martian surface
apparently contains oxidizing agents highly incompatible with
any form of organic life.
The planet closest to the Sun is MERCURY, a planet whose mass
is half as great as that of Mars.  Mercury has only a trace
atmosphere, consisting of such elements as helium, sodium, and
hydrogen.  Its surface is heavily cratered.  The planet
possesses an interesting resonance with its orbital motion,
presenting first one face and then the other during its closest 
approaches to the Sun.
The Outer Planets
The terrestrial planets just described have in common a rocky
composition whose major constituents have high boiling points
and are therefore described as refractory.  It is believed that 
the entire solar system, including the Sun, was formed from the 
gravitational contraction of a large cloud of gas and dust
composed mainly of hydrogen and helium and only a small
percentage of heavier atoms such as oxygen, silicon, and iron.
The Sun's composition, which is about three-quarters hydrogen
and nearly one-quarter helium, with less than two percent heavy 
elements, is believed to be essentially the same as that of the 
original nebula.  The inner planets lost most of their lighter, 
volatile elements early as a result of their proximity to the
hot Sun, whereas the more distant, cold, outer planets were
able to retain their light gases.  The result is that the outer 
planets became far more massive than the terrestrial planets
and were able to hold very extensive atmospheres of light gases 
such as hydrogen, as well as light, icy substances such as
water, ammonia, and methane.
The most massive planet in the solar system, with about
one-thousandth the mass of the Sun and more than 300 times the
mass of the Earth, is JUPITER.  Composed primarily of hydrogen
and helium, Jupiter may have an interior composed of ice (and
other frozen volatiles) and rocks, or both, exceeding several
times one Earth mass of rocky material and three Earth masses
of the ices.  The total amount of material heavier than
hydrogen and helium is unknown but is probably in the range of
10-20 Earth masses.  Jupiter rotates rapidly on its axis, so
that its figure is significantly flattened toward its
equatorial plane, and the gases in its surface show a banded
structure along lines of latitude.  Infrared measurements from
high-flying aircraft on the Earth and from flyby spacecraft
have determined that Jupiter radiates into space about twice as 
much energy as it absorbs from the Sun;  the additional heat
emerges from the interior of the planet.  Spacecraft also
revealed that Jupiter is ringed.
The next planet outward from Jupiter is the strikingly ringed
SATURN, another gas giant also thought to be composed
predominantly of hydrogen and helium.  Its mass is slightly
less than a third that of Jupiter, but it also appears to have
something approaching 20 Earth masses of heavier materials in
the form, presumably, of icy or rocky materials.  Saturn also
rotates rapidly, is highly flattened toward its equatorial
plane, and exhibits a banded structure along latitude lines.
Beyond Saturn are URANUS and NEPTUNE, two planets of similar
size.  Uranus has a mass about 15 times and Neptune a mass
about 17 times that of the Earth.  Hydrogen and helium
predominate in the atmospheres of both planets.  The planetary
interiors lie hidden beneath thick atmospheres, but data from
Voyager 2 suggest that Uranus has a superheated water ocean, up 
to 10,000 km (6,000 mi) deep, surrounding an Earth-size core of 
molten rock materials.  Although Neptune receives comparatively  
little energy from the Sun, it has an active atmosphere and
apparently has some form of internal energy source.  The
rotation period of Uranus is a little more than 17 hour;  that
of Neptune a little longer than 16 hours.  Uranus is unique
among the planets in being tilted on its rotation axis by about 
98 degrees with respect to the plane of the ecliptic, so that
its rotation is retrograde.  Uranus and Neptune both have ring
PLUTO is a planet whose characteristics were largely unknown
until the discovery of its moon, Charon, in 1978.  Astronomers
report that Pluto's diameter is 2,302 km (1,430 mi) and that
Charon's is 1,186 km (737 mi).  The density of the planet is
about the same as that of water, so that it may be composed of
an ice-rock mixture.  Pluto has a rather elliptical orbit that
at times takes the planet closer to the Sun than Neptune.  From 
1979 until 1999, for example, Pluto will be within Neptune's
orbit.  This would ordinarily be a rather unstable state of
affairs, but perturbations of the Pluto orbit caused by Neptune 
occur in such a way that a collision between the two planets
cannot occur.
Astronomers have also observed perturbations in the orbits of
Uranus and Neptune.  Pluto is too small to cause these
irregularities, and the Pioneer spacecraft have detected no
other sources of gravity.  Some scientists hypothesize that a
tenth planet, called "Planet X," is responsible for these

Of the more than 50 known satellites in the solar system, only
three circle the inner planets.  Earth has its abnormally
massive Moon, and Mars has two tiny satellites, DEIMOS and
PHOBOS.  Very dark and heavily cratered, the Martian satellites 
may resemble the chondritic meteorites (fragile, low-density,
stony-type meteorites that contain large amounts of carbon,
water, and other volatile substances).
Most of the outer planets have large swarms of satellites
attending them.  In many cases the satellites are arranged in
regular orbits that are suggestive of miniature solar systems.
Jupiter has four giant satellites, each comparable in mass to
Earth's Moon, called the Galilean satellites for their
discoverer.  The internal densities of these satellites are now 
reasonably well known as the result of measurements made by the 
flyby Pioneer spacecraft.  The innermost two Galilean
satellites, IO and EUROPA, are largely rocky in composition.
On the other hand, the outer two giant satellites, GANYMEDE and 
CALLISTO, are of a lower density, suggestive of a much higher
ice content.  Closer to Jupiter than these Galilean satellites
is a much smaller one, Amalthea.  These five satellites lie in
the plane of Jupiter's equator and have very nearly circular
orbits.  Because of this ordered arrangement, they are called
the regular satellites.  Three further, very small satellites
were discovered by Voyager spacecraft.
Orbiting far from these regular satellites are the irregular
satellites, in two swarms of much smaller bodies, each only a
few kilometers in radius.  Eight of these bodies are so far
known to exist, and there are indications of additional
members.  The orbits of these satellites are inclined at
substantial angles with respect to the plane of Jupiter's
equator, and the orbits themselves are quite elliptical.  Four
of these small satellites rotate in a direct (west to east)
sense, but the others rotate in a retrograde (east to west)
Saturn also has a system of regular satellites.  One of these,
TITAN, is larger than the planet Mercury and is unique among
the satellites in the solar system in that it has a substantial  
atmosphere.  Four other satellites of Saturn have diameters of
more than 1,000 km (600 mi), but the rest are much smaller.
One of them, Phoebe, has a retrograde orbit.  Studies of
Voyager data have brought the total number above 20.
The five satellites of Uranus visible to Earth-based telescopes 
are closely clustered in the plane of the Uranian equator, so
that the plane of their orbits is also rotated 98 degrees to
the plane of the ecliptic.  These satellites are relatively
small, comparable in size to the lesser regular satellites of
Saturn.  Several much smaller satellites were discovered by
Voyager 2.
The unusual system of Neptune contains one major satellite,
TRITON--whose mass is not exactly known but may be comparable
to that of the Moon--which moves in a circular but inclined
retrograde orbit.  Neptune also has a smaller, direct-rotating
A single moon of Pluto was discovered on June 22, 1978, and
named Charon.  It appears to have about 5-10 percent of the
mass of Pluto, meaning that it is the solar system's largest
moon compared to its planet.
The major planets in the solar system are greatly outnumbered
by the swarms of smaller bodies called minor planets, or
asteroids, and by the even more numerous and smaller bodies
known as meteoroids.  Most of the asteroids exist within the
relatively large gap lying between the orbits of Mars and
Jupiter, whereas meteoroids are randomly distributed.  A few
large asteroids have radii of a few hundred kilometers, but
most are much smaller.  The smaller meteoroids produce meteor
trails when they enter the Earth's atmosphere, and the larger
ones form meteorite craters.  A large number of the asteroids
appear similar to the carbonaceous chondritic meteorites and
are probably of relatively lower density than ordinary rocks.
Nearly 2,000 of the asteroids have accurately determined orbits 
and have been given names.  It is generally believed that the
smaller asteroidal bodies have been created in collisions
involving larger ones, so that there probably exist many small
bodies that have not been detected by photographic surveys.
Many asteroids have orbits that cross the orbit of Mars;  some
cross the orbit of the Earth or go even further into the inner
solar system.  These are called the Apollo asteroids.  It has
been suggested that many of the meteorites that strike the
Earth are chips of the Apollo asteroids caused by collisions.
These asteroids can collide with the Earth or one of the other
terrestrial planets, and some of the major craters that exist
on these planets have probably been caused by such collisions.
Other asteroidal bodies, called Trojan asteroids, have been
observed both 60 degrees ahead of Jupiter in its orbit and 60
degrees behind.  These positions of special orbital stability
are called Lagrangian points.
It is possible that swarms of dust particles are concentrated
in the Moon's orbit, both 60 degrees ahead of the motion of the 
Moon and 60 degrees behind it.  These are sometimes called the
L4 and L5 Lagrangian points.  Although there has not been clear 
confirmation of the presence of these dust swarms, they may
exist in a manner similar to that of the Trojan asteroids with
respect to Jupiter.  There have been suggestions that future
human colonies in space might be established at one of these
Lagrangian points.

Until recently it was believed that minor planets were confined 
to the inner solar system.  In 1977, however, an object was
discovered called CHIRON, a body some hundreds of kilometers in 
radius that orbits between Saturn and Uranus.  This object has
since been classified as a huge comet.

Comets are sometimes spectacular objects from the outer regions 
of the solar system, as far away as a substantial fraction of
the distance to the nearest star.  They appear to be typically
a few kilometers in radius and are composed largely of icy
substances.  Their chemistry is, however, clearly complex.  As
a comet enters the inner solar system, it emits large amounts
of volatile materials that are transformed by the energy of
sunlight and of the solar wind into a variety of individual
atoms, molecules, and ions, mostly of the common materials
carbon, nitrogen, oxygen, and hydrogen, and combinations that
include these.  Many complex molecules have been detected by
spectroscopic analysis of comet tails.  Comets also emit a
large number of tiny dust particles.
The Dutch astronomer Jan H.  OORT recognized (about 1950) that
most of the apparently fresh comets coming into the inner solar 
system started from initial distances beyond 50,000
astronomical units (the distance from the Earth to the Sun is
defined as one astronomical unit).  Furthermore, he recognized
that the ease with which planetary perturbations can change the 
orbits of the comets meant that typical comets were unlikely to 
endure many orbital passages through the inner solar system.
Because several comets are observed each year, this means that
there must be a very large reservoir of them in the outer solar 
system.  Oort suggested that a thick shell of cometary material 
surrounds the Sun about 1,000 times farther out than the orbits  
of Neptune and Pluto.  The Dutch-American astronomer Gerard
Kuiper further suggested a nearer ring of cometary material in
the plane of the solar system.  Any disturbance of these clouds 
can send some material plunging into the solar system to be
observed as a comet.
The sun is also encircled by rings, or disks, of interplanetary 
dust.  One, lying in the zone between the orbital paths of
Jupiter and Mars, has long been known and is the cause of
ZODIACAL LIGHT.  Another ring was found in the region of the
asteroids, between Mars and Jupiter, by the Infrared Astronomy
Satellite (IRAS) launched in 1983.  Also detected in 1983, by a 
team of Japanese and Indonesian astronomers, is a third ring
only two solar diameters away from the Sun.  The dust in this
ring is theorized to spiral slowly inward from the outer solar
system, due to differential absorption and reradiation of solar 
energy, until it is vaporized by the Sun and the resulting
gases are driven back by the pressure of solar radiation.
For more than 300 years there has been serious scientific
discussion of the processes and events that led to the
formation of the solar system.  For most of this time lack of
knowledge about the physical conditions in the solar system
prevented a rigorous approach to the problem.  Explanations
were especially sought for the regularity in the directions of
rotation and orbit of objects in the solar system, the slow
rotation of the Sun, and the Titius-Bode law, which states that 
the radii of the planetary orbits increase in a regular fashion 
throughout the solar system.  In a similar fashion, the radii
of the orbits of the regular satellites of Jupiter, Saturn, and 
Uranus increase in a regular manner.  In modern times the slow
rotation of the Sun has been explained as resulting from the
deceleration of its angular motion through its magnetic
interaction with the outflowing solar wind, so that this
feature should not have been considered a constraint on
theories of the origin of the solar system.
The many theories concerning the origin of the solar system
that have been advanced during the last three centuries can be
classified as either dualistic or monistic.  A common feature
of dualistic theories is that another star once passed close to 
the Sun, and tidal perturbations between the two stars drew out 
filaments of gas from which the planets condensed.  Theories of 
this type encounter enormous difficulties in trying to account
for modern information about the solar system, and they have
generally been discarded.  By contrast, monistic theories
envisage a disk of gas and dust, called the primitive solar
nebula, that formed around the Sun.  Many of these theories
speculate that the Sun and the planets formed together from the 
primeval solar nebula.  A photograph taken in 1984 of a nearby
star, Beta Pictoris, appears to show a solar system forming in
this way from a disk of surrounding material.
The large amount of activity that has taken place in the last
20 years in the renewed exploration of the solar system has
also provided a great impetus for renewed studies of the origin 
of the system.  One important component of this research has
been the detailed studies of the properties of meteorites that
has been made possible by modern laboratory instrumentation.

The distribution and abundance of the elements within different 
meteoritic mineral phases has provided much information on the
physical conditions present at the time the solar system began
to form.  Recent discoveries of anomalies in the isotopic
compositions of the elements in certain mineral phases in
meteorites promise to give information about the local galactic 
interstellar environment that led to the formation of the solar 
system.  Investigations of the properties of other planets has
led to the new science of comparative planetology, in which the 
differences observed among the planets not only lead to a
better understanding of the planets, but also pose precise new
questions concerning the mechanisms by which the planets may
have been formed.
Studies of the stars within our galaxy have shown that the age
of our galaxy is much greater than the age of the solar system. 
Therefore, processes observed in the formation of stars within
our galaxy today are likely to be found relevant to the
formation of our solar system.  Stars appear to form in groups
or associations, as a result of the gravitational collapse of
clouds of gas and dust in the interstellar medium.  Modern
monistic theories envisage the gas and dust in the primitive
solar nebula to be the collapsed remnant of a fragment of an
interstellar cloud.
There has been much discussion of how the planets might have
formed from the primeval solar nebula.  In recent years
attention has focused on the possibility that two types of
gravitational instabilities might have played an important role 
in this process.  One type is a gravitational instability in
the gas of the primitive solar nebula, from which there would
be formed a giant gaseous protoplanet.  From the evolution of
such protoplanets there could arise, in the outer solar system, 
the giant planets that are observed today.  In the inner solar
system, the possibility exists that giant gaseous protoplanets
formed rocky cores at their centers, which survived the
stripping away of the gaseous envelopes caused by gravitational 
and thermal forces from the growing Sun.
The other form of gravitational instability involves the
condensed materials in the solar nebula.  Small dust particles
that may have been present in the gas of the solar nebula could  
be expected to settle toward the midplane of the nebula if the
gas were not subject to extensive turbulent churning.
Gravitational instabilities acting on a thin dust layer might
have formed bodies ranging from tens to hundreds of kilometers
in radius.  Collisions among these bodies may have played a
major role in accumulations of material to form the planets.
It must be stressed that all theories of the origin of the
solar system currently being formulated respond to and are
limited by the rapid accumulation of facts about planetary
bodies within the solar system.  Because of the rapid rate of
progress in such studies, it is generally recognized that such
theories are preliminary and simplified, so that ideas and
theories in this area of research can be expected to continue
to evolve rapidly.
Finally, the movement of the solar system as a whole through
space is defined in terms of the CELESTIAL SPHERE, the
imaginary sphere of the heavens that has Earth at its center.
The solar system appears to be moving toward a point on the
sphere at the velocity, relative to nearby stars, of about 20
km/sec (12 mi/sec).  This point, called the solar apex, lies in 
the constellation Hercules near the star Vega, at a right
ascension of about 18 hours and a declination of about 30
degrees north.
A. G. W. Cameron
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Solar System (1978);  Frazier, Kendrick, Solar System (1985);
Hardy, D.  A., Atlas of the Solar System (1982);  Hartmann, W.
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Keynes, Milton, The Solar System (1984);  Moore, Patrick, et
al., The Atlas of the Solar System (1983);  Morrison, David,
and Owen, Tobias, The Planetary System (1988);  O'Leary, Brian, 
and Beatty, J.  Kelly, eds., The New Solar System, 2d ed.
(1982);  Smoluchowski, Roman, et al., eds., The Galaxy and the
Solar System (1987);  Time-Life Book Editors, The Far Planets
(1989) and The Near Planets (1989).

Planet             Mean Distance                Length of
------    ----------------------------------      year
          Astronomical   Millions   Millions    (Earth days
             Units        of km      of mi      and years)


Mercury     0.387           57.9       36.0     88d
Venus       0.723          108.2       67.0     224.7d
Earth       1              149.6       93.0     365.26d
Mars        1.524          227.9      141.6     687 d
Jupiter     5.203          778.3      483.3     11.86 yr
Saturn      9.539        1,427.0      886.4     29.46 yr
Uranus     19.218        2,875.0    1,786.0     84.01 yr
Neptune    30.06         4,496.6    2,794.0     164.8 yr
Pluto      39.44         5,900.0    3,660.0     248.4 yr
        Length of day                  Inclination
        (Earth days,                   of Orbit to
Planet  hours, minutes,    Inclination  Ecliptic,  Eccentricity   
      and seconds)         of Axis    Degrees    of Orbit
Mercury   58.6 d                 7deg     7.00       0.2056
Venus    243 d (retrograde)     3deg24'   3.39       0.0068
Earth    23 hr 56 min 4 sec    23deg27'   ----       0.0167
Mars     24 hr 37 min 23 sec   23deg59'   1.85       0.0934
Jupiter   9 hr 50 min 30 sec    3deg05'   1.30       0.0485
Saturn   10 hr 14 min          26deg44'   2.49       0.0556
Uranus   23 hr 15 min (retr)   97deg54'   0.77       0.0472
Neptune  22 hr                 28deg48'   1.77       0.0086
Pluto     6 d 9 hr              >50deg   16.00       0.0249
        Equatorial Diameter        Mass
Planet    -------------------     (compared       Density
            km        mi          to Earth)       (g)/cm(3)
Mercury    4,880     3,030          0.054          5.4
Venus     12,104     7,517          0.815          5.2
Earth     12,756     7,921            1            5.51
Mars       6,787     4,210          0.107          3.9
Jupiter  143,000    88,800         317.9           1.32
Saturn   120,000    74,500          95.2           0.7
Uranus    51,100    31,750          14.58          1.2
Neptune   49,500    30,750          17.2           1.67
Pluto      2,302     1,186          0.0026        -1

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