||The solar system took shape 4,600,000,000
years ago, when it condensed within a large cloud of
gas and dust. Gravitational attraction holds the planets
in their elliptical orbits around the Sun. Besides the
Earth, five major planets (Mercury, Venus, Mars, Jupiter,
and Saturn) have been known from ancient times. Since
then, only three others have been discovered: Uranus
by accident in 1781, and Neptune and Pluto in 1846 and
1930, respectively, after deliberate searches.
The average Earth–Sun distance was originally defined
as the astronomical unit (a.u.) and provides a convenient
measure for distances within the solar system. The astronomical
unit is now defined dynamically (using Kepler's third law),
and it has the value 1.49597870 ´ 1013 centimetres.
The semimajor axis of the Earth's orbit is 1 + (3.1 ´
10-8) a.u. Mercury, at 0.39 a.u., is the closest planet
to the Sun, while Pluto, at 39.5 a.u., is the farthest.
The planes of the planetary orbits (other than that of Pluto)
are all within a few degrees of the ecliptic, the plane
that contains the Earth's orbit around the Sun. As viewed
from far above the Earth's North Pole, all planets move
in the same (counterclockwise) direction in their orbits.
All of the planets, apart from those closest to the Sun
(Mercury and Venus), have satellites very diverse in appearance,
size, and structure, as revealed through closeup observations
from long-range space probes. Four planets—Jupiter,
Saturn, Neptune, and Uranus—have rings consisting
of small rocks and particles that are confined to disklike
systems as they orbit their parent planets.
Most of the mass of the solar system is concentrated in
the Sun, with its 1.99 ´ 1033 grams. Together, all
of the planets amount to about 2.7 ´ 1030 grams, with
Jupiter alone accounting for 71 percent of this. The solar
system also contains a very large number of much smaller
objects. In order of decreasing size, these are the asteroids
(also called minor planets), comets, meteoroids, and dust
The four terrestrial planets, Mercury, Venus, Earth, and
Mars, along with the Moon, have average densities in the
range 3.9–5.5 grams per cubic centimetre (g/cm3),
setting them apart from the outer planets, whose densities
are all close to 1 g/cm3, the density of water. The compositions
of these two groups of planets must therefore be significantly
different. This is probably attributable to the conditions
that prevailed during the early development of the solar
system. Planetary temperatures now range from about 500°
C on Mercury's surface through the typical 20° C on
Earth to -135° C on Jupiter and down to -230° C
The surfaces of the terrestrial planets and many satellites
show extensive cratering produced by high-speed impacts.
On Earth, with its large quantities of water and an active
atmosphere, many of these cosmic footprints have eroded,
but remnants of very large craters can be seen in satellite
and aerial photographs of the terrestrial surface. On Mercury,
Mars, and the Moon, the absence of water and any significant
atmosphere has left the craters unchanged for billions of
years, apart from disturbances produced by infrequent later
impacts. Cratering on the largest scale seems to have ceased
about 3,000,000,000 years ago, but there is clear evidence
for a continued cosmic drizzle of small particles, with
the larger objects churning (gardening) the lunar surface
and the smallest producing microscopic impact pits in crystals
in the lunar rocks.
Theories Of Origin
The origin of the Earth, Moon, and solar system as a whole
is a problem that has not yet been settled in detail. The
Sun probably formed by condensation of the central region
of a large cloud of gas and dust, with the planets and other
solar-system bodies forming soon after, their composition
strongly influenced by the temperature and density gradients
in the evolving solar nebula. Less-volatile materials could
condense into solids relatively close to the Sun to form
the terrestrial planets. The abundant, volatile lighter
elements could condense only at much greater distances.
The origin of the planetary satellites is not settled.
There is still the question as to the origin of the Moon,
and professional opinion has been oscillating between theories
that see its origin and condensation simultaneous with the
formation of the Earth, to an explanation in terms of a
large impact on the Earth resulting in the expulsion of
material that subsequently formed the Moon. For the outer
planets with their multiple satellites, many very small
and quite unlike one another, the picture is even less clear.
Some of the objects have icy surfaces, while others are
heavily cratered, and at least one, Jupiter's Io, is volcanic.
Some of the satellites may have formed along with their
parent planets, and others may have formed elsewhere and
||During the U.S. Apollo missions, a total sample weight
of 381 kilograms was collected; 300 grams of lunar material
also was returned by three unmanned Soviet Luna space
vehicles. Less than 10 percent of the samples has so
far been distributed for analysis, but planetary science
has been revolutionized by these expeditions. A wide
range of laboratory techniques has been employed on
the lunar samples. The results of the analysis have
enabled investigators to determine the composition and
age of the lunar surface. Seismic techniques have made
it possible to probe the lunar interior. In addition,
a retroreflector left on the Moon's surface by Apollo
astronauts returns a high-power laser beam emitted from
the Earth, enabling researchers to monitor on a regular
basis the Earth–Moon distance to an accuracy of
a few centimetres. This experiment provides data that
can be used in calculations of the dynamics of the Earth–Moon
Study Of Planets
Mercury is too hot to
retain an atmosphere, but Venus' brilliant white appearance
is the result of its being completely enveloped by
thick clouds of carbon dioxide. Below the upper clouds
it has a hostile atmosphere containing clouds of sulfuric
acid droplets. The cloud cover shields the planet's
surface from direct sunlight, but the energy that
does filter through warms the surface, which then
radiates at infrared wavelengths. The long waves of
infrared radiation are trapped by the dense clouds,
resulting in a very high surface temperature of almost
480° C. Radar can penetrate the thick Venusian
clouds and has been used to map the planet's surface.
The Martian atmosphere is very thin, only about 0.006
that of the Earth, and composed mostly of carbon dioxide
(95 percent), with very little water vapour.
The outer planets have atmospheres composed largely of
light gases. For example, hydrogen and helium, along with
some methane and ammonia, have been detected on Jupiter.
Each of the planets rotates on its axis, and nearly all
of them rotate in the same (counterclockwise) direction,
as viewed from above the ecliptic. The two exceptions are
Venus, which rotates in the clockwise direction beneath
its cloud cover, and Uranus, which has its rotational axis
very nearly in the plane of the ecliptic.
Some of the planets have magnetic fields. The Earth's field
extends outward until it is disturbed by the solar wind,
an outward flow of protons and electrons from the Sun that
carries a magnetic field along with it. Through processes
not yet fully understood, protons and electrons from the
solar wind and cosmic rays populate two doughnut-shaped
regions called the Van Allen radiation belts, the inner
of which extends from about 1,000 to 5,000 kilometres above
the Earth's surface and the outer from roughly 15,000 to
25,000 kilometres. In these belts, trapped particles spiral
along paths that take them around the Earth while bouncing
back and forth between the Northern and Southern hemispheres.
During periods of increased solar activity, these regions
of trapped particles are disturbed, and some of the particles
move down into the Earth's atmosphere where they collide
with atoms and molecules to produce auroras.
Study Of Minor Bodies
||Approximately 3,500 asteroids have now
been identified. Most have orbits close to the ecliptic
and move in the asteroid belt located between 2.3 and
3.3 a.u. from the Sun. Only about 250 of these objects
are larger than 100 kilometres, and their total mass
is thought to be roughly 1/2,000 that of the Earth.
Comets are considered to come from a vast reservoir,
the Oort cloud, which orbits the Sun at distances of
30,000 to 100,000 a.u.
More than 600 comets have so far been discovered. Most
make only a single pass through the inner solar system,
but some are deflected by Jupiter or Saturn into orbits
that allow them to return at predictable times. Comet Halley
is the best known of these periodic comets, with its next
return predicted for AD 2060. About 30 comets have periods
of less than 100 years. Comet masses have not been well
determined, but most are thought to be less than 1018 grams,
or 1,000,000,000 times smaller than the Earth.
Even smaller than comets are the meteoroids, lumps of stony
material. Meteoroids vary in size from small rocks to large
boulders weighing a ton or more. A few have orbits that
bring them into the Earth's atmosphere and down to the ground
as meteorites. These are classified into three broad groups:
stony or chondrites (about 93 percent), iron (5.7 percent),
and stony-iron (1.5 percent). Smaller meteoroids that enter
the atmosphere may heat up sufficiently to vaporize and
appear as meteors. Many, perhaps most, of the meteors occur
in showers and follow orbits that seem to be identical with
those of certain comets, thus pointing to a cometary origin.
For example, each May the Earth crosses the orbit of Comet
Halley, and the Eta Aquarid meteor shower becomes visible.
Micrometeorites, the smallest meteoroidal particles, can
be detected from Earth-orbiting satellites or be sufficiently
slowed by atmospheric friction to be collected by specially
equipped aircraft flying in the stratosphere and returned
for laboratory inspection.