The
Solar System
The place in space we call the Solar System consists of a not very
spectacular star in an average spiral galaxy we call the Milky Way Galaxy.
It has nine major planets, at least 60 planetary satellites or moons,
countless asteroids and comets and all space in between. Four of the
planets, Jupiter, Saturn, Neptune and Uranus have ring systems and seven
have one or more satellites, only Mercury and Venus do not possess a
satellite. Thousands of asteroids are in orbits between Mars and Jupiter,
in a region known as the asteroid belt, while most of the several billion
comets travel around the Sun in a spherical region approximately 50,000
times farther out from the Sun than the orbit of the Earth.
The Sun and its planets
Containing more than 99% of the mass of the Solar
System, the Sun lies at the centre of the Solar System; all the planets
and asteroids move around the Sun in elliptical orbits in the same direction
as the Sun rotates, counterclockwise.
The planets are divided into two distinct categories on the basis
of their place in the Solar System. The inner terrestrial planets, Mercury,
Venus, Earth, and Mars, are small high density planets with rocky composition.
They lack ring systems, and have few or no satellites. Among the planets
of the inner Solar System, only the Earth has a strong magnetic field.
In contrast, the outer Jovian planets, Jupiter, Saturn, Uranus, and
Neptune are large low density gaseous planets. The outer Jovian planets
are much more massive than the terrestrial planets and have immense
atmospheres composed mainly of hydrogen (H2) and helium (He).
Each of the outer planets, except for Pluto, has a magnetic field, a
ring system, and many satellites.
Pluto is a unique outer planet in that it is an icy, low-density body
smaller than the Earth's Moon, resembling a giant comet nucleus or an
icy satellite of one of the outer planets. Pluto has no rings and only
a single satellite, Charon.
Satellites or moons
Of the more than 60 satellites known, most orbit the Jovian planets.
They are extremely diverse in composition and represent a wide range
of environments. One of Jupiter's satellites, Io, is wracked by intense
volcanism, while Saturn's largest satellite, Titan, exhibits a primitive
atmosphere denser than that of the Earth. Neptune has a satellite, Triton,
which moves "backwards" in a retrograde orbit and features
plumes of material rising through its tenuous atmosphere from a surface
that is very cold at only 37 K or -236°C. Pictured are Io (top left)
and Titan (right).
Asteroids
The asteroids represent remnants of the planet-building process in the
inner Solar System. The asteroids found in the asteroid belt between
Mars and Jupiter are rocky bodies, ranging in size from the largest
known, Ceres, with a diameter of 930 kilometres to the microscopic dust.
Comets
Comets are very different from asteroids. They are made mainly of ices,
laced with dust and a rich variety of organic compounds. Typically,
a comet is an irregularly shaped object with a nucleus a few kilometres
across. The vast majority of comets spend most of their time at immense
distances from the Sun, way outside the orbit of Pluto, in fact up to
one-third of the way to the nearest star.
Not all comets exist in the dark reaches of outer space. Halley's
Comet has a period of only 76 years, as opposed to the normal several
million years. Halley's Comet is an example of a short-period comet
that has been captured into a smaller orbit by a close encounter with
Jupiter.
Interplanetary space and the solar
wind
Space within the Solar System also contains protons, electrons,
and ions all streaming outward from the Sun in the form of the solar
wind. Giant flares or Sun spots on the Sun's surface expel matter, along
with light and other high-energy radiation into the near space of the
Solar System.
Exactly where the boundary between the Solar System and the interstellar
space lies has not yet been determined, but four spacecraft have recently
passed the orbit of Pluto with velocities that will allow them to escape
from the Solar System. Thus, this boundary may well be crossed in the
near future.
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