| Overview
 Our
solar system consists of the sun and all of its orbiting
objects. These objects belong to various classes, including
planets and their moons and rings; asteroids; comets;
meteors and meteorites; and particles of dust and debris.
The sun, which keeps the other objects in orbit with
its immense gravitational field, alone accounts for
99.8 percent of the mass of the solar system. Jupiter,
the largest planet, represents another 0.1 percent of
the mass, and everything else together makes up the
remaining 0.1 percent. A planet is a body that orbits
the sun (or another star) and produces no light of its
own, but reflects the light of the sun or star. At present,
scientists know of nine planets in our solar system.
They are grouped into three categories: the solid, terrestrial
planets; the giants, gaseous (also know as "Jovian")
planets; and Pluto.
 The
first group of planet consist of Mercury, Venus, Earth,
and Mars, the planet closest to the sun. The next group,
farther from the sun, consists of Jupiter, Saturn Uranus,
and Neptune. The third group consists of a single planet,
Pluto, the smallest planet, farther away even than the
string of gas giants.
Astronomers have speculated about the existence of two
other planets in our solar system: Vulcan, between Mercury
and the sun, and Planet X, beyond Pluto. However, despite
exhaustive effort, neither planet has as yet been found.
A moon is any natural body (as opposed to a man-made
satellite) that's orbits a planet. Seven of our solar
system's planets are accompanied on their journeys around
the sun by moons. In total, these planets are orbited
by sixty-one moons. This number will probably change
as a result of new findings like the recent unconfirmed
sighting of four additional moons around Saturn. Although
moons do not orbit the sun independently, they are still
considered members of the solar system.
History
Today the theory of the solar system's formation considered
most likely to be correct is a modified version of the
eighteenth-century nebular hypothesis. The current theory
states that 4.56 billion years ago the sun and planets
formed from the solar nebula-a cloud of interstellar
gas and dust.
Due to the mutual gravitational attraction of the material
in the nebula, and possibly triggered by shock waves
from a nearby supernova, the nebula eventually collapsed
it on itself. As the nebula contracted, it spun increasingly
rapidly, leading to frequent collisions between dust
grains. These grains stuck together to form ever larger
objects, first pebbles, then boulders, and then planetesimals.
 These
planetesimals continued to stick to solid particles
as well as gas (in what's known as the accretion theory)
and eventually gave way to protoplanets, planets in
their early. As the nebula continued to condense, the
temperature at its core rose to the point where nuclear
fusion could begin. It then became a star (our sun)
and the bodies farther from the core became the planets.
 While
the nebular hypothesis was popular in the 1800s and
the modified nebular theory is preferred today, there
was a period in the early 1900s when another group of
theories were in fashion-the encounter theories. These
theories all stated, in one way or another, that the
planets were created by a collision between a foreign
object (such as another star) and the sun. This resulted
in the ejection material from the sun, which cooled
to form the planets. This theory has been rejected for
two main reasons. One is that such material would have
likely remained very close to the sun and not scattered
at the distances of the planet, and the other is that
solar material would be more likely to dissipate than
to come together.
 Some
scientists at the time believed that the solar system
had been created when another star had passed by the
sun millions of years ago. The star's gravitational
field, they proposed, had pulled material away from
the sun, and this material then formed into the planets.
British astronomer Arthur Stanley Eddington's research
into the structure of stars disproved this popular theory.
Eddington showed that any material pulled from a star's
core would explode into a thin gas when it was removed
from the star's balance of energy production and gravity.
 British
astronomer Fred Hoyle (1915- ), who served as a professor
of astronomy and philosophy at the Institute of Theoretical
Astronomy at Cambridge, England, has made detailed studies
of the nuclear reactions that take place in the core
of a star. He has also researched the gravitational,
electrical, and nuclear fields of stars and the various
elements formed within them. Hoyle is the author of
several books on stars, both technical and for general
readers, as well as a number of science fiction stories
and even a script for an opera. Hoyle proposes that
the solar system was formed out of the remains of an
exploded star that was once paired with our sun.
|
Welcome To W3Network
This site is evolving
into a 
