General Information
Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; the interplanetary medium; and the kuiper belt and oort cloud. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun's nearest known stellar neighbour is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the centre of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is travelling through intergalactic space.
The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. The former planet Pluto, and many of its Kuiper Belt neighbours are a special cases in that their orbits are highly inclined and highly elliptical. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exception is Uranus, which is tipped on its side.
The Origin of the Solar System
Five billion years ago a cloud of hot swirling dust and hydrogen gas gave birth to our Sun and planets. As the cloud spun and collapsed inwards it flattened into a central mass with a surrounding disk. Dust and gases in the disk formed small condensations each spinning about its own centre. Gravitation condensed and heated the central mass. Density increased dramatically and [nuclear] fusion began. Energy was released and our Sun flared into existence. The solar wind of the newly ignited Sun blew away leftover dust and gas in the vicinity of the inner condensations, leaving the rocky inner planets: Mercury, Venus, Earth and Mars. In the outer regions of the disk, the solar wind was weaker. The remaining dust and gas condensed into the larger gaseous planets: Jupiter, Saturn, Uranus and Neptune.
The Earth, Sun, and all the planets and their satellites formed through condensation in an interstellar cloud of gas and dust. The Horsehead Nebula in Orion consists of dust-laden material photographed against a background of hot gas. Deep within the clouds of gas and dust, new stars are forming now, some perhaps with planets. As the cloud of gas and dust that formed the solar system began to contract, it must have acquired some rotation, which led to more rapid rotation as the cloud grew smaller. This rotation tended to support the cloud against contraction in directions perpendicular to the axis of rotation, and thus led to a pancake-like shape for the contracted, rotating cloud.
When the cloud that would form the solar system first began to contract, it must have done so as a condensation with some rotation. The rotation was slow at first but grew more rapid as the cloud shrank, for much the same reason that figure skaters who pull in their arms spin more quickly. The combined result of the contraction and rotation was a spinning, disk-like solar nebula, within which gas and dust had much greater density than they did before the contraction began. The nebula was densest of all at its centre, where the protosun began its final condensation. By the time the sun grew so dense that nuclear fusion reactions began inside it, the pancake-shaped cloud had begun to form agglomeration at various distances from its centre. The rather regular spacing of the planets' orbits from the sun apparently reflects the way in which matter accumulated within the disc-like configuration.
The eight planets orbit the sun in nearly circular trajectories that all (except for Mercury) lie in very nearly the same plane. The Sun contains 99.9 percent of the mass in the solar system, and the four giant planets have the bulk of the 0.1 percent residue. The Earth, largest of the four inner planets, has only 1/318 of Jupiter's mass and 1/329,000 of the sun's mass. The four giant planets differ most strikingly from the four inner planets (Mercury, Venus , Earth, and Mars) in their size and composition. The giant planets are large, gaseous, rarefied, and hydrogen-rich, while the inner planets are small rocky, dense, and hydrogen-poor. Because the giant planets consist mostly of hydrogen and helium, they resemble the universe at large. The inner planets are distinctly different: Though the universe consists mostly of hydrogen, the Earth does not.
A relatively simple explanation exists for the extreme differences between the four giant planets and the four inner planets. As nuclear-fusion reactions began in the sun's deep interior 4.5 billion years ago, the solar nebula close to the sun grew much warmer than the dust and gas at greater distances. This warming had a profound effect on the kinds of material that could condense and accumulate into "planetesimals," the small objects that can collide to form planets. At distances close to the sun - less than about five times the Earth-sun distance (5 astronomical units, or A.U.) - the sun's heat prevented ice from forming. This fact had significant consequences, since ice is potentially the most abundant solid in the universe.
