Born in the darkness of interstellar space some 4.6 billion years ago, out solar system emerged from a contracting molecular cloud of dust and gas. When the core of this cloud became dense enough, gravity triggered the collapse of its inner layers. Matter fell inward for perhaps a million years. Internal heat, trapped by the gases, fired up a protosun and ignited thermonuclear reactions. Rotation drew the material into swirling nebular disk. Its hot innermost constituents drifted farther inward, enlarging the protosun. A powerful gaseosus outflow from the solar inferno blasted away the infalling material, unveiling our newborn Sun in all its brilliance. The cooler, more distant portions of the nebula contained the raw material of a planetary system.
Tiny dust grains in the disk consisted of solid carbon, silicates, and metals as well as ices of volatile materials such as water, carbon dioxide, methane, and ammonia that had condensed as the solar nebula continued to cool. These particles coalesced into rocky or icy clumbs - asteroids or comets - called planetsimals, the building blocks of planets and their satellites. Since planetesimals aggregated at different distances from the sun's heat and light, the composition of the planets varies. Those nearer the sun are more rocky, those farther away more icy.
Four terrestrials, or earthlike, planets inhabit the inner realm of the solar system: Mercury, Venus, Earth, and Mars. Each is a solid sphere with a metallic core surrounded by silicate shells. In times past all were alive with volcanoes and earthquackes and, alongwith other bodies in our system, were created by meteorites; today of the four only Earth is known to be tectonically active. Gases exhaled by volcanoes became the athmospheres of Venus, Earth and Mars. The athmosphere of Mars, primarily carbon dioxide, has a surface pressure less than one-hundredth that of Earth, while the athmosphere of Venus, also mostly CO2, produces a crushing surface pressure nearly a hundred times greater then our own. Mercury, so small and close to the sun that its gases were quickly lost to space, is nearly airless.
Separating the inner and outer planets is a belt of asteroids, flotsam from the early solar system. These fragments of rock and iron range from a thousand kilometers in diameter to less than one.
The four other giant outer planets - Jupiter , Saturn , Uranus, and Neptune - contain 99% of the material in the solar system, excluding the sun. All are huge gaseous spheroigs of hydrogen and helium with some methane, ammonia, water, and other volatiles. Beneath their cloud layers, gaseous hydrogen becomes denser with depth - at one level becoming liquid and at still greater depths in Jupiter and Saturn changing to a metalic state. All probably have cores of metals, silicates, and water.
Three of these giants radiate much more heat back into space than they absorb from the sun. This residue of primordial energy is like the warmth from dying coals. Curiously, Uranus has almost no such excess heat.
Satellites - some large enough to be planets in their own right - either formed along with the giants or were captured later. the four biggest moons of Jupiter have silicate cores and, with the exception of Io, are surounded by shells of ice and possibly liquid water. The surface of Io, belived to be the most volcanically active body in the solar system, spurts liquid sulfur, sulfur dioxide, and perhaps silicates.
Orbiting at the outer edge of the planetary realm, Pluto and its satelite, Charon - more than half of Pluto's size - are considered a double planet. Pluto has a tiny atmosphere made up in part of methane gas, and both bodies probably consist of ices such as water and methane. Pluto may well be an intermediate object between planets and comets. Composed primarily of ices, most comets circle the dark fringes ot the solar system, barely within the sun's gravitational grasp.
For more information contact NASA site.
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