Planet Formation
The Sun formed from a nebula
The inner planets are mostly rock and metal
The outer planets are mostly ice and gas
Earth's Moon formed separately from the rest of the solar system
The solar system was born about 4.5 billion years ago, when something disturbed and compressed a vast cloud of cold gas and dust -- the raw material of stars and planets. The disturbance may have been a collision with another cloud, or a shock wave from an exploding star.
Whatever the cause, the cloud fragmented into smaller, denser pockets of matter, which collapsed inward under the pull of gravity. In perhaps 100,000 years, one of the pockets, called a nebula, condensed into a volume about the size of the present-day solar system. In the dense center of the nebula, a star formed -- our Sun.
The newborn Sun was still surrounded by its nebula, which was spread into a thin disk because the nebula was spinning slowly.
Atoms and molecules within the nebula combined to form larger particles. The Sun determined what kinds of particles could exist. Close to the Sun, solar heat vaporized ices and prevented lightweight elements, like hydrogen and helium, from condensing.
Inner Planets
This zone was dominated by rock and metal, which clumped together into ever-larger bodies, called planetesimals, eventually forming the rocky inner planets:
Outer Planets
In the solar system's outer region, though, it was chilly enough for ices to remain intact. They, too, merged into planetesimals, which in turn came together to form the cores of the giant planets:
Plenty of hydrogen and helium remained in this region far from the Sun. As the giant planets grew, their gravity swept up much of these leftovers, so they grew larger still. Jupiter and Saturn contain the largest percentages of hydrogen and helium, while Uranus and Neptune contain larger fractions of water, ammonia, methane, and carbon monoxide.
Most of the moons probably formed at the same time as their parent planets. Earth's Moon probably formed a bit later, when a body several times as massive as Mars slammed into our planet. The collision blasted a geyser of hot gas and molten rock into orbit around Earth; the material quickly cooled and coalesced to form the Moon.
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