How and when did the solar system form?
The Sun and planets formed about 4.5 billion years ago from a massive cloud of hydrogen, helium, and pinches of heavier elements like oxygen, carbon, nitrogen, silicon, and iron. As the material in the cloud clumped together, its gravity pulled in even more material, so the clumps grew larger and larger. At the center of this cloud, the largest clump became a "protostar" -- a glowing cloud that is not yet a full-fledged star. Gravity compressed the core of this collapsing ball of gas until its own temperature and pressure were high enough to begin fusing hydrogen into helium at its core, causing the Sun to release energy. Our star was born.
The material left over from the Sun's creation existed as gases and tiny mineral grains (dust) throughout the cloud, and frozen gases in the cooler, outer parts of the cloud. In the inner, hotter regions the dust grains of heavier elements came together to form Mercury, Venus, Earth, and Mars (the terrestrial planets). In colder regions farther from the Sun, frozen water, methane, and ammonia added their material to the growing planets. These outer planets grew so large so quickly that their gravities attracted the nearby hydrogen and helium gases as well. These gases are now the main ingredients in Jupiter, Saturn, Uranus, and Neptune: giant planets that may have small, rocky cores beneath thousands of miles of clouds.
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