The solar system formed 4.6 billion years ago from a collapsing molecular cloud, with the Sun and planets forming at the same time. Planets formed by accretion, with rocky planets closer to the Sun and gas giants farther away. Jupiter’s gravity created the asteroid belt, and beyond that is the Kuiper belt and interstellar space.
The solar system, which includes the Sun and all the planets that orbit it, originated about 4.6 billion years ago. This figure was derived from radiocarbon dating of meteorites, such as the Canyon Diablo meteorite, which is approximately 4.6 billion years old.
Scientists believe that the Sun and the rest of the solar system formed at about the same time, when a giant molecular cloud – which would have been several light-years in diameter – gravitationally collapsed into a condensed mass, forming several stars beyond to our Sun. This is called the nebular theory, and while it is not perfect, it explains most of the current structure of our solar system.
When the density within the collapsing cloud reached some extreme level, nuclear fusion would be initiated and the Sun would be born. Orbiting the Sun would be a disk of debris that eventually condenses into spheres that make up the planets. Also called a protoplanetary disk, many of these have been observed in distant nebulae within our Galaxy. The densest part of these disks are areas called Bok globules, where stars are believed to have been born. The actual star birth process is hidden from our telescopes by the opaque dust that surrounds it.
Planets formed by accretion, where dust-sized chunks of matter orbiting the early Sun accumulated into planetesimals and eventually full-fledged planets. This accretion process would have taken at least 30 million years, possibly more. The Earth itself formed about 4.57 billion years ago, about 30 million years after the origin of the solar system.
Closer to the Sun, the temperature was too high for volatile substances such as water and methane to condense, so small rocky planets, made of iron and silicates, formed here. Today, these make up the inner solar system and include Mercury, Venus, Earth and Mars. Farther from the Sun, the temperature was low enough for the volatiles to condense and the great gas giants formed. One gas giant, Jupiter, is so large that its gravitational pull continues to shatter the rocks between its orbit and that of Mars, creating the asteroid belt. These gas giants make up the outer solar system. Beyond the outer system is another asteroid belt, the Kuiper belt, and beyond that, interstellar space.
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