Solar System

The solar system formed from a giant cloud of dust and gas. Over 99.8% of the mass ended up in the Sun, our nearest stellar neighbor. Most of the rest was distributed into the nine planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto - with Jupiter comprising over twice as much mass as the rest of the planets put together.

This was the view of our solar system until 1801 when the first asteroid was discovered. Over two hundred years later, over 300,000 bits of rocky debris left over from the formation of our solar system have been found; some have been discovered to lie over 13 times farther from the Sun than Pluto. Nearly a century before the first asteroid was found, comets were realized to be phenomena that occur outside of Earth's atmosphere and that they are small rock-ice balls that orbit the Sun just like everything else in the solar system.

Our understanding of our local region of space has evolved significantly over the past few centuries, and it continues to change today. In 1992, the first object that spends its entire year beyond Pluto - a Kuiper Belt Object - was found. In 2005, a Kuiper Belt Object larger than Pluto was discovered. Space probes operating at and on Mars in the past few years have completely revised our understanding of Mars, and in 2004, the first probe to sample particles from the Sun returned to Earth.

Our current picture of the solar system has one central star, eight main planets and their associated moons, a large number of asteroids between Mars and Jupiter and more asteroids that lie throughout the solar system, and then a gradually increasing disk of asteroids and comets beyond the orbit of Pluto in the Kuiper Belt. Beyond that lies the Oort Cloud, which is a hypothesized cloud of comets that might extend half-way to the next star. This section contains information on all of this and more.

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