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Our Solar system Scientists believe the solar system began forming 10 to 12 billion years ago as swirling gas and dust formed a thick core. The core, with most of the mass, collapsed around 5 or 6 billion years ago and later became the Sun. The remaining matter swirled into a disk. Some of it crashed together and formed planets. That's the main theory, at least, and though most scientists think that's how it happened, there are a few other explanations. Planets are part of our solar system. Comets, asteroids and the planets' moons are part of the solar system, too. These things all orbit around a star we call the Sun. And it's all held together by gravity. Mercury is the closest planet to the Sun. Because Mercury is so close to the sun it is difficult to spot from Earth. Venus, the second planet from the Sun, bakes under twice as much solar radiation as Earth. Venus reaches temperatures of 480° degrees Centigrade! Earth is the third planet from the Sun - and a lot of people's favourite! Like the other three planets of the inner solar system our planet is made up mostly of rock and metal. The fourth planet from the Sun has always made us humans do a lot of wondering and hoping. While scientists haven't proven there's any life on Mars, the dusty red planet is still a fascinating place. The fifth planet from the Sun is mostly a huge ball of gas. But at Jupiter's centre is a core of rock many times the mass of Earth. The whole planet is so massive it could hold all the other planets put together! Saturn is the sixth planet from the Sun and has a rocky core and a lot of gas on its surface. Saturn is known for its rings. Uranus, the seventh planet from the Sun, is a lot like its neighbours, with a cloudy surface, strong winds, and a small rocky core. Neptune is the eighth planet from the Sun. It has a rocky core surrounded by ice, hydrogen, helium and methane. Pluto, which is smaller than our Moon, is a cold, dark, frozen place. We don't know very much about the tiny planet. Scientists think it is made of rock and ice. Check that you understand what these words mean in this document To swirl: to move quickly in a circular motion. Core: centre. To collapse: to break down. Orbit: a (usually elliptical) path followed by one celestial body in its revolution about another. Gravity: the force that attracts things to the ground. Fascinating: very interesting. Massive: huge. For further information visit: Solar System Tour Solar System – National Geographic