The Solar System

The Solar system consists of the Sun and eight orbiting planets (or nine if you include Pluto). The four planets closest to the Sun are rocky, with all except Mercury (the closest) possessing an appreciable atmosphere. The other planets are gaseous bodies, with perhaps a core consisting of ice or rock or a combination. The composition reflects a temperature gradient during the formation of the solar system. The inner planets were the in the hottest region, so are composed of less volatile materials, while the outer planets were formed in a cooler region so are composed of more volatile materials. Between the rocky and gaseous planets is the asteroid belt, consisting of rocky bodies which never formed into or became part of a planet.

The orbit of each planet is an ellipse around the Sun as focus. Many of the orbits are almost circular, but the orbit of Pluto is highly elliptical, and Pluto is for part of it's orbit, closer to the Sun than Neptune.

Distance in the Solar System are vast. The Earth is about 150,000,000,000 m (metres) from the Sun on average and Pluto is about 6,000,000,000,000 m . Even the closest planet, Mercury is about 58,000,000,000 m from the Sun on average.

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