The solar system consists of the Sun and those bodies, such as planets, satellites, asteroids, and comets, which are trapped by the Sun's gravity into a variety of orbits around it. The Milky Way Galaxy had been in existence for billions of years when the Sun was born. Millions of stars had been born and died. In the gaseous swirls of the Galaxy, they left a rich "soup" of chemical elements in the form of gases and dust. These stars provided the raw materials for future stars and planets. A cloud of gas and dust began to contract, possibly triggered by the explosive death of a nearby star. As it spun, it's core became denser and heated up until it reached nuclear ignition point. The Sun was born.
The new Sun did not use up all of the available material in the rolling gas cloud from which it was formed. While the Sun began to shine with fusion reactions, particles in the cloud around it began to coalesce to form other much smaller bodies. These were the planets. Most scientists think the planets originated in the same gas and dust cloud from which the Sun condensed.
Almost certainly, as the Sun was formed, it was surrounded by a flattened, rotating disk made up of many clumps of material. A long series of collisions helped to coalesce some of these clumps into sizeable bodies. As time passed, they attracted any smaller fragments that came within range of their fields of gravity. Some heated up enough to melt as they contracted. The heavier materials then became concentrated in the central core of the objects.
The nine planets, of which the Earth is one, orbit the Sun at different speeds. Compared to the Sun, the planets are tiny. All together, they have only one-thousandth of the mass of their parent star, the Sun. Just as the planets orbit around the Sun, the moons or satellites of the Solar System rotate around their parent planets.
All planets are dark; the light that appears to shine from them is the reflected light from the Sun. The stars, on the other hand, shine with the luminosity of their own burning energy. When early stargazers began to look closely at the sky, they traced the patterns of the stars in their fixed constellations and noted the apparent movement of the constellations across the sky. By the early 1500s Nicolaus Copernicus developed the idea that the Sun was the center of the universe, with the planets orbiting around the Sun. This Sun-centered, or "heliocentric," version of the Solar System overturned the old traditional doctrine that the Earth was the center of the Solar System
