The word "solar" refers to the sun; the sun is one of the 150 billion stars of the Milky Way. It moves through space taking with it a larger family of objects. The whole group is called the solar system. Our solar system is elliptical in shape. The sun is the center of the solar system. Solar system is always in motion. Its largest and most important members are the nine known planets and their moons, along with smaller objects called comets, asteroids, and meteoroids that orbit the sun. The sun is the biggest object in our solar system. It contains 99.8% of the solar system's mass. Many scientists believe that our Solar System is over 4.6 billion years old. Scientists believe that the solar system was formed when a cloud of gas and dust in space was disturbed, may be by the explosion of a nearby star called SUPERNOVA. This explosion made waves in space that squeezed the cloud of gas and dust. Squeezing made the cloud start to collapse, as gravity pulled the gas and dust together, forming a solar nebula. The sun's nuclear fires ignited at the dense center of this nebula. The planets were born in the swirling currents of the great cloud. The planets Mercury, Venus, Earth, and Mars, evolved as globes of rock that are present near the Sun. They were too small and their gravitational fields too weak to capture. However, far from the sun, the massive planets Jupiter and Saturn, with powerful gravitational fields, did attract and hold thick gaseous atmospheres of Hydrogen and Helium.