About 5 billion years ago, a vast, cold cloud of interstellar gas and dust, called a nebula, began to collapse. A star formed at the center, surrounded by a spinning disk of gas and dust. Small grains condensed, accreted into smaller rocky bodies, and ultimately grew into planets. Thus was born our solar system—the Sun and all the planets, moons, comets, asteroids, and meteoroids that revolve around it.
This solar nebula theory is well supported by modern observations of other nebulas, disks, and planets, and by radiometric dating. Samples of the Moon, Earth, Mars, and meteorites (the remnants of meteoroids that entered the atmosphere as meteors and landed on Earth) have a similar age of 4.56 billion years.
What initially caused the nebula to begin to collapse remains unknown. Some scientists suggest the death of a distant massive star in a violent, luminous explosion called a supernova sent a shock wave through space, compressing the cloud. Others propose instead that radiation emitted by a nearby massive star prior to its death could have nudged the nebula to collapse.
The debate centers on evidence from meteorites, which preserve a record of the chemical composition and conditions at the time of solar system formation. The following video describes this chemical composition evidence of meteorites and its relationship to the hypothesis on the formation of our solar system.
"Origins of the Solar System." Teachers' Domain. 31 Mar. 2011. Web. 14 Sep. 2011. <http://www.teachersdomain.org/resource/nsn11.sci.ess.eiu.solarorigins/>.