Current theory holds that our Solar System formed 4.56 billion years ago from a rotating cloud of gas and dust, the solar nebula. Gravitational attraction of solar nebula material led to the formation of the Sun, center of the Solar System and location of the greatest percentage of its mass. Material not incorporated into the Sun aggregated into far smaller bodies, planetesimals, each a few kilometers in diameter. The Sun is a G spectral type star of average size. The age of the Solar System is determined not by the study of Earth rocks but by radioisotopic dating of meteorites, the most primordial materials to which we have access.
As the early Solar System evolved, planetesimals accreted with other planetesimals to form the planets. Nine planets formed (8 with a parsimonious view) at varying distances from the Sun, each planet orbiting the Sun at its own pace, each consisting of a spherical mass of gas and melted rock. As they cooled during the first tens of millions of years of the Solar System, the smaller or terrestrial planets (Mercury, Venus, Earth and Mars) lost their volatile gases (carbon dioxide, water, methane, and ammonia) to surrounding Solar System space. They formed solid, crusty surfaces over molten cores. The larger planets, called giant, jovian, or gaseous (Jupiter, Saturn, Uranus, and Neptune), consisted mostly of hydrogen, helium and other light gases and retained their gaseous condition. The ninth planet, Pluto, is farthest from the Sun and is unlike any of its neighboring jovian planets. Its status as planet is discussed below. These major planets have 60 satellite bodies (moons) in orbit which are themselves structurally diverse.
A vast number of original planetesimals did not aggregate to form the Sun or the planets. They remained as smaller bodies, called planetoids. Mankind has been aware of one category of planetoid, the comets, for more than 2000 years. Comets are grouped into two broad types, the long-period comets and the short-period comets, and they reside in different regions of the Solar System. Taken together, they comprise the most distant objects from the Sun in the Solar System. Another category of planetoid, the asteroids, was discovered only 200 years ago. Most of these reside in a belt between the orbits of Mars and Jupiter. We know very little about the asteroids and comets during the earliest period of formation of the Solar System. However, they may have played a critical role in the formation of an Earth hospitable to life. It is believed that a period of heavy cometary bombardment of Earth during its first half billion or so years delivered the vast quantities of water to the planet which now comprise its oceans and atmospheric water.
Earth’s Moon underwent a 200 million year period of massive bombardment by asteroids and comets beginning approximately 4 billion years ago. Known as the Late Heavy Bombardment (LHB) or the Lunar Cataclysm, these colossal collisions with the Moon led to the formation of its great craters. We have every reason to believe that the inner planets were subjected to the same bombardment through their own gravitational attraction of planetoids. One theory, however, holds that only the Earth-Moon system experienced the LHB during this period. Discovery in Antarctica of a meteorite (described below) dated at 4.0 billion years provides some evidence that Mars experienced the LHB, so it is likely that all the terrestrial planets did. This early period involved the largest and most abundant collisions to occur in the history of the Solar System. Life originated on Earth shortly after the end of the LHB, at least 3.8 billion years ago.