Our solar system is about 4,567 million years old. Earth's Moon formed 4,450 million years ago, just 50 million years after the Earth's formation. How do we know this? The age of these events has been determined using radioactive dating techniques that rely on the precise decay rate of radioactive elements. The solar system formed from the gravitational collapse of a large rotating nebula. The collapsing mass that collected in the center formed the Sun, and the remaining matter formed a protoplanetary disk that eventually coalesced to form the planets, moons, asteroids and comets.
Our solar system has eight planets. Mercury, Venus, Earth and Mars are called terrestrial planets because they are composed mainly of silicate rocks or metals, such as found on the Earth. The giant planets Jupiter, Saturn, Uranus and Neptune consist mainly of gases and low-boiling-point materials. All the planets, except Earth, have names from Greek or Roman mythology.
In ancient times, the configuration of the planets in the sky and their positions in the constellations of the zodiac was thought to influence the biological, mental and emotional states of individuals. Astrology is a pseudoscience that presumes to foretell the future based on the movements and relative positions of celestial bodies.
Pluto was discovered in 1930 and it was considered the ninth planet of the solar system until 2006 when it was reclassified as a dwarf planet. Many objects similar to Pluto orbit the Sun beyond Neptune in a region called the Kuiper belt. Beyond the Kuiper belt there is an extended shell of icy objects called the Oort Cloud which is thought to be the origin of most long-period comets.
The planets and the large moons formed by accretion, a process in which celestial objects collided to form larger bodies. The craters of the Moon are the scars of some of these collisions. On the Earth, volcanism and erosion by wind and water have erased most of the evidence of cosmic impacts.