<human>:  What is the history of the solar system?
<bot>: The solar system is thought to have formed about 4.5 billion years ago from a giant, rotating cloud of gas and dust known as the solar nebula. Over time, gravity caused the matter within the nebula to clump together and form the planets, moons, asteroids and comets that make up the solar system. The Sun is the largest and most massive object in the solar system, and its gravity is responsible for holding the planets and other objects in their orbits. The planets in the solar system are divided into two categories: the inner planets, including Mercury, Venus, Earth, and Mars; and the outer planets, including Jupiter, Saturn, Uranus, and Neptune. Beyond Neptune lies a region populated by icy objects known as the Kuiper Belt. In addition, a group of icy objects and comets known as the Oort cloud are thought to exist at the far edges of the solar system.