The study of the origin and evolution of the Solar System is based on laboratory analysis of meteorites and other extraterrestrial materials, and on astronomical observations of star-forming regions today. A virtual consensus holds that the Solar System formed from the collapse of interstellar cloud material under its own gravity about 4.56 Ga ago. This process led initially to a dusty disc forming around the newly formed Sun. The material in the disc accreted into small planetary bodies, called planetesimals, which in turn made up the building blocks of planets. Some planetesimals survived as asteroids. Fragments of these bodies make up the majority of meteorites that fall to Earth. The most common type of meteorite are chondrites. These formed from accretion of material from the dusty disc. Most chondrites were heated, probably by radioactive decay, and became metamorphosed. Other meteorite types include those made from basalts and iron metal alloys; these testify to extensive heating to produce melting on some planetesimals. Impacts of large (>100 km bodies) were responsible for forming much of the diversity seen among planets. Indeed, impact of a Mars-sized body into the early Earth is believed to have caused the formation of the terrestrial Moon.