Scientists believe that the sun and the planets in our solar system formed from a vast cloud of dust and gas called the solar nebula . Important evidence for this view comes from meteorites like the one called Allende (named for the village near where it was found), which contains tiny beads of magnesium-silicate minerals called chondrules , set in a matrix composed of a wide variety of tiny mineral grains and organic compounds. Embedded in this matrix are other strange objects. Among them are white inclusions (foreign bodies enclosed in a mineral or rock) composed mainly of oxides of aluminum and calcium known as calcium-aluminum inclusions (CAls). The CAIs solidified at a very high temperature and thus must have been the very first solid objects to have formed in the solar nebula, when it was still very hot. The ages of several CAls have been determined by radiometric dating, a technique by which age is determined from the content of radioactive elements and their decay products. Radiometric dating established that the chondrules are 4.566 billion years old, making them the oldest known objects in the solar system,

Allende is important for other reasons. First, it and other carbonaceous chondrites have a composition similar (except for hydrogen, helium, and other gaseous elements) to that of the Sun's visible outer layer, and for this reason they may be the best available representatives of the bulk composition of the solar nebula. This match in composition is what allows us to use carbonaceous chondrites to deduce what happened in the early solar system. Second, Allende contains a few hardy particles that survived the events leading up to the formation of the solar nebula. They include particles, typically only a few millionths to a few thousandths of a millimeter across, of diamond, silicon carbide, graphite, and corundum (aluminum oxide). The ratios of the different forms of carbon and of other elements in these grains are much different from the ratios found in anything formed in the solar system. This is why the particles are believed to be relics of old stars that existed before the birth of the solar system.

As the solar nebula cooled, magnesium-iron minerals- -the common stuff of Earth- -began to form. Most of this material condensed relatively close to the "protosun" (the dense core of matter that formed at the center of the solar nebula) in the region now occupied by Mercury, Venus, Earth, and Mars. Together, these are known as the rocky, or terrestrial, planets precisely because they are made of rocks composed of silicates. More distant from the Sun, temperatures were low enough for elements to freeze, so that Jupiter, Saturn, Uranus and Neptune- -the so-called gas-rich giants of that part of the solar system- -as well as Pluto, would end up consisting of higher proportions of these ices. This is getting ahead of the story, however. During the early stage when the solar nebula was cooling, in the region of the future terrestrial planets, small particles formed progressively larger aggregates as they collided with one another, and larger bodies swept up more material and thus grew more rapidly than smaller bodies. The result of this process was many billions of "planetesimals," or solid bodies each 1 kilometer (0.62 mile) or so across, rotating around the protosun.

At some point after the formation of planetesimals, the seminal event of the solar system occurred: the Sun ignited to become a star. As dust collapsed in the center of the solar nebula, the gravitational pressure there became high enough to cause a nuclear process known as fusion, in which hydrogen is converted into helium atoms and, in doing so, releases energy. The ignition swept away what gas and small particles remained in the inner solar system, leaving only bodies too large to be affected- -the planetesimals. The timing is one of the numerous accidents that led to our existence. Had the Sun ignited before the formation of the planetesimals, all matter in the solar system would have been swept away, and there would have been no Earth or rocky planets!