"As soon as we could make a more careful analysis of the situation, it turned out that: first, the sky was dark as before but was beginning to be not so dark; second, the surface where we were was all bumpy and crusty, an ice so dirty it was revolting, which was rapidly dissolving because the temperature was rising at full speed; and, third, there was what we would later have called a source of light, that is, a mass that was becoming incandescent, separated from us by an enormous empty space, and it seemed to be trying out all the colors one by one, in iridescent fits and starts. And there was more: in the midst of the sky, between us and that incandescent mass, a couple of islands, brightly lighted and vague, which whirled in the void with our uncles on them and other people, reduced to distant shadows, letting out a kind of chirping noise.

"So the better part was done: the heart of the nebula, contracting, had developed warmth and light, and now there was the Sun. All the rest went on revolving nearby, divided and clotted into various pieces, Mercury, Venus, the Earth, and others farther on, and whoever was on them, stayed where he was. And, above all, it was deathly hot."

When contemplating the formation of the solar system, there are two important points to take into consideration: the planets all revolve about the sun in the same direction, and their orbits are located nearly in the same plane. These two characteristics suggest that the planets were formed in the sun's orbit, not captured by the sun's gravitational pull, and were most likely formed from the same mass of matter. The theory that is currently accepted is that the sun and the planets originally started as a large rotating cloud of matter which has been dubbed the solar nebula. Because all of these bits of matter gravitationally attracted each other, they tended to condense toward the center of the cloud. Since a falling object gains speed as it falls, pieces of matter being pulled toward the center of the cloud picked up a lot of kinetic energy. When they finally hit the dense clump of matter in the center, their kinetic energy was converted to thermal energy. The area at the center, called the protosun, got denser and hotter until there was enough heat present for nuclear reactions to begin, transforming the protosun into a star.

The matter surrounding the sun eventually flattened into a disk. Not all of the matter was pulled to the center because the rotation of the solar nebula provided a centrifugal force that counteracted the protosun's gravitational pull. In the inner solar system, this matter coalesced into clumps called planetesimals, which collided and combined to form protoplanets (about the size of the Moon), which themselves combined to form planets of the size they are today. The energy from the collisions would have provided enough heat for some of the solid material to melt and rearrange itself, so that heavier elements fell toward the centers of the planets and remained there. Past Mars, the temperature was cold enough that many elements that existed as gases in the inner solar system could only exist as solids (since the pressure was too low for liquids to form). This means there was much more solid material present in the outer solar system, so the planets that formed were much larger. These larger planets had enough mass to capture huge amounts of gases and retain them as atmospheres, creating the "gas giants" such as Jupiter and Saturn. These outer planets were likely massive enough to form their own rotating disks of matter, from which it is theorized their moons were formed. Because it obviously did not form in this way, and because it resembles small objects that orbit the sun at a much farther distance, Pluto is not considered to be a planet by many astronomers.

The sun, like all stars, is powered by a process called nuclear fusion. Nuclear fusion occurs when the nuclei of two different atoms combine to form one heavier atom. For elements lighter than iron, this process is accompanied by a release of energy. Atomic nuclei, since they are positively charged, feel a repulsive electromagnetic force from each other. This force is strong enough at normal temperatures to keep the nuclei from colliding. At high enough temperatures, however, such as those present in the interior of a star, the nuclei possess enough kinetic energy to overcome this repulsion and collide with one another. When the nuclei come into contact with one another, the strong nuclear force (which is only felt over very short distances) will bind the two nuclei together to form a larger nucleus, releasing energy as it does so. Since the energy released is greater than the energy required to start the reaction (i.e. an exothermic reaction), the process of nuclear fusion powers further collisions, creating a chain reaction that will continue until all of the base nuclei are used up. In stars about the size of our sun and smaller, hydrogen atoms can be fused together to form helium atoms. In hotter and more massive stars, heavier atoms are fused together. It is by this process that most of the heavier elements were made, as the big bang only accounts for a tiny portion of them.