This section describes the most widely accepted views on the origin of our solar system. The theory summarized here represents the most consistent set of ideas available to explain what we know about our solar system today. The figure below ▼ provides a useful perspective on the size and the scale of our solar system.

The Universe Begins

Our scenario begins about 13.7 billion years ago, with the Big Bang, an almost incomprehensible event in which space itself, along with all the matter and energy of the universe, exploded in an instant from tiny to huge dimensions. As the universe continued to expand, subatomic particles condensed to form hydrogen and helium gas, which later cooled and clumped to form the first stars and galaxies. It was in one of these galaxies, the Milky Way, that our solar system, including planet Earth, took form.

The Solar System Forms

Earth is one of eight planets that, along with dozens of moons and numerous smaller bodies, revolve around the Sun. The orderly nature of our solar system helped scientists determine that Earth and the other planets formed at essentially the same time and from the same primordial material as the Sun. The nebular theory proposes that the bodies of our solar system evolved from an enormous rotating cloud called the solar nebula ▼. Besides the hydrogen and helium atoms generated during the Big Bang, the solar nebula consisted of microscopic dust grains and other matter ejected ultimately from long-dead stars. (Nuclear fusion in stars converts hydrogen and helium into the other elements found in the universe.)

Nearly 5 billion years ago, something—perhaps a shock wave from an exploding star (supernova)—caused this nebula to start collapsing in response to its own gravitation. As it collapsed, it evolved from a huge, vaguely rotating cloud to a much smaller, fast-spinning disk. The cloud flattened into a disk because that was the path of least resistance, that is, for the same reason it is easier to move along with a crowd of circling ice skaters than to cross their path. The orbital plane within the cloud that started out with the largest amount of matter gradually, through collisions and other interactions, incorporated gas and particles that originally had other orbits until all the matter orbited in one plane. The disk spun faster as it shrank, much as ice skaters spin faster when they draw their arms toward their bodies. Most of the cloud’s matter ended up in the center of the disk, where it formed the protosun (pre-Sun). Astronomers have observed many such disks around newborn stars in neighboring regions of our galaxy.

The protosun and inner disk were heated by the gravitational energy of infalling matter. In the inner disk, temperatures became high enough to cause the dust grains to evaporate. However, at distances beyond the orbit of Mars, the temperatures probably remained quite low. At -200°C (-328°F), the tiny particles in the outer portion of the nebula were likely covered with a thick layer of frozen water, carbon dioxide, ammonia, and methane. The disk also contained appreciable amounts of the lighter gases hydrogen and helium.

The Inner Planets Form

The formation of the Sun marked the end of the period of contraction and, thus, the end of gravitational heating. Temperatures in the region where the inner planets now reside began to decline. The decrease in temperature caused substances with high melting points to condense into tiny particles that began to coalesce (join together). Materials such as iron, and nickel, and the elements of which the rock-forming minerals are composed—silicon, calcium, sodium, and so forth—formed metallic and rocky clumps that orbited the Sun ▲. Repeated collisions caused these masses to coalesce into larger asteroid-size bodies, called planetesimals, which in a few tens of millions of years accreted into the four inner planets we call Mercury, Venus, Earth, and Mars ▼. Not all of these clumps of matter were incorporated into the planetesimals. Those rocky and metallic pieces that remained in orbit are called asteroids and meteors. Meteors become meteorites if they impact Earth’s surface.

As more and more material was swept up by these growing planetesimals, the high-velocity impact of nebular debris caused their temperatures to rise. Because of their relatively high temperatures and weak gravitational fields, the inner planetesimals were unable to accumulate much of the lighter components of the nebular cloud. The lightest of these components, hydrogen and helium, were eventually whisked from the inner solar system by the solar wind.

The Outer Planets Develop

At the same time that the inner planets were forming, the larger, outer planets (Jupiter, Saturn, Uranus, and Neptune), along with their extensive satellite systems, were also developing. Because of low temperatures far from the Sun, the material from which these planets formed contained a high percentage of ices—water, carbon dioxide, ammonia, and methane—as well as rocky and metallic debris. The accumulation of ices accounts in part for the large size and low density of the outer planets. The two most massive planets, Jupiter and Saturn, had a surface gravity sufficient to attract and hold large quantities of even the lightest elements—hydrogen and helium.

• The nebular theory describes the formation of the solar system. The planets and Sun began forming about  billion years ago from a large cloud of dust and gases.

• As the cloud contracted, it rotated faster and assumed a disk shape. Material that was gravitationally pulled toward the center became the protosun. Within the rotating disk, solid matter gradually cohered to form objects called planetesimals, which grew as they swept up more and more of the cloud’s debris.

• Because of their high temperatures and weak gravitational fields, the inner planets were unable to accumulate and retain large quantities of the light elements in the disk. Because of the very cold temperatures existing far from the Sun, the large outer planets include huge amounts of lighter materials. These substances account for the comparatively large sizes and low densities of the outer planets.

• nebular theory: The basic idea that the Sun and planets formed from the same cloud of gas and dust in interstellar space.