Our Sun, planets, and other objects in the solar system formed from a gigantic cloud of gas and dust more than 4.5 billion years ago. According to this accepted model of solar system formation, a disturbance caused the cloud of material to contract through gravitational forces; this led to the formation of a protostar (an early stage of star formation; "proto" indicates "first") in the center of a spinning disk of matter. As the disk of gas and dust cooled, matter condensed, and planets formed through the process of accretion—dust grains collided with each other to form clumps, which in turn collided to form larger and larger bodies.

Small objects in the solar system are known as planetesimals. Early in the history of the solar system, planetesimals collided with each other to form larger bodies (protoplanets). In turn, protoplanets collided with each other to form the eight planets we see today. When objects collided, the resulting body had more mass and, therefore, exerted a stronger gravitational pull on the material around it, causing it to attract more material. As clumps of matter grew larger and larger, the object became massive enough that the force of its own gravity caused it to assume a roughly spherical shape. Smaller objects always have irregular shapes.

In the young solar system, there were hundreds of protoplanets and innumerable planetesimals. Collisions among the objects eventually caused the number of protoplanets to decrease as the size of the accreted bodies grew.

The terrestrial planets of the inner solar system (Mercury, Venus, Earth, and Mars) are relatively small and rocky compared to the other planets in the solar system. Because it was too hot for volatile compounds (like water) to condense in the inner solar system, rocky planetesimals formed from compounds with high melting points, such as iron and rocky silicates.

Beyond the frost line—the distance from the Sun beyond which certain compounds can remain solid—icy compounds such as water, carbon dioxide, and methane were abundant, so the outer planets were able to grow much larger than the inner planets. The outer planets (Jupiter, Saturn, Uranus, and Neptune) became massive enough that they could attract large amounts of gaseous hydrogen and helium. These four planets contain about 99 percent of the mass orbiting the Sun and are known as gas giants. Leftover planetesimals that were not incorporated into planets became asteroids, comets, and Kuiper Belt objects.

In astrobiology—the study of the origin of life on Earth and the search for extraterrestrial life—scientists have been primarily focused on rocky planets and moons that may have conditions similar to those on Earth. But learning more about objects such as asteroids and comets is also important, since they are remnants from the formation of the solar system—literally, time capsules of the past. They may hold the key to understanding the ingredients that were available when life began.

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