Like all families, the members of our solar system family share a common origins story. Their story started even before our solar system formed 4.56 billion years ago. Their story started when the story started for every single thing in our universe. Our universe was born from the Big Bang about 13.5 billion years ago. The first stars lived out their lives and eventually exploded, sending "star stuff" out into the cosmos. That original stellar material was recycled as another generation of stars, and many of these, too, exploded at the end of their lives. Our Sun is thought to be a third-generation star and our entire solar system is made of the recycled star stuff of previous star generations.
Our solar system began forming about 4.6 billion years ago within a concentration of interstellar dust and hydrogen gas called a molecular cloud. The cloud contracted under its own gravity and our proto-Sun formed in the hot dense center. The remainder of the cloud formed a swirling disk called the solar nebula.
Within the solar nebula, scientists believe that dust and ice particles embedded in the gas moved, occasionally colliding and clumping together. Through this process, called "accretion," these microscopic particles formed larger bodies that eventually became planetesimals with sizes up to a few kilometers across. In the inner, hotter part of the solar nebula, planetesimals were composed mostly of silicates and metals. In the outer, cooler portion of the nebula, water ice was the dominant component.
The Sun's light warmed the objects in our solar system, especially those in the inner solar system. There, it was too warm for lightweight volatiles, such as water and ammonia, to condense. In addition, particles from the Sun (the solar wind) pushed volatiles out of the inner solar system. When the volatiles reached the cold temperatures of the outer solar system -- out beyond an invisible boundary called the "frost line" -- they condensed onto the nascent giant planets. Thus, the outer planets had rocks, metals, and volatiles available to accumulate, while the relatively warm, "windy" inner region was stripped of all but the densest materials, like rock and metal. For more information about the compositions of the planets see A Family Affair.
When the nascent planets grew from a few kilometers to a few hundred kilometers across, they became massive enough that their gravity influenced each other's motions. This increased the frequency of collisions, through which the largest bodies grew most rapidly. During this "childhood" stage of growth, the bodies are referred to as planetesimals. Eventually, regions of the nebula were dominated by large protoplanets. The interiors of these more mature bodies were becoming ordered -- differentiated -- into protoplanets. The process of collision and accretion continued until only four large bodies remained in the inner solar system -- Mercury, Venus, Earth, and Mars, the terrestrial planets. In the cold outer solar nebula, much larger protoplanets formed. The largest ones swept up other protoplanets, planetesimals, and nebular gas, leading to the formation of Jupiter, Saturn, Uranus, and Neptune.
Shortly after Earth formed, the Moon did. A large object (about half as wide as Earth) collided with our world. The off-center cosmic smash-up increased Earth's spin, and its energy disintegrated the impacting object, melted Earth's outer layers, and flung debris into orbit around Earth. This material formed a ring of gas, dust and molten rock around Earth. In less than a hundred years -- an incredibly short time for the formation of an entire world -- this debris clumped (accreted), growing larger and larger to form our Moon!
The solar system's birth story is an unfolding tale. The processes of solar nebula collapse and accretion explain why there is so much space in space, where we find the various types of planets and other small bodies, and why the planets all lie in about the same plane and orbit the Sun in the same direction. Despite their diversity, all the planets, dwarf planets, comets, and asteroids in the solar system formed together, along with the Sun, as a system. Scientists still have many questions about our solar system's story. How did the planets form quickly enough to escape the blast of the early Sun's intense solar wind, which would have swept gas and dust out of the growing planets' reach?
Fortunately, vital clues are scattered throughout the solar system -- from the oldest rocks on the Earth, Earth's Moon, Mars and the asteroids to the frozen outer reaches of the Kuiper Belt. NASA robotic missions are examining these distant worlds, making new discoveries that will help to fill in the pages of this story. Ongoing research is examining the early solar nebula's composition and conditions such as radiation, by studying grains from comets, from the Sun, and the composition of Jupiter (for more information about comets, see Small Bodies, Big Impacts). Comets are made of the ices and dust from the original nebula that formed our solar system, and can tell us more about how our planets formed. The EPOXI mission, which will flew past Comet Hartley 2 on November 4, 2010, studied the structure, composition and formation history of cometary nuclei, in order to learn more about the origin of the solar system.
Scientists have examined meteorites to learn more about the primitive material that made up the solar nebula. Recent research on the radioactive isotopes within meteorites suggests a nearby supernova explosion may have influenced the environment and materials in the early solar system, seeding it with materials and perhaps triggering it to collapse into a swirling accretion disk.
Our understanding of the solar system's formation is also being guided by the new worlds discovered around distant stars. The unusual orbits of many of these distant planets have sparked a hot topic of research: that planets' orbits may shift -- migrate -- early after their formation. This "planetary migration" is the best explanation for these newly discovered "hot Jupiters"-- massive gas giants orbiting extremely close to their stars.
Planetary migration is caused by gravitational interactions between the gas in the solar nebula and the young planets, and also by gravitational interactions between the planets and the remaining planetesimals. These interactions transfer angular momentum between the objects, causing the planet too either give up energy, and move closer toward the star, or gain energy, and migrate outward.
One model for our own solar system suggests that our giant planets' orbits shifted dramatically early in the solar system's history, with Jupiter's orbit migrating slightly inward toward the Sun, and those of Saturn, Neptune, and Uranus expanding farther from the Sun. These dramatic movements gave us the order of the planets and smaller bodies that we are familiar with today, and caused many smaller bodies (such as comets) to scatter out into the Kuiper belt and Oort cloud.
Planetary migration may also account for an intense period of bombardment throughout the inner solar system, around 3.9 billion years ago. Some computer models suggest that interactions of Jupiter and Saturn's orbits destabilized the orbits of asteroids and comets in the outer solar system, causing them to pelt the inner solar system. Scientists are continuing to investigate how the gravitational influences of the giant planets interacted to dramatically reshape our solar system.
Did the planets form in their present locations, or did the giant planets form closer to the Sun and, through complex gravitational interactions, migrate to their orbits of today? More detailed understanding of the dates of impacts and the materials in the early solar system may provide us with final answers.