Our understanding of the solar system has come in a way that can best be described as:

Solar system formation began approximately 4.5 billion years ago, when gravity pulled a cloud of dust and gas together to form our solar system.

Scientists can't directly study how our own solar system formed, but combining observations of young stellar systems in a range of wavelengths with computer simulations has led to models of what could have happened so many years ago.

How did the sun form?

The solar system is anchored by our sun.

Before the solar system existed, a massive concentration of interstellar gas and dust created a molecular cloud that would form the sun's birthplace. Cold temperatures caused the gas to clump together, growing steadily denser. The densest parts of the cloud began to collapse under their own gravity, perhaps with a nudge from a nearby stellar explosion, forming a wealth of young stellar objects known as protostars.

Gravity continued to collapse the material onto the infant solar system, creating a star and a disk of material from which the planets would form. Eventually, the newborn sun encompassed more than 99% of the solar system's mass, according to NASA (opens in new tab). When pressure inside the star grew so powerful that fusion kicked in, turning hydrogen to helium, the star began to blast a stellar wind that helped clear out the debris and stopped it from falling inward.

Although gas and dust shroud young stars in visible wavelengths, infrared telescopes have probed many clouds in the Milky Way galaxy to study the environment of other newborn stars. Scientists have applied what they've seen in other systems to our own star.

How did the planets form?

The planets, moons, asteroids and everything else in the solar system formed from the small fraction of material in the region that wasn't incorporated in the young sun. This material formed a massive disk around the baby star, which surrounded it for about 100 million years — an eyeblink in astronomical terms.

During that time, planets and moons formed out of the disk. Among the planets, Jupiter likely formed first, perhaps as soon as a million years into the solar system's life, scientists have argued (opens in new tab).

Scientists have developed three different models to explain how planets in and out of the solar system may have formed. The first and most widely accepted model, core accretion, works well with the formation of the rocky terrestrial planets but has problems with giant planets. The second, pebble accretion, could allow planets to quickly form from the tiniest materials. The third, the disk instability method, may account for the creation of giant planets. 

The core accretion model

Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.

With the rise of the sun, the remaining material began to clump together. Small particles drew together, bound by the force of gravity, into larger particles, according to the core accretion model. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create terrestrial worlds. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants. In this way, asteroids, comets, planets and moons were created.

Some exoplanet observations seem to confirm core accretion as the dominant formation process. Stars with more "metals" — a term astronomers use for elements other than hydrogen and helium — in their cores have more giant planets than their metal-poor cousins. According to NASA (opens in new tab), core accretion suggests that small, rocky worlds should be more common than the large gas giants.

The 2005 discovery of a giant planet with a massive core orbiting the sun-like star HD 149026 is an example of an exoplanet that helped strengthen the case for core accretion. The planet's core is about 70 times more massive than Earth, scientists found; they believe that is too large to have formed from a collapsing cloud, according to a NASA statement about the research (opens in new tab).

Pebble accretion

The biggest challenge to core accretion is time — building massive gas giants fast enough to grab the lighter components of their atmosphere. Research published in 2015 probed how smaller, pebble-size objects fused together to build giant planets up to 1,000 times faster than earlier studies.

"This is the first model that we know about that you start out with a pretty simple structure for the solar nebula from which planets form, and end up with the giant-planet system that we see," study lead author Harold Levison, an astronomer at SwRI, told Space.com at the time.

In 2012, researchers Michiel Lambrechts and Anders Johansen of Lund University in Sweden proposed that tiny rubble, once written off, held the key to rapidly building giant planets. "They showed that the leftover pebbles from this formation process, which previously were thought to be unimportant, could actually be a huge solution to the planet-forming problem," Levison said.

In simulations that Levison and his team developed, larger objects acted like bullies, snatching away pebbles from the mid-size masses to grow at a far faster rate. "The bigger guy basically bullies the smaller one so they can eat all the pebbles themselves, and they can continue to grow up to form the cores of the giant planets," study co-author Katherine Kretke, also from SwRI, told Space.com.

The disk instability model

Other models struggle to explain the formation of the gas giants. According to core accretion models, the process would take several million years, longer than the light gases were available in the early solar system.

"Giant planets form really fast, in a few million years," Kevin Walsh, a researcher at the Southwest Research Institute (SwRI) in Boulder, Colorado, told Space.com. "That creates a time limit because the gas disk around the sun only lasts 4 to 5 million years."

A relatively new theory called disk instability addresses this challenge. In the disk instability model of planet formation, clumps of dust and gas are bound together early in the life of the solar system. Over time, these clumps slowly compact into a giant planet.

Planets can form in this way in as little as 1,000 years, the models suggest, allowing them to trap the rapidly vanishing lighter gases. They also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.

As scientists continue to study planets inside of the solar system, as well as around other stars, they will better understand how gas giants formed.

Planets on the move

Originally, scientists thought that planets formed in their current locations in the solar system. But the discovery of exoplanets shook things up, revealing that at least some of the most massive worlds could migrate through their neighborhoods.

In 2005, a trio of papers published in the journal Nature (opens in new tab) outlined an idea the researchers called the Nice model (opens in new tab), after the city in France where they first discussed it. This model proposes that in the early days of the solar system, the giant planets were bound in near-circular orbits much more compact than they are today. A large disk of rocks and ices surrounded them, stretching out to about 35 times the Earth-sun distance, just beyond Neptune's present orbit.

As the planets interacted with smaller bodies, they scattered most of these objects toward the sun. The process caused the massive planets to trade energy with the smaller objects, sending the Saturn, Neptune and Uranus farther out into the solar system. Eventually the small objects reached Jupiter, which sent them flying to the edge of the solar system or completely out of it. 

Movement between Jupiter and Saturn drove Uranus and Neptune into even more eccentric orbits, sending the pair through the remaining disk of ices. Some of the material was flung inward, where it crashed into the terrestrial planets during the Late Heavy Bombardment. Other material was hurled outward, creating the Kuiper Belt. 

As they moved slowly outward, Neptune and Uranus traded places. Eventually, interactions with the remaining debris caused the pair to settle into more circular paths as they reached their current distance from the sun.

Along the way, our solar system may have lost members: It's possible that one or even two other giant planets were kicked out (opens in new tab) of the neighborhood by all this movement. Astronomer David Nesvorny of SwRI has modeled the early solar system in search of clues that could lead toward understanding its early history.

"In the early days, the solar system was very different, with many more planets, perhaps as massive as Neptune, forming and being scattered to different places," Nesvorny told Space.com 

Where's the water?

Even after the planets had formed, the solar system itself wasn't quite recognizable. Earth stands out from the planets because of its high water content, which many scientists suspect contributed to the evolution of life.

But the planet's current location was too warm for it to collect water in the early solar system, suggesting that the life-giving liquid may have been delivered after Earth formed.

Just one hitch: scientists still don't know where that water might have come from. Originally, researchers suspected comets carried it to Earth, but several missions, including six that flew by Halley’s comet in the 1980s and the European Space Agency's more recent Rosetta spacecraft, revealed that the composition of the icy material from the outskirts of the solar system didn't quite match Earth's.

The asteroid belt is another potential source of water. Several meteorites have shown evidence of alteration, changes made early in their lifetimes that hint that water in some form interacted with their surface. Impacts from meteorites could be another source of water for the planet.

Recently, some scientists have even challenged the notion that the early Earth was too hot to collect water. They argue that, if the planet formed fast enough, it could have collected the necessary water from icy grains before they evaporated.

Whatever process brought water to Earth likely did so to Venus and Mars as well. But rising temperatures on Venus and a thinning atmosphere on Mars kept these worlds from retaining their water, resulting in the dry planets we know today.

Additional resources

• Read NASA's description (opens in new tab) of how the solar system formed, or watch an animation (opens in new tab) on the topic.
• Read a description (opens in new tab) of how stars and planets form from ALMA, which specializes in observing the disks planets are born from.
• Scientists have learned about planet formation by comparing (opens in new tab) worlds in our solar system with exoplanets.

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