Approximately 4.5 billion years ago, gravity pulled a cloud of dust and gas together to form our solar system. While scientists aren't certain of the exact nature of the process, observations of young stellar systems combined with computer simulations have allowed them to develop three models of what could have happened so many years ago.

Birth of the sun
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 its own gravity, forming a wealth of young stellar objects known as protostars. Gravity continued to collapse the material onto the infant object, creating a star and a disk of material from which the planets would form. When fusion kicked in, 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 of the Milky Way Galaxy's clouds to reveal the natal environment of other stars. Scientists have applied what they've seen in other systems to our own star.

After the sun formed, a massive disk of material surrounded it for around 100 million years. That may sound like more than enough time for the planets to form, but in astronomical terms, it's an eye blink. As the newborn sun heated the disk, gas evaporated quickly, giving the newborn planets and moons only a short amount of time to scoop it up.

Formation models
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. 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, core accretion suggests that small, rocky worlds should be more common than the more massive 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.

"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry in a press release. Henry, an astronomer at Tennessee State University, Nashville, detected the dimming of the star.

In 2017, the European Space Agency plans to launch the CHaracterising ExOPlanet Satellite (CHEOPS), which will study exoplanets ranging in sizes from super-Earths to Neptune. Studying these distant worlds may help determine how planets in the solar system formed.

"In the core accretion scenario, the core of a planet must reach a critical mass before it is able to accrete gas in a runaway fashion," said the CHEOPS team. "This critical mass depends upon many physical variables, among the most important of which is the rate of planetesimals accretion."

By studying how growing planets accrete material, CHEOPS will provide insight into how worlds grow.

The disk instability model
But the need for a rapid formation for the giant gas planets is one of the problems of core accretion. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.

"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."

According to a relatively new theory, disk instability, 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. These planets can form faster than their core accretion rivals, sometimes in as little as 1,000 years, 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.

Pebble accretion
The biggest challenge to core accretion is time — building massive gas giants fast enough to grab the lighter components of their atmosphere. Recent research probed how smaller, pebble-sized 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 in 2015.

In 2012, researchers Michiel Lambrechts and Anders Johansen of Lund University in Sweden proposed that tiny pebbles, 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.

Levison and his team built on that research to model more precisely how the tiny pebbles could form planets seen in the galaxy today. While previous simulations, both large and medium-sized objects consumed their pebble-sized cousins at a relatively constant rate, Levison's simulations suggest that the larger objects acted more like bullies, snatching away pebbles from the mid-sized masses to grow at a far faster rate.

"The larger objects now tend to scatter the smaller ones more than the smaller ones scatter them back, so the smaller ones end up getting scattered out of the pebble disk," study co-author Katherine Kretke, also from SwRI, told Space.com. "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."

A Nice model
Originally, scientists thought that planets formed in the same part of the solar system they reside in today. The discovery of exoplanets shook things up, revealing that at least some of the most massive objects could migrate.

In 2005, a trio of papers published in the journal Nature proposed that 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. They called this the Nice model, after the city in France where they first discussed it.

As the planets interacted with the smaller bodies, they scattered most of them toward the sun. The process caused them to trade energy with the 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, it's possible that one or even two other giant planets were kicked out of the system. 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

Water gatherer
The solar system didn't wrap up its formation process after the planets formed. 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 it was grown.

But scientists still don't know the source of that water. Originally, they suspected comets, but several missions, including six that flew by Halley’s comet in the 1980s and the more recent European Space Agency's Rosetta satellite, revealed that the composition of the icy material from the outskirts of the solar system didn't quite match Earth's.

The asteroid belt makes another potential source of water. Several meteoriteshave 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 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 the icy grains before they evaporated.

While Earth held onto its water, Venus and Mars would have likely been exposed to the important liquid in much the same way. Rising temperatures on Venus and an evaporating atmosphere on Mars kept them from retaining their water, however, resulting in the dry planets we know today.

Around 4.5 billion years ago, gravity pulled a cloud of dust and gas together to form our solar system. While scientists aren't certain of the exact nature of the process, observations of young stellar systems combined with computer simulations have allowed them to develop three models of what could have happened so many years ago.

 

 

Birth of the sun  

 
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 its own gravity, forming a wealth of young stellar objects known as protostars. Gravity continued to collapse the material onto the infant object, creating a star and a disk of material from which the planets would form. When fusion kicked in, 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 of the Milky Way Galaxy's clouds to reveal the natal environment of other stars. Scientists have applied what they've seen in other systems to our own star. 

Main goal: Understand the reason for the different sizes, compositions, and orbital and rotational motions of planets (Terrestrial, Jovian) and minor objects (Pluto and similar objects, asteroids, comets), including exceptions to the general patterns. The theory should produce a scenario similar to the one for other stellar systems.

|   Stages of Star System FormationStarting point: A cloud of interstellar gas and dust, the "solar nebula"; Most of it (98%) is hydrogen and helium, but it includes atoms and dust grains of heavier material, formed in previous generations of stars.Onset of formation: The nebula is already thicker than the average interstellar region, and possibly part of a chaotic region of starbirth; Because of some disturbance that compresses it, such as a supernova explosion, it starts a gradual process of collapse.

Contraction: The cloud starts collapsing under its own gravity; over 100,000 years, it shrinks down to 100 AU, heats up (thermal energy), and compresses in the center.Accretion disk: The matter around the center spins up and flattens into a disk, while heat vaporizes the dust.Protostar: Forms in the center, when the core becomes opaque; later will become the Sun. (The gas orbiting the protostar in some cases may start to compress under its own gravity, producing a double star.)Condensation: The disk radiates away its energy and cools off; some gas condenses into tiny dust grains of metal, rock and, far enough from the forming star, outside the "snow line", ice (differentiation).Planetesimals: Dust grains stick to each other (ice helps) and sweep their paths, forming larger particles; this accretion goes on until the particles become the size of boulders or small asteroids, which attract matter with their gravity. |

| Protoplanets: The larger particles' growth accelerates, and they accumulate all of the solid matter close to their own orbit. In 100,000 to 20,000,000 yr, the protoplanets' size is large asteroid/lunar size in the inner solar system, and several times the Earth's size in the outer solar system (lower temperature).Question: What is the relative importance of gravitational instabilities and core accretion in the formation process for gas giants?Solar wind: After about 1,000,000 yr, it sweeps away the leftover gas. If a protoplanet is already large enough, its gravity pulls in the surrounding gas, and it becomes a gas giant (leftover gas and dust around it condenses into moons); if not, it remains a rocky or icy body.Fragmentation: In 10 to 100 million years, while larger planetesimals become more massive, smaller ones break into smaller pieces when they collide, end up as meteorites on larger objects, and/or their orbits are altered.

ResultOverview: After a billion years of clean-up and meteoritic bombardment, you end up with ten or so planets, in stable orbits; The protostar turned into a star when the core became hot enough.Catastrophes: Needed to explain specific isolated features and exceptions. The planets, their surfaces and atmospheres may be heavily modified by the last, big collision they experience.Examples: For Earth, our Moon and the presence of water, brought by comets (the original Earth could not have retained it); Also, composition of Mercury, Venus' rotation, Uranus' tilt.Debris: Some planetesimals remain in the asteroid belt (a would-be planet, if not for Jupiter) and the Kuiper belt; others are thrown outwards by "gravity assist" during close encounters (Oort cloud); Some dust remains in a dust disk in the plane of the solar system; we see it from the zodiacal light it scatters..How big is it? Pluto's orbit at 40 AU, Kuiper Belt between 30 and 100 AU or so, the Oort Cloud extends out to 50,000-100,000; The nearest star is at about 300,000.What Evidence Do We Have?Earth and Moon rocks: They can be dated using their radioactive elements; The oldest ones are about 4.5 billion years old.Meteorites: The oldest objects in our solar system are 4.57-Gyr old, mm-sized grains found in some meteorites; Some even give us evidence that a star exploded in our neighborhood around the time the solar system formed, and the Sun may have been part of a cluster.Exploration and experiments: Spacecraft have been sent to observe asteroids made of "primitive rock" (like NEAR, with asteroid Eros, and Hayabusa, to asteroid Itokawa) and comets (Rosetta), and collect samples of solar wind (Genesis); Conditions have been recreated in a Space Shuttle flight.Solar neighborhood: Its configuration also shows evidence for some kind of past explosion; For example, we seem to be inside a bubble with walls about 70 light years away; Further away, we can see other (proto)planetary systems where the process is happening right now. 

Approximately 4.5 billion years ago, gravity pulled a cloud of dust and gas together to form our solar system. While scientists aren't certain of the exact nature of the process, observations of young stellar systems combined with computer simulations have allowed them to develop three models of what could have happened so many years ago.

Birth of the sun

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 its own gravity, forming a wealth of young stellar objects known as protostars. Gravity continued to collapse the material onto the infant object, creating a star and a disk of material from which the planets would form. When fusion kicked in, 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 of the Milky Way Galaxy's clouds to reveal the natal environment of other stars. Scientists have applied what they've seen in other systems to our own star.

After the sun formed, a massive disk of material surrounded it for around 100 million years. That may sound like more than enough time for the planets to form, but in astronomical terms, it's an eye blink. As the newborn sun heated the disk, gas evaporated quickly, giving the newborn planets and moons only a short amount of time to scoop it up.

Formation models

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. 

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. 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, core accretion suggests that small, rocky worlds should be more common than the more massive 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.

"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry in a press release. Henry, an astronomer at Tennessee State University, Nashville, detected the dimming of the star.

In 2017, the European Space Agency plans to launch the CHaracterising ExOPlanet Satellite (CHEOPS), which will study exoplanets ranging in sizes from super-Earths to Neptune. Studying these distant worlds may help determine how planets in the solar system formed.

"In the core accretion scenario, the core of a planet must reach a critical mass before it is able to accrete gas in a runaway fashion," said the CHEOPS team. "This critical mass depends upon many physical variables, among the most important of which is the rate of planetesimals accretion."

By studying how growing planets accrete material, CHEOPS will provide insight into how worlds grow.

But the need for a rapid formation for the giant gas planets is one of the problems of core accretion. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.

"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."

According to a relatively new theory, disk instability, 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. These planets can form faster than their core accretion rivals, sometimes in as little as 1,000 years, 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.

The biggest challenge to core accretion is time — building massive gas giants fast enough to grab the lighter components of their atmosphere. Recent research probed how smaller, pebble-sized 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 in 2015.

In 2012, researchers Michiel Lambrechts and Anders Johansen of Lund University in Sweden proposed that tiny pebbles, 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.

Levison and his team built on that research to model more precisely how the tiny pebbles could form planets seen in the galaxy today. While previous simulations, both large and medium-sized objects consumed their pebble-sized cousins at a relatively constant rate, Levison's simulations suggest that the larger objects acted more like bullies, snatching away pebbles from the mid-sized masses to grow at a far faster rate.

"The larger objects now tend to scatter the smaller ones more than the smaller ones scatter them back, so the smaller ones end up getting scattered out of the pebble disk," study co-author Katherine Kretke, also from SwRI, told Space.com. "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." 

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypotesis. In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud. 
 

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc.

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost line). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid belt, Kuiper belt, and Oort cloud.

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972). In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Problems:

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.