How Did the Outer Planets Form?

The formation of outer planets was the result of various processes. These processes include accretion collapsing and impacts. This article will discuss these processes. The outer planets formed because they were massive enough to attract solar wind. Since they were located in the colder regions of the solar system they could retain this energy more easily. This resulted in the formation of large icy planetesimals.

Large icy planetesimals

Many planetary systems are believed to have been formed by large ice planetesimals that accumulated early on the outer planets. These bodies formed at such a young age that they would have avoided the drag of gas as they moved towards the star. This process of planetesimal formation is possible even for large planets that have grown to tens of kilometers in size.

These ice-like particles could have originated from the nebular gas of the outer planets which is composed of heavier elements such as carbon. This material is present on the outer planets as well which explains the presence of metals and rocks. As planetesimals form in a disk of gas and dust they begin to grow accumulating more material as they enlarge.

Once these ice-rich objects have accumulated on the outer planets they may have collided with each other and formed satellites. They may have formed in the same part of an ancient solar nebula as comets. After the dramatic process of planet formation only eight stable planets remained. As these planets moved their motions reflected their formation history.

This phenomenon may not have been observed until the outer planets had become fully developed. The icy planetesimals survived in the coldest regions of the nebula because the density of the disk was so low. Consequently they were unable to accrete the surrounding gas and therefore remain small dirty snowballs. Today these icy planetesimals make up the family of Kuiper belt comets.


While many theories have been proposed to explain the formation of outer planets accretion of solids is the most widely accepted explanation. Accretion is how the giant planets in our solar system formed. This process began when a protoplanetary disk was formed which was devoid of solid materials. As the protoplanetary disk cooled and weakened more solid materials accreted into the protoplanetary disk. The resulting planets formed massive solid cores.

The formation of outer planets takes place via accretion the process of accumulation of matter from the protosun. As the protoplanets grow in mass they slowly accrete to form larger bodies. These bodies are then fragmented resulting in a disk of matter around the protosun. The outer planets are formed by collision of these fragmented disks which is why the Sun is so hot so accretion can happen so quickly.

Young stars are known to have a planetary disk but the initial conditions for accretion are complicated. Young stars need to be sufficiently massive for accretion to take place. In order to explain the formation of planets the process must be both fast and effective. The protoplanetary disk contains approximately 300 Earth masses of solids. However only about 100 Earth masses are incorporated into the planets.


The Collapse of outer planets is a process that occurs when the mass of a star collides with diffuse dust and gas forming a nebula. It is possible to infer the formation of planets from the chemical composition of meteorites which may contain traces of material injected into the growing solar system a few million years ago. This phenomenon is similar to the formation of stars where an intense supernova explosion swept the dispersed atoms closer together forming planets.

The Collapse of outer planets hypothesis explains the basic facts about the Solar System including the fact that all planets orbit the sun in nearly the same direction and revolve at similar rates. Interestingly all the planets’ rotation axes are almost perpendicular to the orbital plane. Moreover this theory explains why the planets are positioned in such a way that their orbits lie near the plane of the sun’s limb.

Earlier scientists believed that the Solar System had formed from a molecular cloud or nebula billions of years ago. It isn’t entirely clear what happened billions of years ago but the details of the process are now better understood thanks to observations of other star systems and studies of meteorites. It has also become clear that our planet has moons and may even have formed before the Sun. This discovery provides important clues as to how the Earth formed.


A similar mechanism could explain why Neptune and Uranus start off so similar. If one of the outer planets was to be struck by a body the size of three Earth masses an impact could alter the internal structure of Uranus. Although the interior of Uranus would not be altered the impact would tilt it. However it would be impossible to tell what caused this type of impact without studying the interior of the planet.

The majority of impacts occur on the planet of origin and inward from it. Small numbers of meteoroids are perturbed into orbits that allow them to impact planets outward from their initial planet. The impact rate of outer planets decreases with the distance from the planet of origin. Therefore the Sun and ejections from the system are the outer boundaries. The results from these simulations suggest that impacts on the outer planets are rarer and longer-lived than impact rates on the inner planets.

The discovery of life on Mars raises the possibility of impacts on the outer planets. The presence of life on the outer planets may increase the number of potentially life-bearing rock transfers between planets. For example the impact of Comet Shoemaker-Levy on Jupiter caused large holes in the cloudtops of the planet. This was the first major impact in the solar system. Further this impact has triggered an increase in the number of comets that may contain the genetic material for life.

Irregular satellites

In the early stages of planet formation there was only one big chunk in our solar system so we don’t have many moons to study. However the massive planets at the center of our solar system accumulated enough mass to draw asteroids into their orbits. At the end of the formation process the Earth collided with one final large chunk ejecting material to form the moon. As planets formed they moved closer to the sun and eventually settled into orbits.

The outer planets formed in a similar manner to the inner ones though they were farther away from the sun and did not lose any of the gases they gathered along the way. The heavier elements of hydrogen and helium however increased the volume and surface area of the planets. For example the Jovian planets both have similar mass and surface area to the planets we observe today. Their similarities with each other point to a common origin of these planets.

The first planets in the solar system formed fast and close to the Sun. The rocky planets begin to form closer to the Sun as they cool. As these planets form the materials closer to the Sun accumulate forming the inner planets. Meanwhile those farther away were formed from materials accreted from interplanetary meteoroids or smaller moons of Jupiter. However this isn’t definitive as it’s not possible to predict how planets formed in this way.

Water on the outer planets

The outer planets are thought to be in the ‘habitable zone’ – where liquid water could exist. Their high pressure and higher temperatures mean they are unlikely to have liquid water. But water on the outer planets remains a fascinating mystery. There’s still a lot to learn about the icy worlds. Even the most powerful telescopes aren’t 100% certain that water exists on outer planets so scientists are looking for more evidence.

The outer planets formed in an area of condensed gas far from the Sun. These planets’ moons such as Europa are rich in ice. The ice and dust inside their disks grew to create a larger planet. It is thought that water ice made up most of the solid matter beyond the ice line. Despite this scientists think that the water on the outer planets came from water-bearing asteroids and comets.

The gradient in D/H of water on the outer planets’ surface is the result of radial mixing and re-equilibration between H2 and H2O. Some models predict that water D/H ratios increase monotonically with radial distance. However this is a gross oversimplification. The basic prediction that planetesimals formed further away from the Sun should have a greater concentration of D than H in their water ice is supported by H isotopes.