How Was The Planet Earth Formed?

How Was The Planet Earth Formed?

Despite The Various Theories, There Are Still Many Uncertainties About The Exact Process Of Earth Formation. However, Scientists Have Proposed Three Dominant Hypotheses.

Despite all that is known about the solar system, scientists are still unsure how the planets formed. Although the orbit of planets around the stars of galaxies is entirely accepted, how planets form is still a matter of debate among scientists. Currently, no theory has been able to fully answer all the questions raised about the formation of the Earth.

The first and most widely accepted theory, accretion (accretion), explains the formation of rocky planets well but has some ambiguities about how gas giants form. The second theory, disk instability, can partially explain the formation of gas giants.

By studying exoplanets, researchers can obtain more data about the formation of Earth and other planets in the solar system.

Today's land and the land of the past
An artist rendering of today’s Earth (right) versus the Earth of the past about 4.5 billion years ago (left).

Nucleus accretion model

About 4.6 billion years ago, the solar system was a cloud of gas and dust known as the solar nebula. With the rotation of this nebula and the gravitational collapse of the material inside it, the Sun was formed in the center of the gas cloud. With the formation of the Sun, the remaining material in the nebula gradually piled up. The tiny particles stuck together under the influence of the Sun’s gravity, creating larger particles. The solar wind carried lighter particles, such as hydrogen and helium, away from the nebula’s center, leaving heavy, rocky particles near the Sun, from which the rocky and terrestrial planets formed. But since the solar wind’s strength decreases at long distances, these particles had the opportunity to merge to form gas giants. In this way, asteroids, comets, planets, and their moons were formed.

Initially, the rocky core of the Earth was formed by the collision and fusion of heavy elements. The denser material sank towards the center of the Earth, and the lighter material formed the Earth’s crust. Due to gravity, gas molecules are trapped around the Earth and create the Earth’s atmosphere. Probably, in this period, the Earth’s magnetic field was created.

In the early period of the Earth’s evolution, a large celestial body collided with it, which caused large pieces of the Earth to be torn off and enter the surrounding space; due to the Earth’s gravity, a large part of these materials was fused and formed the moon, which is At that time, it began to orbit the Earth.

The movement of materials beneath the Earth’s crust caused the formation of tectonic plates, so the direction of large pieces of rock on the Earth led to the construction of the crust. The wear and collision of these plates caused the creation of mountains and volcanoes, which released more gas into the Earth’s atmosphere.

Today, the number of asteroids and comets that enter the solar system is minimal. However, these objects were more when the Sun and planets were young. The collision between these cosmic bodies has probably led to water accumulation on the Earth’s surface.

The formation of planetsThe final stages of planet formation with protoplanets and asteroids are shown in this illustration.

Earth is located in an area known as the life belt or Goldilocks, A region close to the star where the temperature is suitable for forming liquid water on the planet’s surface. In other words, the Earth is neither cold enough for water to freeze nor hot sufficient to evaporate. According to many scientists, the presence of the Earth in such an area and the presence of liquid water have played an essential role in forming known life.

By studying and observing exoplanets, scientists have come to believe that the core accretion model is more compatible with the formation of planets. Stars with more “metal” in their cores host more massive planets than their less-metal counterparts. However, ers use metal for chemical elements heavier than hydrogen and helium. According to NASA, the accretion core model proves that small rocky worlds should be more common than heavier gas giants.

One of the findings that stabilize the validity of the core accretion model was the discovery in 2005 of a massive planet with a heavy core orbiting a Sun-like star called HD 149026. According to Greg Henry, an astronomer at the University of Tennessee, this finding confirms the core accretion theory for forming planets and the abundance of these types of worlds.

In 2019, the European Space Agency launched the Exoplanet Survey Satellite (CHEOPS), whose main goal was to study exoplanets in the super-Earth to Neptune dimension range. The purpose of such missions is to investigate distant worlds and collect information about the possibility of different formations of planets in other star systems.

According to the CHEOPS group, in the core accretion scenario, the planet’s core must reach a certain mass to collect the surrounding gas. This specific mass depends on several physical variables, such as the accretion rate of the asteroid.

Disk instability model

Although the core accretion model can well explain the formation of rocky planets near the Sun, according to must proliferate to hold large amounts of gas. However,  according to this theory, the simulations could not recreate this state. In these simulations, the accumulation process of gases takes several million years. On the other hand, the core accretion model is not the only explanation for the formation of planets.

According to the relatively new theory of disc instability theory, dust and gas cluster together early in the formation solar system’s appearance, and these masses compact together to form giant planets. In this model, planets form faster than in the accretion model, so sometimes planets’ formation times several thousand years. This short time is an excellent opportunity to absorb light and volatile gases. With this model, the possibility of planets falling into the Sun is also ruled out because these objects reach stability in their orbits faster.

If disk instability is the primary planet formation process, we should see large and large numbers of planets forming. Four giant planets located significantly from the star HD 9700 are objective evidence to prove the disk instability theory. The exoplanet Femalhut B, which completes its highly elliptical orbit around its star in approximately 1700 Earth years, can be an example of disk instability. But in the latter case, interactions with neighboring bodies could also force the planet into distant orbit by interaction, showing the Moon and Earth systems early in their formation.

Pebble accretion

The biggest challenge facing the nuclear accretion model is the issue of time. Gas giants must form quickly, or they will miss the opportunity to trap light elements. But we said that the performed simulations estimate the formation time of gas giants to be several million years. However, a newer model called pebble augmentation can fill this gap. According to this model, smaller pebble-sized objects merge to form more giant planets at a rate a thousand times faster than other models.

According to Harold Levison, an astronomer at SwRI and lead author of the Pebble Accretion Model study, this model starts with a straightforward structure for the solar nebula and ends up with the massive system we know.

In 2012, researchers Michel Lambrecht and Anders Johannes of Lund University in Sweden hypothesized that small pebbles from the formation process could be the correct solution to the problem of planet formation.

Based on this research, Levison and his team were able to model the formation of the planets we see today using pebbles. While previous simulations showed that large or medium-sized celestial bodies captured pebbles at a roughly constant rate, according to Lewis’ simulations, stones are trapped faster.

Scientists are trying to gain more knowledge about the formation process of the Earth and its neighbors by continuing their investigations on the planets inside and outside the solar system.

Dust disk of a young star
Imaging the dusty disk around a young star

starting ground

The early Earth was very different from today’s Earth. In the beginning, the Earth’s surface was filled with molten magma. Over a few hundred million years, our planet began to cool, and oceans of liquid water formed. Heavy elements were sent to the seas and magma to the center of the Earth. In this way, the Earth found several layers, the outermost layer of which contains lighter materials, and denser and heavier materials moved towards the center of the Earth.

According to scientists, the Earth reached its current state in three stages. The first stage, according to the previous article’s previous sections, was the planet Earth’s formation stage of the construction of the Earth was probably the collision of the Earth with a protoplanet, which occurred about 4.5 billion years ago and, according to some possibilities, became the cause of the formation of the Earth’s moon. But the final stage was the bombardment of the Earth with meteorites.

Earth’s early atmosphere was probably a mixture of hydrogen and helium. With planetary changes and crust formation, many volcanic eruptions occurred. These volcanoes release water vapor, ammonia, and carbon dioxide into the Earth’s atmosphere. Slowly the oceans formed, and the first life evolved in the oceans.

Asteroid bombardment

Asteroid hitting the earth
Earth was subjected to asteroid bombardment in its early formation.

In the early stages of Earth’s formation, asteroids constantly bombarded the Earth. Scientists believe that the asteroids that hit the Earth, the Moon, and other inner planets of the solar system contain large amounts of water in their minerals, which are essential for forming life. These asteroids seemed to the Earth at a very high speed and left pieces behind. According to some researchers, nearly 30% of the water in asteroids remains in parts of rocks on Earth.

A few hundred million years after this process, about 2.2 to 2.7 billion years ago, photosynthetic bacteria evolved. Through photosynthesis, these bacteria released oxygen into the Earth’s atmosphere, and in this way, within a few hundred million years, the composition of the Earth’s atmosphere became what we know today. Nearly 78% of the Earth’s current atmosphere is nitrogen, and 21% is oxygen.

How did the Earth's atmosphere become breathable?


Despite many uncertainties about the formation of Earth and other solar system planets, three dominant models have been proposed: core accretion, disk instability, and accretion. The core accretion model applies more to rocky planets, and the disc instability model applies to gas planets.

However, the third model, i.e., the increase of gravel, has compensated for many of the problems of the first two models, including the speed of material accumulation. None of the models are definitive, and researchers will continue investigating this field.