Planet formation theory


Formation of the Solar System

Accretion of solid bodies led to formation planetesimals – protoplanets – planets

Planet formation is a much talked upon topic amongst scientists, although most agree with Viktor Safronov’s planet formation theory. Safronov proposed the planetesimal hypothesis, which states, planets are created from dust particles which undergo collisions and stick together, eventually forming larger and larger bodies. The dust particles collide due to the gravitational forces exerted on upon the surrounding gasses by the protoplanetary disk. The protoplanetary disk is a rotating disk of matter which includes gas and dust, which surrounds newly formed stars.As the bodies increase in sizes to upwards of 1 kilometer, their own gravitational forces help them in supplementary growth into protoplanets. Another theory which tries to explain the formation of the universe was the gravitational collapse theory, which says planets form much like stars through the swift contraction of hydrogen clouds in the nebula. Although this theory provides a model of the creation of the universe, it doesn’t fit the available data, and cannot explain the creation of terrestrial-mass planets. 1


Terrestrial inner planets vs. Outer giant gaseous planets

Our solar system consists of 8 planets, which orbit around the sun. The four inner planets are Mercury, Venus, Earth, and Mars. Theses planets are composed of heavier elements such as Iron, Silicon, Magnesium, Sulphur, and Nickel, as such they are all terrestrial planets. The four outer planets are Jupiter, Saturn, Uranus, and Neptune. They are composed of lighter elements, mostly consisting of Hydrogen and Helium. The differences between the outer gaseous planets and the inner terrestrial planets can all be explained by the theory of accretion and of the activity of the protoplanetary disc. Accretion is the process by which debris and dust are joint together through collisions due to gravitational forces within the protoplanetary disc. Findings show that gaseous components of begin to dispel soon after the star birth, and the nebula is fully depleted within just several million years. As such in many cases, accretion would take place at a greater rate than the depletion of gases, allowing protoplanets of critical mass about 10 times that of Earth, to form within this time. Outer gas giants are formed within the ice line of the solar system where there is a high concentration of ice and gases allowing many collisions to place and as such create a planet of large critical mass. On the other hand, Terrestrial planets form much slower than gaseous planets, since they are composed in the ice line, a region which give little orbital space and mostly rocky and metallic materials. Their slow accretion rates are due to the fact that this region does not possess as much ice and gasses as the other regions. After the process of accretion, protoplanets of sizes of several hundred kilometres are formed, and these planets face many more collisions, eventually becoming Terrestrial planets. 1


Formation of planetary rings

Planetary rings are rings of dust, rocks, and other small particles orbiting around a planet in a flat circular motion, as seen in figure 1. These planetary rings are found in the four outer gaseous planets Jupiter, Saturn, Uranus, and Neptune. A prime example of this would be the ever famous rings of Saturn. The source of particles for these planetary rings can be traced back to the numerous collisions of Icy planetesimals which caused fragmentations and so bring forth many smaller pieces. Although the real mystery which plagues the minds of scientists, is how the material which make up these enormous rings were brought to their present position inside the gravitational radius of the planets. This process may have been different for every ring system. For instance, Jupiter’s rings are to be in a mature state, with balance between the loss of particles, and the production of new ones, supplied from the planets inner moons. On the other hand, scientists are divided over their beliefs on how the rings of Saturn were formed. Some believe the rings are remnants of the protoplanetary process, whiles others believe the rings have just come into existence only several million years ago. 1

Asteroid belt, between , Mars and Jupiter

Asteroids are small irregularly shaped bodies of material composed of rocks and metals, which rotate the sun in a region between Mars and Jupiter. Asteroids are also known as Small Solar System Bodies (SSSBs), along with other irregular shaped materials such as comets. One theory suggests that asteroids are the remains of a larger protoplanet which faced a cataclysmic collision resulting in its numerous partitions, yet this theory fails to explain why there are exactly so many asteroids in the asteroid belt. Therefore scientists widely consider the following as the correct hypothesis on the creation of the asteroid belt. It is believed that asteroids are the remnants of protoplanetary matter formed by the protoplanetary disc, which can be seen by the immense number of asteroids in our solar system. Asteroids are leftover since they were within the immense gravitational radius of Jupiter, causing them to undergo destructive collisions, and even at times slinging them at a course beyond the gravitational energies of our solar system. Therefore, no planetesimals could be formed and thus, the rocky matter remains continuously orbiting the sun.1

Formation of Earth’s moon

We as humans need the moon. For it is the Moon`s orbit around the Earth that gives our planet a stable rotation, preventing polar shifts, which could lead to catastrophic results in global climate. Some scientists even believe the moon was a fundamental factor in the evolutionary cycle of life on earth. This is because the moon is of the main causes of Tides on Earth, and these very tides were a fundamental factor in the migration of amphibians into coastal regions, and eventually onto land. 2 Approximately 4.5 billion years ago, around the time of Earth`s formation, other protoplanetary bodies were also growing. At one point a protoplanetary body hit the Earth, sending massive amounts of debris into outer space, as seen in figure 2 (below). This debris started orbiting the Earth and eventually amassed into the moon. This theory is widely accepted since it is able to explain why the moon is not composed of an iron. The moon does not contain an iron core since, at the time of the collision the Earth had already depleted all of the Iron into its core, and the protoplanets iron core fused with the Earth`s core, thus only rocky matter rid of all iron was released into space. Secondly, the Earth and the moon share the same isotope composition, which can only be if the moon were formed from material from the Earth. Other theories which tried to explain the formation of the moon are the following. First, it was proposed that the Moon is a separate planetary body which formed around Earth, and went into its orbit, yet this theory failed to explain why the moon contains a core that lacks iron. Second, it was hypothesized that the moon formed elsewhere in the solar system at a place with little iron, and was gravitated into an orbit around earth, but this theory could not justify the fact that the moon`s oxygen isotope composition was the same as Earths. Third, it was proposed that the Earth had spun so fast that it spun off the moon, explaining the equivalent isotope composition and iron-less core, yet it failed when proper calculation were done which concluded that the moons present orbit could not take place if this hypothesis were to be correct.

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  • Unusual rotation of Uranus


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