The solar system formed from a cloud of gas and dust, with the Sun forming as part of the cloud collapsed under gravity. Earth was created about 4.54 billion years ago from material in the accretion disk. Stars form from giant molecular clouds, and planets can form from excess gas or dust particles clumping together. Rocky planets like Earth needed heavier elements that were created within stars by nuclear fusion and supernova explosions. The Earth’s formation involved massive impacts, including the collision that may have created the Moon.
The creation of the Earth is closely linked to the formation of the solar system about five billion years ago. The solar system condensed from a huge cloud of gas and dust, with the Sun forming as part of the cloud collapsed in on itself under the influence of gravity to a point where nuclear fusion could begin. The gravitational field of the Sun has attracted large amounts of material, which has formed a disk-shaped structure around it, known as an accretion disk. Earth, like the other planets, was created about 4.54 billion years ago when some of the material in this disk came together to form a spherical body. At some point early in its history, a smaller planet is thought to have collided with this body, increasing its size and resulting in the formation of the Moon.
Star formation
Stars form from huge clouds of gas, mainly hydrogen, known as giant molecular clouds, as they are made up of molecules. The first stars began to appear when the universe was cold enough for hydrogen molecules to form. The parts of these clouds where the density is somewhat higher accumulate more gas through gravitational attraction, forming globular regions of relatively high density. These are known as “Bok’s globules,” after astronomer Bart Bok, and can be seen today in other parts of the galaxy. Parts of these globules further condensed under gravity, until the nuclei of hydrogen atoms were compressed so much that nuclear fusion took place, resulting in the birth of a star.
The density fluctuations that lead to the collapse of parts of a giant molecular cloud may be small variations that are present from the start. Alternatively, some events can compress parts of the cloud. One possibility is that the cloud could pass through the arm of a galaxy, where there is a higher density of preexisting stars. Another is the compression of shock waves from a nearby supernova.
Formation of the planet
The material surrounding a new star orbits it, eventually settling into an accretion disk. From this material, planets can form in two ways. Excess hydrogen, along with small amounts of other gases, can condense in giant planets like Jupiter and Saturn. The quantities of gas involved are not enough for gravity to cause nuclear fusion, so they remain planets rather than stars. The other, much slower way is for the dust particles to clump together, forming larger masses that collide with each other and merge to form asteroids and planets.
Rocky planets like Earth could not have formed as part of the first wave of star formation as no suitable material was available. At this point there was only hydrogen and helium, both gases, and a trace of lithium, a very light metal. The heavier elements needed to form rock were created within stars by nuclear fusion. This process, however, can only create the elements up to and including iron. There are many elements heavier than iron present on Earth and some of them are essential for human life.
Elements heavier than iron can only be produced by a supernova explosion. It follows that there must have been at least one supernova in the vicinity of the solar system before its formation. It may be that this is what triggered the collapse of the molecular cloud that formed the sun and planets.
The Formation of the Earth
The processes that form star systems are still ongoing and can be observed, in various stages, in other parts of our galaxy. The formation of the solar system is thought to have followed a similar pattern. There are, however, some particular events that helped shape the Earth as we know it today.
It is not known precisely what mechanism caused part of a molecular cloud in the Sun and its accretion disk to collapse. Whatever the cause, when the center became dense enough, it ignited to become the Sun. Streams of particles – known as the “solar wind” – from the new star banished the gases to the outer solar system, where they formed the gas giant planets. Chunks of rocky material remained in the inner solar system, where they could grow into planets.
Once the Earth formed, it started to warm up. This phenomenon was due to a combination of the decay of radioactive elements, the continuous compression of the planet’s material by gravity and meteorite impacts. As the material melted, different elements became mobile and heavier elements, such as iron, gravitated towards the center, forming the nucleus responsible for the earth’s magnetic field. Lighter materials, such as silicates, floated to the surface, forming the crust. The relatively thin, solid crust atop the denser molten material gave rise to plate tectonics and volcanism.
Our planet’s early history was not a smooth one, but it did involve a series of events featuring massive impacts. The largest of these collisions may have created the Moon. Strong evidence suggests that shortly after its formation, the planet was struck by a Mars-sized body called Theia, which may have formed at a Lagrange point – a point of gravitational equilibrium – in Earth’s orbit. This collision would have ejected many gigatons of material which would then go into orbit and merge to form the Moon.
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