About 4.6 billion years ago, within the spinning disk of dust and gas that surrounded the newly forming Sun, small chunks of material collided and amalgamated, culminating in the collisions of larger bodies called protoplanets. These collisions, in turn, led to the release of tremendous amounts of heat energy, and as a result Earth was born in a hot, molten state.
The origins of our planet are closely tied to the formation of the whole Solar System—the Sun, the eight planets orbiting it, and many other bodies, such as comets and asteroids. Astronomers now agree that the Solar System started forming about 4.6 billion years ago out of an immense, slowly spinning cloud of gas and dust within the Milky Way galaxy.
Gradually, gravity caused the cloud to contract and spin faster, and as its central region became denser, it also became hotter. This region eventually became the Sun. Surrounding the central region was a spinning disk of gas, dust, and ice.
Within the disk, grains of ice and dust stuck together to form solid particles of ever-increasing size— pebbles, rocks, boulders, and eventually bodies called planetesimals, which can be anywhere in size from several feet to hundreds of miles wide. These came together through collisions to form protoplanets, which were roughly the size of our present-day Moon.
Protoplanets underwent a series of violent collisions to form the four inner planets (Mercury, Venus, Earth, and Mars) and the cores of the outer giant planets, such as Jupiter.
It is not known how many protoplanets came together to form Earth, but it may have been a dozen or so. With each collision, a tremendous amount of heat was generated as the kinetic energy of the colliding bodies
converted to heat energy. In addition, as the number of protoplanets diminished, and their size grew, each one contracted under the influence of its own gravity, a process that also generated heat.
Eventually, a final collision is thought to have occurred between an object almost the size of Earth—a precursor of our planet that is sometimes called proto-Earth or young Earth —and a protoplanet about the
size of Mars, known as Theia. The result of this final collision was the Earth-Moon system, comprising Earth itself and its orbiting moon.
CORE FORMATION
It is likely that a process called differentiation occurred in each of the larger protoplanets that came together to form Earth. Differentiation, which is only possible in molten bodies, involves heavier materials, such as iron and nickel, sinking to the middle to form a core.
With each major protoplanet collision, enough heat would have been generated to keep the combined body molten, and the two cores would have quickly merged. This means that Earth is likely to have had a core soon after it came into existence.
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