After the sun shone, the formation of the planets of the solar system began. But it took another hundred million years for the moon to form. There are three theories about how our satellite could have been born: the giant collision hypothesis, the conformity theory, and the capture theory.
Giant collision hypothesis
This is the predominant theory held by the scientific community. Like other planets, the Earth was formed from the remaining cloud of dust and gas orbiting the young Sun. The early solar system was a hot spot where several bodies formed that never reached full planetary status. According to the giant collision hypothesis, one of them crashed into the Earth shortly after the formation of the young planet.
It was a Mars-sized body known as Theia. The object collided with the Earth, ejecting evaporated particles of the young planet's crust into space. Gravity brought the ejected particles together to form the moon. This birth explains why the Moon is composed primarily of lighter elements, which makes it less dense than the Earth - the material that formed it came from the crust, and the rocky core of the planet remained intact. As the material gathered around what was left of Theia's core, it concentrated near the plane of the Earth's ecliptic - the path the sun travels across the sky and where the lunar orbit is today.
Conformity theory
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According to this theory, gravity facilitated the simultaneous unification of material in the early solar system into the Moon and Earth. Such a moon should be very similar to the planet, and its location should coincide with the current one. But while the Earth and the Moon are mostly made of the same material, the Moon is much less dense than our planet, which would be unlikely if both bodies began to form their cores from the same heavy elements.
Capture theory
Perhaps the gravity of the Earth caught on a passing body, as happened with other moons in the solar system, such as the Martian moons Phobos and Deimos. According to the capture theory, a rocky body formed in other parts of the solar system could be launched into orbit around the Earth. Capture theory explains the differences in the composition of the Earth and the Moon. However, such satellites often have an unusual shape, rather than spherical like that of the Moon, and their paths, unlike the lunar, do not tend to align with the ecliptic of their mother planet.
Although the theory of co-formation and capture theory explain some aspects of the Moon's existence, they leave many questions unanswered. The giant collision hypothesis covers most of them, making it the most popular among scientists.