The Moon is Earth's only natural satellite, orbiting at an average distance of 384,400 km. Scientists have proposed several hypotheses to explain how the Moon formed approximately 4.5 billion years ago. Four scientists present their viewpoints below.
The Moon formed as a result of a giant impact between the early Earth and a Mars-sized object, sometimes called Theia. This collision occurred approximately 4.5 billion years ago, when the solar system was still forming. The impact ejected a massive amount of debris into orbit around Earth. Over time, this debris coalesced to form the Moon.
Evidence supporting this hypothesis includes the Moon's relatively low iron content compared to Earth; the iron-rich core of Theia would have merged with Earth's core during the impact, while the iron-poor mantle material formed the debris disk. Additionally, the angular momentum of the Earth-Moon system is consistent with a large, off-center impact.
The Moon and Earth formed together from the same cloud of gas and dust that created our solar system. As the protoplanetary disk around our young Sun condensed, Earth and the Moon simultaneously accreted from neighboring regions of this disk. This explains why the Moon orbits in nearly the same plane as Earth's orbit around the Sun.
This co-formation model is supported by similarities in the oxygen isotope ratios between Earth and Moon rocks. If the Moon had formed from a different body (like Theia), we would expect different isotope ratios. The nearly identical ratios suggest both bodies formed from the same source material.
The Moon originally formed elsewhere in the solar system and was later captured by Earth's gravitational field. When the Moon passed close enough to Earth, gravitational forces slowed it sufficiently to trap it in orbit. This capture scenario would explain why the Moon's orbit is slightly elliptical and inclined relative to Earth's equator.
Some lunar rocks contain minerals that are rare on Earth but common in the outer solar system, suggesting the Moon may have originated farther from the Sun. The capture hypothesis also explains the Moon's relatively large size compared to Earth; it formed independently and could have grown larger than a satellite that formed in place.
The Moon formed from Earth itself through a process called fission. Early in Earth's history, our planet was rotating extremely rapidly — possibly completing one rotation every 2–3 hours. This rapid rotation created a bulge at the equator, and eventually, a portion of Earth's outer layers was flung off into space. This material became the Moon.
The fission hypothesis explains the chemical similarities between Earth and Moon rocks; they came from the same body. It also explains why the Moon is gradually moving away from Earth (currently about 3.8 cm per year) — it has been moving outward since it was ejected billions of years ago. The Pacific Ocean basin may mark the location from which the Moon was ejected.