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A new study challenges the prevailing idea about how Earth's Moon formed, claiming that our planet may have "captured" the Moon in an unconventional process rather than being formed as a result of a collision with a protoplanet.
Until now, the widely accepted idea was that the Moon formed after a giant celestial body called "Theia" collided with Earth early in the history of the solar system. This theory was based on the results of studies of lunar materials brought back by the Apollo missions, which showed a chemical similarity between the Earth and the Moon, supporting the hypothesis that the Moon formed from the debris of the Earth after the collision.
However, researchers from Pennsylvania State University have recently put forward a new hypothesis that challenges this theory.
The research team, led by Professor Darren Williams, says that the Earth may have "captured" the Moon through a phenomenon known as "binary exchange capture". According to this hypothesis, the moon was initially part of a "binary pair" of celestial bodies that orbited each other. When this pair passed close to Earth, our planet pulled one of the bodies into its orbit, while the other body was pushed out into space.
This theory is supported by similarities with other cases in the solar system, such as the moon "Triton" that orbits Neptune. The researchers point out that Triton, like Earth's moon, has an abnormally tilted orbit at an angle of 67 degrees, which reinforces the idea that celestial bodies can be captured by their planets in similar circumstances.
According to the study, Earth may have captured a celestial body weighing between 1 and 10% of the Earth's mass, which is consistent with the moon's mass of 1.2% of Earth's mass.
The Moon is thought to have started out in an elliptical orbit as it approached Earth, but tidal effects on the Moon's orbit over thousands of years "helped to stabilise its orbit and gradually make it circular".
The study also shows how tidal effects have caused the Moon to move away from Earth by 3cm each year, explaining why it is constantly moving away from our planet.
This model provides an explanation for many phenomena that would not have been well explained by the traditional collision theory, such as the tilt of the Moon's orbit and some chemical differences between the Earth and the Moon.
Although the researchers admit that proving this hypothesis is difficult due to the need for "unreasonable events" to occur simultaneously, they believe that "binary exchange capture" is a viable alternative to the standard collision theory and deserves further research.
Williams says: "This discovery opens up many new questions that could enhance our understanding of the Moon's history, and provide opportunities for further research in this area."
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