New Theory Could Explain Where Our Moon Came From

The study was published Monday in the journal Nature.

The new theory claims that the Moon and the Earth originated from one and the same material, that split apart as a result of a collision.

"In the giant-impact hypothesis for lunar origin, the Moon accreted from an equatorial circum-terrestrial disk; however, the current lunar orbital inclination of five degrees requires a subsequent dynamical process that is still unclear," the study reports.

It has been long believed that a "giant impact" phase during the formation of Solar System saw hot planet-size objects colliding with one another. A Mars-sized object collided with a body that later became the Earth. A big part of the body's material was cast off and condensed, later becoming the Moon. The earth got a five-hour day, but the Moon continuously receded from the Earth, slowing its rotation to 24 hours.

However, there are some points that are not explained by the classic theory. One is the similar composition of the Earth and its satellite.

"The giant-impact theory has been challenged by the Moon's unexpectedly Earth-like isotopic composition," the researchers wrote. Another challenge is the five-degree inclination of the Moon's current orbit, which should have been over the Earth's equator if it condensed from the material rotating around Earth's equator.

The alternative theory was proposed by Sarah Stewart of the University of California, Davis; Matija Ćuk, a scientist at the SETI Institute in Mountain View, California; Douglas Hamilton at the University of Maryland; and Simon Lock, of Harvard University. They suggest that some of the angular momentum of the Earth-Moon system was transferred to the Earth-Sun system, allowing more energetic collision at the beginning of the process that resulted in mixing the material of Earth and its impactor. The material then condensed, giving rise to the Earth and the Moon. This theory goes far in explaining their similar composition.

"[…] the solar perturbations on the Moon's orbit naturally induce a large lunar inclination and remove angular momentum from the Earth-Moon system," the article states. This flipped the Earth upright with the Moon orbiting at a high angle to the equator. Over millions of years, that angle dropped to about five degrees.

Thus, the new theory also explains how the Moon got to its current position in the Solar System.