The Moon is believed to have formed as a result of Earth colliding with a minor planet known as Theia approximately 4.5 billion years ago. But now experts have put forward a new theory about the origin of the Moon. The study suggests that the young Earth may have caught up with the Moon during a close encounter with the binary system.

Similarities between Earth and Moon

During six missions to the moon from 1969 to 1972, Apollo astronauts collected more than 800 pounds of moon rock and soil. Chemical and isotopic analysis of these samples revealed their similarities to Earth’s rock and soil. They were calcium-rich, basaltic, and dated to about 60 million years after the formation of the solar system.

Rejection of the theory of collision of planets

Planetary scientists gathered at the Kohn Conference in Hawaii in 1984 agreed, based on Apollo data, that the Moon was formed from debris from a massive impact. This explanation for the origin of the moon has shaped scientific understanding for decades. But two researchers from Pennsylvania State University are challenging this long-standing narrative.

The study, led by Professor Darren Williams and Professor Michael Zugger, suggests that the Moon was captured during a close collision between the Earth and a pair of rocky bodies.

Unsolved questions about the origin of the Moon

“The Kona conference set the 40-year story,” Williams said. However, some unresolved problems still remain. One of them is related to the orbit of the Moon. If the Moon was formed from debris from collisions of planets and coalesced into a ring around the Earth, it should orbit directly above the planet’s equator. In contrast, the Moon’s orbit is not aligned with the Earth’s equator but coincides with the Sun.

Did Earth’s gravity capture the moon?

In the binary exchange capture theory proposed by Williams and Zugger, Earth’s gravity separated two objects in the binary system and captured the Moon as the other object moved away. The captured Moon then reached the orbit we observe today.

This process is not unheard of in the solar system. Williams cited Neptune’s largest moon, Triton, as evidence of such an event.

Triton is believed to have been brought into orbit from the Kuiper Belt, where about 10% of objects consist of binary pairs. Triton’s retrograde orbit (the opposite of Neptune’s rotation) and the planet’s 67-degree tilt from its equator suggest a capture event.

Dynamics of the Moon’s orbit

Williams and Zugger calculated that the Earth could capture a satellite even larger than the Moon (potentially an object the size of Mercury or Mars). But they suggested that the resulting trajectory was not stable enough to last. Researchers explained that initially the Moon’s orbit looked like an elongated ellipse, not a circle. Over time, tides on Earth affected the shape of the orbit, causing it to change.

“Today, the Earth’s tide is ahead of the Moon’s tide,” Williams said. “The tide speeds up the orbit. That gives it a pulse, a little push. Over time, the moon moves a little further away.”

The Moon’s elliptical orbit became circular

However, when the Moon approached Earth immediately after capture, the tides had the opposite effect. Williams and Zugger predict that the initial elliptical orbit will narrow and become increasingly circular over thousands of years.

Eventually, the moon’s rotation became fixed in the Moon’s orbit around the Earth, a situation that continues today. At this point the tidal evolution probably changed and the Moon began to slowly move away from the Earth.

Gravitational force of Earth and Sun

Williams explained that each year the moon moves about three centimeters away from Earth. The Moon is currently 239,000 miles away, and at that distance it is subjected to a strong gravitational pull from both the Sun and the Earth.

“The moon is now so far away that both the sun and the earth are competing for its attention,” Williams said. “They’re both striving for it.”

The formation of the Moon still remains a mystery

Williams and Zugger’s calculations show that a satellite captured by binary exchange could exhibit the same behavior as an Earth satellite. However, Williams acknowledged that this theory is not necessarily the final word.

“No one knows how the Moon was formed. Over the past four decades, we’ve had one chance to understand how it got to this point. Now we have two. This opens up a treasure trove of new questions and opportunities for further study,” he concluded. The study was published on: Planetary Science Journal.