Scientists Propose Radical New Theory on Moon’s Origins

For centuries, scientists have debated the origins of Earth's only natural satellite, the Moon.

Since the 1980s, the predominant theory has posited that the Moon formed from debris following an explosive collision between early Earth and a protoplanet named Theia.

However, a new study by researchers from Penn State University challenges this "collision" model, proposing instead that Earth might have captured the Moon as it drifted nearby in a process called binary-exchange capture.

The study suggests that the Moon could have initially been part of a "terrestrial binary" – a pair of rocky bodies orbiting one another.

As the binary passed within Earth's gravitational pull, the Moon was pulled into orbit, while the other body was cast out into space.

Lead researcher Professor Darren Williams stated, "No one knows how the Moon was formed. For the last four decades, we have had one possibility for how it got there. Now, we have two."

In 1984, the Kona Conference in Hawaii sought to reach a consensus on the Moon’s formation, drawing on data from lunar material brought back by NASA's Apollo missions.

Scientists concluded then that the Moon was created from debris after a large celestial body collided with early Earth, as evidenced by the Moon’s similar yet not identical chemical composition to Earth.

Although this theory has gained traction due to its alignment with the Moon’s known chemical makeup, it fails to account for all details, such as the Moon's seven-degree tilt away from Earth's equatorial plane.

Williams and his team explored binary-exchange capture as an alternative, noting that it could explain the Moon's tilted orbit.

This phenomenon mirrors Neptune's capture of Triton, its largest moon, which is thought to have been pulled from the Kuiper Belt, where about 10% of objects are binaries.

Triton’s orbit is tilted 67 degrees from Neptune's equator, paralleling the Moon’s inclination around Earth.

According to the study, Earth could have feasibly captured an object between 1-10% of its mass; the Moon, at 1.2% of Earth's mass, fits this range.

For this to occur, the binary object would need to pass within 80,000 miles of Earth at speeds below 6,700 miles per hour.

Though this proximity and speed might seem fast, they would appear moderate on a cosmic scale.

Initially, the Moon would have orbited Earth in an elongated, comet-like path.

The study suggests that tidal forces gradually stabilized the Moon’s orbit, transforming it from a wild ellipse to its current close, circular orbit over millennia.

As Professor Williams explains, "Today, the Earth tide is ahead of the Moon. High tide accelerates the orbit. It gives it a pulse, a little bit of boost. Over time, the Moon drifts a bit farther away."

The Moon now drifts about 3 centimeters farther from Earth each year, influenced by the combined pulls of Earth and the Sun.

The binary-exchange capture theory has advantages over the collision model, as it accounts for both the Moon's orbital tilt and the unique chemical isotopes present on the Moon but absent on Earth.

Though Professor Williams acknowledges that proving this theory remains challenging and would require several "improbable events" to occur in tandem, he asserts that binary-exchange capture offers a credible alternative to the traditional collision hypothesis.

According to Williams, "This opens a treasure trove of new questions and opportunities for further study," as planetary binaries may have been more common in the early solar system, potentially leading to the Moon’s creation.