Moon Has Been Captured by Earth not Born, New Study Suggests

For centuries, the origin of the Moon has been a subject of scientific research. Still, there is no consensus among scientists about how it was formed. The most widely accepted theory today is the Giant Impact Hypothesis, which suggests that a Mars-sized cosmic body hit the young Earth, creating a debris ring that over years formed the Moon. However, researchers from Penn State offer an alternative explanation – the Moon has been captured from space.

Binary-exchange Capture

According to the new research published in The Planetary Science Journal by Darren Williams, professor of astronomy and astrophysics at Penn State Behrend, and Michael Zugger, a senior research engineer at the Applied Research Lab at Penn State, Moon was a part of a binary system, together with another rocky body. Earth’s gravity separated the objects, ejecting one of them and leaving the other one (Moon) as a satellite in orbit around the planet.

Moon’s Orbit Confirms the Theory

The main argument supporting the new theory is the orbit of the Moon. A natural satellite that forms from a planetary collision, is likely to orbit above the planet’s equator. Earth’s moon orbits in a different plane, more aligned with the Sun.

Captured Just Like Triton

Darren Williams and Michael Zugger compare Moon to Neptune’s largest moon – Tritone. It is believed to be pulled into orbit from the Kuiper Belt, where 1 of every 10 objects is thought to be a binary. Triton orbits Neptune in a retrograde orbit, moving in the opposite direction of the planet’s rotation. Its orbit is also significantly tilted, angled 67 degrees from Neptune’s equator. This may be evidence to support the Moon origin theory.

In the paper researchers wrote that when the Moon was captured, its orbit was as an elongated ellipse. Over time, influenced by extreme tides, the shape of the orbit changed to be more circular.

Moon Drifts Away From Us?

Researchers from Penn State also emphasise that high tide accelerates the Moon’s orbit, giving it a boost. Over time, the Moon drifts a bit farther away. According to their calculations, every year the Moon moves 3 centimetres farther from Earth. 

The calculations show that a binary-exchange captured satellite could behave similarly to the Moon. However, for now it is not possible to be sure that it is the way it was formed. The new study provokes discussions about the Moon’s origin and opens up opportunities for further research.