Study Suggests Earth Captured the Moon from Space, Rather Than Forming It

For nearly four decades, the widely accepted theory has been that the Moon formed from debris resulting from a massive collision between Earth and another celestial body. However, a new study presents an alternative possibility: What if the Moon wasn’t formed this way, but instead was a separate object from space that Earth’s gravity managed to capture?

During the Apollo missions between 1969 and 1972, astronauts collected over 800 pounds of lunar rocks and soil for analysis. These samples were found to be compositionally similar to materials on Earth, leading scientists to conclude that the Moon originated from Earth, forming around 60 million years after the solar system came into existence. At the 1984 Kona Conference, scientists largely agreed that the Moon had resulted from a collision with a young Earth.

But two Penn State researchers, Darren Williams and Michael Zugger, challenge this long-standing theory. Their study, published in *The Planetary Science Journal*, suggests that the Moon may not have formed from this collision after all. Instead, they propose the Moon might have been part of a binary system with Earth—two space bodies orbiting each other—until Earth’s gravity pulled the Moon in and ejected the other object.

“The Kona Conference shaped the prevailing narrative for 40 years,” said Williams, a professor of astronomy and astrophysics at Penn State Behrend. But he and Zugger, a senior research engineer at Penn State’s Applied Research Lab, argue that this explanation leaves unresolved issues.

Why the Moon likely came from elsewhere
One key issue is the Moon’s current position. If the Moon had formed from a collision, the debris would have clumped together and formed a ring orbiting above Earth’s equator. Yet, the Moon orbits in a plane more aligned with the Sun than with Earth’s equator.

“The Moon’s orbit is more in sync with the Sun than Earth’s equator,” Williams explained.

In their binary-exchange capture theory, Williams and Zugger argue that Earth’s gravity captured one of the two objects in the binary system, making it the Moon. The other object was repelled.

They also highlight another similar case within our solar system: Triton, Neptune’s largest moon. Current research suggests Triton was captured into Neptune’s orbit from the Kuiper Belt, a region filled with binary objects.

Initially, according to Williams and Zugger, the Moon’s orbit around Earth was highly elliptical. Over time, extreme tidal forces altered its shape, eventually becoming the near-circular orbit we see today. The researchers calculated the tidal effects and the Moon’s orbit, determining that over thousands of years, the Moon’s orbit contracted and became more circular.