Dr. Darren Williams, professor of astronomy and astrophysics, has a new theory of how the moon came to orbit Earth.
Behrend astronomer publishes new hypothesis about the moon’s origin
Over six missions to the moon, from 1969 to 1972, Apollo astronauts collected more than 800 pounds of lunar rock and soil. Chemical and isotopic analysis of that material showed that it was similar to the rock and soil on Earth: calcium-rich, basaltic, and dating to about sixty million years after the solar system formed.
Using that data, the planetary scientists who gathered at the Kona Conference in Hawaii in 1984 came to the consensus that the moon formed from debris after a collision on the young Earth.
But that might not be the moon’s true origin story. New research published in The Planetary Science Journal by Darren Williams, professor of astronomy and astrophysics, and Michael Zugger, a senior research engineer at Penn State’s Applied Research Lab, offers another possibility: That the moon was captured during a close encounter between a young Earth and a terrestrial binary—the moon and another rocky object.
Questions Lingered
“The Kona Conference set the narrative for forty years,” Williams said.
Questions lingered, however. For example, a moon that forms from a planetary collision, taking shape as debris clumps together in a ring, should orbit above the planet’s equator. Earth’s moon orbits in a different plane.
“The moon is more in line with the sun than it is with the Earth’s equator,” Williams said. In the alternative binary-exchange capture theory, Earth’s gravity separated the binary, snagging one of the objects—the moon—and making it a satellite that orbits in its current plane.
There is evidence of this happening elsewhere in the solar system. Scientists believe that Triton, the largest of Neptune’s moons, was pulled into orbit from the Kuiper Belt, where one of every ten 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 sixty-seven degrees from Neptune’s equator.
Williams and Zugger determined that Earth could have captured a satellite even larger than the moon— an object the size of Mercury or Mars—but the resulting orbit might not have been stable.
The problem is that the “capture” orbit—the one the moon follows—began as an elongated ellipse, rather than a circle. Over time, influenced by extreme tides, the shape of the orbit changed.
“Today, the Earth tide is ahead of the moon,” Williams said. “High tide accelerates the orbit. It gives it a pulse, a little bit of boost. Over time, the moon drifts a bit further away.”
The effect is reversed if the moon is closer to Earth, as it would have been immediately after capture. By calculating tidal changes and the orbit’s size and shape, Williams determined that the moon’s initial elliptical orbit contracted over a timescale of thousands of years. The orbit also became more circular, rounding its path until the lunar spin locked into its orbit around the Earth, as it is today.
At that point, Williams said, the tidal evolution likely reversed, and the moon began to gradually drift away.
Every year, he said, the moon moves three centimeters farther from Earth. At its current distance from Earth—239,000 miles—the moon now feels a significant tug from the sun’s gravity.
“The moon is now so far away that both the sun and Earth are competing for its attention,” Williams said. “Both are pulling on it.”
A Second Theory
His calculations show that, mathematically, a binary-exchange captured satellite could behave as Earth’s moon does. But he’s not certain that’s how the moon came to be.
“No one knows for sure how the moon was formed,” he said. “For the last four decades, we have had one possibility for how it got there. Now, we have two. This opens a treasure trove of new questions and opportunities for further study.”
Total Eclipse, Lifelong Impact
The 2024 total solar eclipse was science at 2,400 mph. As the moon’s shadow swept across Erie, which experienced nearly four minutes of totality—the moon completely obscuring the sun’s light—a large crowd at the Behrend soccer complex began to cheer.
“In that moment, everyone was an astronomer,” said Dr. Darren Williams, professor of astronomy and astrophysics.
We asked Williams and Jim Gavio, director of the Yahn Planetarium, what they hoped people would take from this once-in-a-lifetime celestial experience:
Williams: “My hope is that, as we shared that experience, people rediscovered some of the childlike curiosity and wonder for things we haven’t seen before, or don’t fully understand. That’s how you begin to appreciate the process of science.”
Gavio: “Many of the children who experienced that moment will remember it for their entire lives. That’s an opportunity: An eclipse makes science fun and accessible and might lead more people to pursue their dreams in astronomy or other types of science.”