Despite being our planet’s closest neighbour, scientists have been grappling with the mystery of our moon’s origin for hundreds of years.
Since the 1980s, experts have assumed that our lunar satellite was formed in the aftermath of an explosive impact with the protoplanet Theia early in Earth’s history.
But now, a study suggests that this so-called ‘collision’ theory may not be correct after all.
Instead, researchers from Penn State University say that the Earth might have neatly nabbed the moon as it drifted by in a process called binary-exchange capture.
This radical theory suggests that the moon might have started life as one part of a ‘terrestrial binary’, a pair of rocky objects orbiting one another.
As this pair passed within Earth’s pull, the moon was yanked into orbit while the second body was catapulted out into space.
Lead researcher Professor Darren Williams says: ‘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.’
A new theory suggests that the moon might not have been formed by a collision with an ancient protoplanet but through a process called ‘binary-exchange capture’
Researchers suggest that the moon might have started out as one part of a planetary binary, two large rocky bodies which orbit each other as they drift through space (file photo)
In 1984, scientists gathered for the Kona Conference in Hawaii to come to a consensus on how the moon had formed.
Using the 800lbs (363kg) of lunar material taken back by NASA’s Apollo missions, the scientists found that the moon had a similar but not entirely identical chemical composition to Earth.
From this evidence, they came to the conclusion that the moon must have been formed out of debris knocked loose when a celestial body slammed into the young Earth.
This theory was popular because it fits well with much of what we now know about the chemical composition of the moon – but it doesn’t explain all the details.
For example, Professor Williams and his co-author point out that, if the moon were formed from a ring of debris slowly condensing into a sphere, we should expect to find it orbiting above the equator.
However, the moon’s orbit is actually tilted onto a totally different plane about seven degrees away from the equatorial plane.
To find an alternative explanation for why this might be the case, the researchers looked at a phenomenon called binary-exchange capture.
This suggests that Earth might have snagged one of a pair of passing rocky bodies and made it into its satellite.
Earlier theories suggest that the moon was formed when a protoplanet called Theia collided with the young Earth. Scientists believe the moon was formed as the resultant ring of debris collected into a sphere
In support of this idea, Professor Williams points to the example of Triton, Neptune’s largest moon.
Current theories suggest that Triton was pulled into Neptune from the Kuiper Belt where one in 10 objects are thought to be a binary.
Just like our moon, Triton orbits at a significantly tilted angle, leaning 67 degrees away from the planet’s equator.
And, according to mathematical models, it is quite plausible that the same thing could have happened to our moon.
In their paper, published in The Planetary Science Journal, the researchers calculate that Earth could have captured an object between one and 10 per cent of its total mass.
At just 1.2 per cent of the Earth’s mass, the moon falls comfortably within this range.
The only caveat is that the planetary binary would have had to have passed within just 80,000 miles (128,750km) of Earth at a speed below 6,700 miles per hour (10,800km).
While that might seem incredibly fast, in the scale of the solar system that is the equivalent of a leisurely stroll.
Researchers point out that if the moon were formed from a collision we should expect it to orbit Earth around the equator. But the moon’s orbital plane is angled at about 7 degrees from the equator
Using a mathematical model (pictured), the scientists calculated that the Earth could capture an object between one and 10 times its mass if it was travelling at the right speed and the other part of the binary had a large enough mass. As this graph shows, the Earth is easily capable of capturing something as large as the moon at speeds of up to 3km per second
The problem is that, even at these sedate speeds, when it first arrived, the moon’s orbit would have been massively elliptical much like that of a comet around the sun.
However, the researchers also show how this orbit would have evolved under the influence of tidal forces.
As the moon races around the Earth, the tides would have slightly lagged behind its orbit, exerting a gravitational pull which would have slowly tamed its wild orbit.
Over thousands of years, that constant tug would have made the orbit more regular and circular until it settled into the close orbit it currently has.
Professor Williams says: ‘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.’
Now the moon is so far out that both the Sun and Earth pull on it, leading it to drift about 3cm further away every year.
This theory has some key advantages in that it explains why the moon’s orbit is so tilted and accounts for the presence of certain chemical isotopes found on the moon and not on Earth.
Scientists believe that Triton (pictured), the largest moon of Neptune, was originally a planetary binary in the Keuper belt before being captured
As this graph shows, when the moon first arrived, its orbit would be shaped like an oval rather than the circle we are more familiar with
The researchers admit that their theory would be hard to prove and relies on several ‘implausible events’ happening at once.
However, Professor Williams maintains that binary-exchange capture is a viable alternative to the standard collision theory and one that merits further consideration.
The researchers argue that planetary binaries may have been more common in the early solar system and could have plausibly gone on to create the moon.
Professor Williams adds: ‘This opens a treasure trove of new questions and opportunities for further study.’
