Scientists have long suspected that the moon was created in a colossal collision between the young Earth and a planet-sized body known as 'Theia'.

But exactly where that body formed, and what it was made of has been far harder to pin down.

A new study, published in the journal Science, examines the chemical fingerprints of rocks collected on Earth and samples brought back from the lunar surface by Apollo astronauts.

By measuring the ratios of different forms of iron and other metals, researchers have been able to reconstruct the likely composition of Theia and track its origins.

At the Royal Observatory Greenwich, astronomer Jake Foster explains the long-standing theory.

Foster is not involved in the new study, but says it builds on decades of work to understand the moon’s earliest history.

"Theia, we believe, is an ancient planet that smashed into the Earth about four and a half billion years ago. It was probably around the size of Mars, and when it smashed into Earth, it essentially demolished itself. It kicked up a lot of debris into space, along with some of the Earth. And over time, that debris coalesced together to form the moon," he said.

Scientists already knew that Earth and moon rocks are almost identical in their metal isotope ratios.

These isotopes act like chemical fingerprints, and the match shows the two bodies formed from the same shared material after the impact.

But until now, that similarity also made it difficult to work out anything about Theia itself, because scientists couldn't tell how much of the material came from early Earth and how much came from the impactor.

That suggested the two bodies share common material, but it did not reveal exactly where Theia came from.

The difficulty has always been that Theia was destroyed completely, leaving only scattered clues within the moon and within the outer layers of our own planet.

The new research attempts a kind of planetary reverse engineering.

By looking at isotopes of iron, chromium, zirconium and molybdenum, scientists modelled hundreds of possible scenarios for the early Earth and Theia, asking which combinations could produce the identical isotope signatures seen today.

Those isotopes exist in slightly different patterns in different regions of the Solar System, because materials closer to the Sun formed under different temperatures and conditions than those further out.

By comparing these patterns, researchers concluded that Theia most likely originated in the inner Solar System, even closer to the Sun than the early Earth.

"One of the things about this research that does make it so exciting is not only is it helping to paint a picture of what happened to the Earth four and a half billion years ago, but also it's able to almost exactly pinpoint where this object came from. We're talking about a planet that does not exist anymore. Theia has not existed in four and half billion years, it was completely vaporised essentially and yet with a good amount of precision we can figure out where it came from," he said.

The study suggests that while the early Earth itself probably formed from material similar to known meteorite families, Theia may have included rock types not seen in surviving meteorite samples today.

This supports the idea that Theia developed very close to the Sun, where different building materials were available in the earliest days of the Solar System.

Understanding Theia’s origin helps scientists reconstruct the major stages of planetary formation, including how Earth gained its mass, how its iron core developed and why the composition of the Moon mirrors our own so closely.

AP video by Mustakim Hasnath