Billions of years ago, a planet named Theia – about as big as Mars – crashed into Earth while our planet was in the early stages of its evolution. This is known as the giant-impact hypothesis, and is widely accepted by scientists as the event that created the moon.

Essentially, they say, the collision ejected materials from the surface of Earth into space, which formed into a disc of dusty debris thanks to our planet’s gravitational pull and eventually coalesced into a single, larger body – the one that we see in our night sky to this day, and keep trying to land on for some reason. The big question is: where did Theia go after the crash? What did it leave behind?

4.5 billion years later (give or take) researchers claim to have unearthed some answers. According to a study led by geophysicist Dr. Qian Yuan (detailed in Nature), relics of the ancient alien planet might still be buried beneath Earth’s crust, preserved over the years in the planet’s gooey mantle. Handily, this would also explain anomalies in Earth’s composition that have puzzled scientists for some time.

Around the 1970s and 80s, scientists identified two mysterious structures in Earth’s mantle called LLVPs (or “large low-shear-velocity provinces”). These structures, located under Africa and the Pacific Ocean, are thousands of kilometres wide and may go up to 1,000 kilometres deep. They’re also unusual, because their composition is different to that of the surrounding mantle, making them stand out when scientists measure seismic waves that pass through the planet, but until now we haven’t really understood why.

According to the new theory, Theia is actually the reason behind these anomalous “blobs” under the surface of Earth. In fact, the blobs are Theia – or what’s left of it, which would explain why they’re made of different stuff to the rest of the planet. Backing up this bold claim are cutting-edge simulations of a giant, high-speed impact like the one between the ancient planet and our own, which produced breakthrough results for Yuan and his fellow researchers.

The simulations seem to confirm that fragments of Theia’s mantle could have sunk into the hot, dense lower mantle of Earth on impact. Measuring several kilometres across, the blobs of buried Theia mantle material (or “TMM”) could then have sunk into other, viscous matter that rests in the fiery cauldron above Earth’s core, and remained there ever since.

If the theory proves correct, then the answer to what remains of Theia’s explosive impact – besides the moon – has, quite literally, been right beneath our feet this whole time. This could also help to figure out the interiors of other planetary bodies, since giant impacts are thought to be fairly common during the late stages of planetary accretion, as smaller solids come together to form massive objects like Earth or Mars.

That said, the researchers, who span a variety of disciplines and institutions, are careful to note that the alien fragments under the surface of Earth are still just a theory, albeit quite a convincing one. Just as groundbreaking models (no pun intended) paved the way for their new ideas about what’s going on under our feet, new technology might throw up new theories further down the line.

It’s no surprise that the subject is a source of much debate, either. In theory, the fallout from a giant interplanetary impact holds some big implications for the evolution of our home planet, and some say it may even be key to understanding the origins of life itself.

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