Hiyah Zaidi
Published November 21, 2025 2:50pm
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Around 4.5 billion years ago a large celestial body called Theia collided with the young Earth. However, experts have never been sure how the collision unfolded and what exactly happened afterwards. What we do know is that as a result, the size, composition and orbit of the Earth changed — and that we gained the Moon, Earth’s now constant companion. (Picture: Getty)
There are so many questions to answer, like what was Theia, what was it made of and how did it crash into the Earth? Theia was completely destroyed in the collision, but to this day we can still see traces of it in both the Earth and Moon. Now, research led by the Max Planck Institute for Solar System Research (MPS) and the University of Chicago has tried to figure out where this body came from. (Picture: Getty)
The researchers say that Theia was once on a stable orbit within the inner solar system, sitting just slightly closer to the Sun than Earth is today. And for the first 100 million years of the solar system's existence, Earth had a neighbour. Speaking to the Daily Mail, lead author Dr Timo Hopp said: ‘It was on a relatively stable orbit around the Sun. We infer that this must have been closer to the Sun than Earth, however, that is all we can say.’ (Picture: Getty)
The researchers say that when Theia collided with the Earth, all the material that survived the impact was swallowed up into the Earth or the Moon, and if any debris did escape, it was shot out of a stable orbit and has long since disappeared. So, they looked at isotopes. These are variants of elements that have different numbers of neutrons in their atomic nucleus, and when the solar system formed, the isotopes of the various elements weren't distributed evenly. (Picture: Getty)
So, rocks in the outer reaches of the solar system have a different ratio of isotopes to those found closer to the Sun. The researchers then took extremely precise measurements of the iron isotopes in rocks from the Earth, Moon rocks brought back by the Apollo missions, and several asteroids. They found that the Moon and Earth had identical ratios of iron isotopes, matching what had previously been found for other elements. (Picture: Getty)
This means that Theia and early–Earth must have mixed together so thoroughly that they became indistinguishable. But this also makes it difficult to work out how much of Theia ended up in the Moon and how much became a part of Earth. The researchers also looked at the different scenarios that could have led to this situation. They say that if Theia were similar to meteors that formed in the cooler outer edges of the solar system, proto–Earth would have needed to have a totally improbable mix of isotopes. However, both Theia and the proto–Earth were most likely made out of rocky 'non–carbonaceous' meteors from the innermost regions of the solar system. (Picture: Getty)
The researchers suggest that Theia would have orbited the Sun for around a hundred million years before the gravitational pull of Jupiter knocked it from its orbit, sending it to crash into Earth. Dr Hopp said: ‘Theia was likely one of tens to hundreds of planetary embryos that collided to form the planets. The similar isotopic composition makes it also impossible to directly measure the initial composition of Theia.’ (Picture: Getty)
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