What Would the Planet That Smashed Into Earth and Created the Moon Have Been Like?

Scientists announced that they have found traces of Theia in moon rocks

moon collision
An artist's rendering of the collision that created the moon Paul Wootton/ /Science Photo Library/Corbis

Scientists announced this week that they had found evidence of the planetary body that slammed into the earth over four billion years ago, creating the moon. In analyzing lunar rocks collected on the Apollo missions, they found that the moon rocks contained different ratios of oxygen isotopes 17 and 16 than their earthly counterparts, showing that some percentage of the moon likely had to come from somewhere else. 

Daniel Herwartz, lead author of the study told Space.com:

"The differences are small and difficult to detect, but they are there," Herwartz said. "We now get an idea of the composition of Theia."

That was the name given to the Mars-sized planet in 2000 by Alex Halliday. Most scientists 14 years ago had started to accept the giant impact hypothesis, first proposed in the 1970s, and when Halliday proposed calling the planet Theia, the name caught on. 

But what people couldn’t figure out was where all the evidence for Theia had gone. The earth and moon have very similar chemical compositions. So similar, if fact, that it’s been a huge puzzle for scientists trying to prove the Giant Impact Theory. With this new research there is finally some difference. Or is there?

There is still considerable scientific research looking in to moon formation, along with a lot of debate, so it's no surprise really that not everyone in the scientific community agrees that the differing oxygen isotopes are conclusive enough evidence for Theia. 

Halliday, now at Oxford, is among the skeptics. He thinks that the isotope difference between Earth and the moon rocks found by Herwartz is just too small when compared to the larger differences found in meteorites from other planetary bodies. As he told the BBC

"It raises the question of how well the meteorites from Mars and the asteroid belt in the outer Solar System are representative of the inner Solar System? We do not have samples from Mercury or Venus.

"They may well be similar to the Earth. If that is the case then all the arguments over the similarities of the Earth and the Moon fall away," he told BBC News.

So then, with the scant evidence we have, what would theoretical Theia have looked like? Like most newborn planets, she was probably a seething mass of rock, about the size of Mars, ricocheting through the early inner solar system. Because the chemical compositions of the rocks we do have (which, admittedly, are a very small sample size) are so similar, it has been suggested that Theia grew up right next to Earth, competing for the same bits of rocks, dust and debris. 

The gravitational pulls between the slowly growing planets in the solar system tugged on each other, causing them to careen around in far more unstable orbits than we have today. Eventually, Theia and Earth got just a little too close, and smashed together, giving us the moon. 

Update, June 9, 2014: This headline has been updated for clarification purposes.

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