What do Theia, Vulcan, Nibiru, Phaëton, and Antichthon have in common? They are all hypothetical planets that do not currently exist. Antichthon is the “counter-Earth” – a planet claimed to be in the same orbit as the Earth but always on the opposite side of the sun, so we can’t see it.  We know Antichthon does not exist because its gravity would be apparent.

Phaëton was the hypothetical planet between Mars and Jupiter that broke apart to form the asteroid belt. Phaëton likely never existed, and the asteroid belt simply failed to ever form a single planet. Nibiru is the planet, not taken seriously by any scientists, that some believe will collide with the Earth sometime this century (predictions have already failed multiple times). Vulcan was hypothesized to orbit within the orbit of Mercury, invented to explain anomalies in the orbit of Mercury that were later explained by general relativity.

Theia is unique among this list of hypothetical planets in that it probably actually existed. It was the Mars-sized planet that struck the proto-Earth 4.5 billion years ago, creating the current Earth-Moon system. This is, at least, the currently most accepted theory of the origin of the moon. It is supported by computer models, and explains many observations about the moon.

In this model, during the early chaotic period of the solar system, Theia crashed into proto-Earth, although a glancing blow, that threw up a mixture of material that was about 70-90% Theia and the rest Earth material. This mixture then coalesced into the moon. One model holds, in fact, that two moons formed, and then later joined together.

There is one problem with the Theia hypothesis of the moon’s origin – material from the moon appears to be all earth and no Theia (at least until the new study which prompted this post, more on that below). Evidence so far suggests that objects that form in different parts of the solar system will have different compositions, specifically they will have different isotopes.

For a quick background, an isotope is an element with the same number of protons but different number of neutrons in the nucleus. The number of protons determines the element, so oxygen is defined as having 8 protons – its number on the periodic table. The most common isotope of oxygen is O-16, which has 8 protons and 8 neutrons. There are also other isotopes, however, such as O-17 with 9 neutrons, and O-18 with 10 neutrons.

The ratio of oxygen isotopes in rocks is very consistent. Rocks from earth have a certain ratio, and rocks from Mars have a different ratio. Astronomers use this known ratio, in fact, to determine if a meteorite came from Mars.

Therefore, if every planet in the solar system has its own signature oxygen isotope ratio, Theia must have had it too, and we should see that signature in the moon, but we don’t This is a problem for the impact hypothesis of the moon’s origin. It’s not a fatal problem, because there are lots of possible fixes, but still a mystery to be solved.

Now, however, researchers publishing in Science report that they have used a more sensitive technique to measure the oxygen isotope ratio in three samples of moon rock – rocks brought back from the moon during the Apollo missions. Science news reports:

Their conclusion was that the lunar samples had an O-17 to O-16 ratio that was 12 parts per million higher than rocks derived from Earth’s mantle. This difference “supports the view that the Moon formed by a giant collision of the proto-Earth with [an impactor],” the team writes. “It is a relief that a [disparity in ratios] has been found, since the total absence of difference between Earth and moon would be hard to explain,” comments planetary scientist David Stevenson of the California Institute of Technology in Pasadena, in an e-mail.

That’s pretty exciting, and I suppose it’s a relief to proponents of the impact hypothesis. Still, this is a small difference. The measurements also suggest that the moon is a 50-50 mix of Earth and Theia, which is different than most computer models predict.

The difference is also very small. Some say it’s too small to really answer the question. Also, the sample size is very small. Basically we need to get more moon rocks, including from deeper below the surface, to see if this ratio holds up.

Impact proponents also point out that it is possible Theia had a similar isotope ratio to Earth. We don’t currently have samples from Mercury of Venus, so we don’t know how different they are from the Earth. It’s possible that the isotope ratio in the inner solar system is all very similar. Theia may have formed in an orbit very close to Earth’s (which makes sense as they did eventually impact) and therefore have had a similar isotopic signature.

Conclusion

The origins of the moon is a fascinating scientific story, one I have been following for years. It seems that this new evidence is further support for the impact hypothesis, although there is certainly room for further research.

In fact, one of the things I love about this story, more than the question itself, is how scientists go about answering that question. As you can see, every explanation is countered by possible alternate explanations with a description of the type of evidence that should resolve the issue, or at least get us closer.

How different are oxygen isotope signatures in different parts of the solar system?

What are the signatures for Mercury and Venus?

How do different impact models affect the ratio of Earth to Theia in the compositions of the moon?

Will this oxygen isotope ratio recently found in moon rocks hold up with a larger sample size?

Are the surface rocks on the moon contaminated or altered by later impacts, and will deeper rocks have the same ratio?

The question is probed from every possible angle, and answered with various types of evidence. Scientists don’t just focus on the parts of the theory that fit and make sense, but rather they focus particularly on those parts that don’t current fit. Weaknesses in the theory point to areas requiring further research – to more knowledge about the history and nature of our solar system.

This is the culture of science. You may notice a stark contrast to those who only pretend to be doing science.