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How the Moon Supports the Privileged Planet Hypothesis

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Three astronomers offer "state of the Moon" addresses in Nature, revealing a troubling state of affairs: the Moon’s position and composition have so far baffled efforts to propose an unguided sequence of events that might have formed it. Since Apollo, multiple scenarios have been proposed, only to be rejected as either physically impossible or statistically improbable. In this, design advocates might see the Moon dropping through two stages of Dembski’s Design Filter: chance and natural law.

Moon origin theories have been on a roller coaster ride since the 1960s. Planetary scientists were optimistic that the Apollo missions would help decide among three leading hypotheses: capture, fission, and accretion. After Apollo, all three were rejected, leaving theorists without a theory until the "giant impact" hypothesis came along in the 1980s. Till recently, the scenario of a Mars-sized object striking the Earth at a glancing blow was hailed as accepted truth. TV documentaries animated the event handsomely, in vivid color. However, new observations have cast doubt on the idea.

For one thing, the size and speed of the impactor had to be finely tuned. Too large or too fast would not leave sufficient material behind to re-form into the Earth and Moon. Worse, though, are findings that elements are too similar in the two bodies. Isotopic ratios of oxygen, titanium, silicon, chromium, tungsten and other elements — both volatile and refractory — are nearly identical to Earth in the lunar samples, contrary to what all the impact models predict (a Moon composed primarily of impactor material). Observations and theory both discount such a possibility from likely impactors beyond the Earth. Robin Canup writes in Nature:

The main challenge is to simultaneously account for the pair’s dynamics — in particular, the total angular momentum contained in the Moon’s orbit and Earth’s 24-hour day — while also reconciling their many compositional similarities and few key differences. The collision of a large impactor with Earth can supply the needed angular momentum, but it also creates a disk of material derived largely from the impactor. If the infalling body had a different composition from Earth, as seems probable given that most objects in the inner Solar System do, then why is the composition of the Moon so similar to the outer portions of our planet? (Emphasis added.)

Growing discontent with the standard model has left astronomers scrambling to come up with alternative impact models. Maybe two "half-Earths" collided, leaving the Moon behind. Maybe the impactor scoured a variety of depths from the Earth’s mantle. Maybe a fast "hit-and-run" impactor took all its own material with it. Each of these proposals creates new problems. Getting rid of excess angular momentum would require a finely tuned resonance between the Earth, Moon and Sun involving precession to transfer the excess to the Moon. Keeping the refractory elements molten long enough to mix in the dust disk is unlikely. Each new "patch" to the impact theory creates new improbabilities.

This is where remarks by the astronomers reveal that they do not wish to entertain the hypothesis that our Moon somehow reflects the working out of a design. They don’t say so explicitly, but look at these comments: in Canup’s article "Planetary Science: Lunar Conspiracies" in Nature, the subtitle reads:

Current theories on the formation of the Moon owe too much to cosmic coincidences, says Robin Canup. She calls for better models and a mission to Venus.

The call to a mission from Venus reflects the exasperation with current models. If Venus turns out to have similar elemental composition to Earth, Canup hopes that it will rescue the impact hypothesis by suggesting another Venus (instead of a Mars) impacted the Earth, giving rise to the moon in the process.

Canup knows our moon is important for life:

The Moon is more than just a familiar sight in our skies. It dictates conditions on Earth. The Moon is large enough to stabilize our planet’s rotation, holding Earth’s polar axis steady to within a few degrees. Without it, the current Earth’s tilt would vary chaotically by tens of degrees. Such large variations might not preclude life, but would lead to a vastly different climate.

After rejecting the standard impact model, Canup entertained an "elegant solution" only to immediately discount it for new difficulties it raises. She does the same with several other scenarios that have been proposed. Each new model created to solve problems makes the scenario more contrived.

But it remains unclear whether the resonance mechanism needed to slow Earth’s rotation in these more extreme scenarios is likely or requires an improbably narrow range of conditions. In other words, is the origin of our Moon a rarer event than we believed, or are we missing something?

Canup states that "No current impact model stands out as more compelling than the rest." All are equally improbable, in other words. Indeed, they are:

It remains troubling that all of the current impact models invoke a process after the impact to effectively erase a primary outcome of the event — either by changing the disk’s composition through mixing for the canonical impact, or by changing Earth’s spin rate for the high-angular-momentum narratives.

Sequences of events do occur in nature, and yet we strive to avoid such complexity in our models. We seek the simplest possible solution, as a matter of scientific aesthetics and because simple solutions are often more probable. As the number of steps increases, the likelihood of a particular sequence decreases. Current impact models are more complex and seem less probable than the original giant-impact concept.

Tim Elliott and Sarah T. Stewart don’t add any new scientific hypotheses to their comments in another Nature article, but their philosophical comments are just as revealing. Elliott says:

Not since NASA’s scientists definitively announced that the lunar white stuff was non-dairy has the Moon faced such an identity crisis. Ironically, it seems that our satellite is compositionally too similar to Earth for a simple explanation of its origins…. As emphasized in a Royal Society meeting in September that debated the origin of the Moon, the compositional differences between Earth and the Moon that would be expected as a consequence [of the impact model] are increasingly at odds with diverse, high-precision isotopic observations.

He describes the "dilemma" of "isotopic embarrassments" and the scenarios invoked to explain them as "highly implausible":

New dynamical models that can produce the Moon from the proto-Earth do not have the inherent simplicity of the canonical giant-impact scenario, and some argue that there are crucial flaws in such models. The sequence of conditions that currently seems necessary in these revised versions of lunar formation have led to philosophical disquiet. From a naive geochemical perspective, however, the isotopic similarity of Earth and the Moon holds an obvious appeal; the proto-Earth represents an abundant local source of material from which to build the Moon. Whether or not this comfort of availability can be meshed with the rigours of celestial mechanics remains to be seen.

Sarah T. Stewart describes two models that attempt to rescue natural models from the "stalemate" created by the isotopic similarities of Earth and Moon. This rescue, however, comes at a cost:

Within our current understanding of planetary and satellite formation processes, each stage of lunar evolution is plausible. But, with the nested levels of dependency in a multi-stage model, is the probability of the required sequence of events vanishingly small? Is there an alternative solution of greater simplicity and universality? Ultimately, the current detailed interrogation of lunar origin may demand answers that have an unexpected level of complexity.


We see these three astronomers coming face to face with realities that contradict their philosophical preferences. They want simplicity. They want plausibility. They want elegance. Even if "each stage of lunar evolution is plausible" alone, proposing a complex sequence of events to get our Moon, they know, multiplies the improbabilities until they become "vanishingly small" — yet the collapse of all the simple, plausible models leaves them with no alternative. They might well be stuck with "an unexpected level of complexity."

It’s as if all the parts of a Rube Goldberg machine came together naturally to flip an egg. Even if each stage were plausible on its own (a dropping ball, a flipping coin), the arrangement for function strains credibility when design is excluded. The fact is, we have a Moon that stabilizes the tilt of Earth’s axis and modulates the climate. They didn’t mention that it also regulates the ocean tides and provides the highly improbable (yet scientifically fruitful) occurrence of total solar eclipses.

For those disinclined to recognize evidence of cosmic design, the Moon is just one headache among many. The Illustra documentary The Privileged Planet lists some twenty other factors that "conspire" to give Earth not only ideal conditions for life but to make our world the ideal platform for scientific discovery. The improbabilities of each of these factors multiply together, yielding a conservative estimate of one chance in 1015 of their all coming together at the same place and time. And that’s before considering the cosmic coincidences, like the fine-tuning of the constants of physics, discussed later in the film.

The words of these astronomers show that it is not the evidence that keeps them from inferring design. "Philosophical disquiet" or "embarrassment" should not prevent good science from boldly considering Canup’s question, "Are we missing something?"

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