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Dr. Denton, Call Your Office: Is Earth’s Position a Lucky Accident?

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As we celebrate the online premiere of Privileged Species, featuring Discovery Institute’s Michael Denton, here’s an opportunity to compare design science with its materialist competition. Which makes the more plausible case?

Two astronomers, one from Caltech and one from Lick Observatory, have just published a paper in PNAS that portrays Earth’s position as a lucky accident. Konstantin Batygin and Greg Laughlin notice that our solar system is rather peculiar as planetary systems go. Many stars have “super-Earths” and “hot Jupiters” orbiting close in, but our sun does not. The inner region of our solar system is empty of large planets. What could have caused that?

The Solar System is an unusual member of the galactic planetary census in that it lacks planets that reside in close proximity to the Sun. In this work, we propose that the primordial nebula-driven process responsible for retention of Jupiter and Saturn at large orbital radii and sculpting Mars’ low mass is also responsible for clearing out the Solar System’s innermost region. Cumulatively, our results place the Solar System and the mechanisms that shaped its unique orbital architecture into a broader, extrasolar context. [Emphasis added.]

Their strategy, they indicate, is just a “proposal.” A news release from Caltech calls it a “possible scenario,” i.e., a mere “hypothesis” that will require more checking. It may have passed peer review, but it is not a theory, a demonstration, or an explanation. What observational facts do they bring to support their proposal?

“Our work suggests that Jupiter’s inward-outward migration could have destroyed a first generation of planets and set the stage for the formation of the mass-depleted terrestrial planets that our solar system has today,” says Batygin, an assistant professor of planetary science. “All of this fits beautifully with other recent developments in understanding how the solar system evolved, while filling in some gaps.”

(Note in passing that both the paper and the news release use the word “evolution” to describe the origin of planets. There have been some who have chastised Darwin skeptics for misusing the word, which, they complain, should be reserved for what happens after life originated. But that is a digression.)

Here’s the best thing they offer as empirical evidence: extrasolar planetary systems don’t look like ours. Notice where observation leaves off and assumption takes over:

Thanks to recent surveys of exoplanets — planets in solar systems other than our own — we know that about half of sun-like stars in our galactic neighborhood have orbiting planets. Yet those systems look nothing like our own. In our solar system, very little lies within Mercury’s orbit; there is only a little debris — probably near-Earth asteroids that moved further inward — but certainly no planets. That is in sharp contrast with what astronomers see in most planetary systems. These systems typically have one or more planets that are substantially more massive than Earth orbiting closer to their suns than Mercury does, but very few objects at distances beyond.

“Indeed, it appears that the solar system today is not the common representative of the galactic planetary census. Instead we are something of an outlier,” says Batygin. “But there is no reason to think that the dominant mode of planet formation throughout the galaxy should not have occurred here. It is more likely that subsequent changes have altered its original makeup.”

Observation: Earth is unique. Conclusion: Its position evolved by an unguided process. Sticking to that strategy of argument, they constructed a model with a migrating Jupiter. Nobody has seen Jupiter move, but if it did, it might have careened inward and flung back outward, sweeping away all the super-Earths that must have been there. The leftover debris became Mercury, Venus, Earth (by chance in the habitable zone), and Mars.

Why didn’t Jupiter careen into the sun? Well, by happenstance, Saturn was handy. Jupiter was on a gravitational conveyor belt into the sun, but just in time, another lucky accident happened:

“Jupiter would have continued on that belt, eventually being dumped onto the sun if not for Saturn,” explains Batygin. Saturn formed after Jupiter but got pulled toward the sun at a faster rate, allowing it to catch up. Once the two massive planets got close enough, they locked into a special kind of relationship called an orbital resonance….

“That resonance allowed the two planets to open up a mutual gap in the disk, and they started playing this game where they traded angular momentum and energy with one another, almost to a beat,” says Batygin. Eventually, that back and forth would have caused all of the gas between the two worlds to be pushed out, a situation that would have reversed the planets’ migration direction and sent them back outward in the solar system.

This scenario wouldn’t have worked, except that there were super-Earths in the inner belt. No one ever saw them — they have long since vanished without a trace — but they must have been there. Why? Because Batygin’s proposal/scenario/game wouldn’t work without them. That justifies some speculation:

He says the answer could lie in primordial super-Earths. The empty hole of the inner solar system corresponds almost exactly to the orbital neighborhood where super-Earths are typically found around other stars. It is therefore reasonable to speculate that this region was cleared out in the primordial solar system by a group of first-generation planets that did not survive….

“It’s a very effective physical process,” says Batygin. “You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the sun.

Where are those “recent developments in understanding how the solar system formed” that “all of this fits beautifully with”? The article doesn’t say much about them, other than noting that most extrasolar systems seem to form with more material in the inner orbits than ours. They also have planets with a lot of hydrogen in their atmospheres — which raises the question why ours doesn’t:

From that point, it would take millions of years for those planetesimals to clump together and eventually form the terrestrial planets — a scenario that fits nicely with measurements that suggest that Earth formed 100-200 million years after the birth of the sun. Since the primordial disk of hydrogen and helium gas would have been long gone by that time, this could also explain why Earth lacks a hydrogen atmosphere. “We formed from this volatile-depleted debris,” says Batygin.

And that sets us apart in another way from the majority of exoplanets. Batygin expects that most exoplanets — which are mostly super-Earths — have substantial hydrogen atmospheres, because they formed at a point in the evolution of their planetary disk when the gas would have still been abundant. “Ultimately, what this means is that planets truly like Earth are intrinsically not very common,” he says.

What’s wrong with this picture? It is lopsided, favoring speculation and assumption. Earth is unique, they admit. Earth doesn’t look like any other planetary system. But it must have formed by unguided evolutionary processes, so they “propose” a “scenario” that gets our unique arrangement out of a series of lucky accidents. Here’s the problem: the sequence of lucky accidents is so contrived, it really doesn’t improve on a simpler proposal: “I guess we were just lucky.” Batygin and Laughlin have constructed a Rube Goldberg scenario that is so dependent on factors being in the right place at the right time, an observer would be justified in inferring that it was unnatural.

Michael Denton’s Turn

Now compare their scenario with Denton’s design inference. Privileged Species is tilted strongly toward observational evidence. Throughout the documentary, specific details about wavelengths, atmospheric conditions, properties of molecules, and many other things are presented as evidence. No convoluted scenario is needed: just a simple, logical inference. The specified complexity presented justifies the inference that complex life exists for a purpose and plan, according to our uniform experience of the way designers work.

The other side presents a long chain of if-then statements: If all planets form by unguided processes — if our debris disk had a Jupiter form at 5 AU — if a Saturn formed later — if the inner disk was populated by super-Earths and debris — if Jupiter started migrating at a certain time — if it cleaned out the hydrogen-containing debris — if Saturn tagged along at the right time and pulled Jupiter back — if there was enough debris left to form an Earth — then, perhaps, maybe, it might be reasonable to speculate that Earth formed and landed right in the habitable zone with a non-hydrogenous atmosphere. (They leave the origin of the moon, oceans, and life to others.)

Denton says: specified complexity is observed at hierarchical levels: that implies design. Denton’s view would pass the “impartial rational observer” test, don’t you think?

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