Next generation space telescopes from NASA and the European Space Agency are ushering in a golden age of deep space exploration, putting us on the verge of answering an old question: Are there other thinking, speaking creatures living on faraway planets?
Taking the question in steps and focusing on our corner of the universe, astrobiologists are asking: Do other stars in our galaxy have rocky, earthlike planets at the right distance from their host stars to nurture life? If so, are any of them actually inhabited? And if so, by what? Mere bacteria? Algae? An array of exotic plants and animals? For many in the hunt, the Holy Grail is to discover an extra-solar planet with advanced aliens who can communicate with us across the vast chasm of space via radio transmissions.
Those hoping for this aren’t the tinfoil-hat brigade either. They are mainstream scientists and scholars working at top universities and other prominent institutions. Many believe the Milky Way is teeming with advanced life. After all, there are hundreds of billions of stars in the Milky Way. If even just one in a million has an earthlike planet with advanced life on it, then our galaxy could contain hundreds of thousands of alien civilizations.
But that’s just the thing. Is it one in a million? One in a thousand? One in a billion trillion? Maybe we are the only inhabited planet in the entire universe. Without data and careful reasoning as to what a planet requires for habitability, the issue is just an empty math game where you fill in the divisor based on wishful thinking, then present the result of the division problem as if you have discovered something.
A Habitable Planet: The Recipe
Astrobiologists move beyond pure guesswork by investigating the question, What factors are needed for a habitable planet and how likely is each such factor to obtain for any given star system? Initially, the list of ingredients was short. Astrobiologists generally agreed that you need a relatively stable star of the right sort. You need a rocky planet (no gas giants need apply). And the planet needs to be the right distance from its host star to allow for liquid water on the planet’s surface. If that’s all that’s needed, then we can expect there to be legions of habitable planets in the Milky Way.
But as astrobiologists dug deeper into the matter, the list of necessary conditions for habitability grew and grew, with the odds of getting all of them together in any given star system shrinking geometrically with each new independent factor.
In the 20th anniversary edition of The Privileged Planet, astrobiologist Guillermo Gonzalez and philosopher Jay Richards update the list of factors necessary for a habitable planet. The bullet points below are drawn from Appendix A of the book, with the wording here either verbatim or lightly modified for clarity outside the larger explanatory context of the book:
- Orbits an early G dwarf type star1 that is at least a few billion years old
- Orbits a star in the galactic habitable zone2
- Orbits a star near the corotation circle3 and with a low eccentricity4 galactic orbit
- Orbits a star outside the spiral arms
- Is a terrestrial planet the right distance from the host star to have liquid water on the planet’s surface (a distance known as the circumstellar habitable zone)
- Is near enough the inner edge of the circumstellar habitable zone to allow high oxygen and low carbon dioxide concentrations in the atmosphere
- Orbits a star with no more than a few giant planets comparable in mass to Jupiter and that are in large, nearly circular orbits (so that the gas giants do not mess up the earthlike planet’s orbit)
- Has a low eccentricity orbit outside of regions around a star that would destabilize a planet’s rotation axis and orbit due to the gravity of giant planets
- Is in the right mass range
- Has the right concentration of sulfur in its core
- Has a large moon and the right planetary rotation period to avoid chaotic variations in its obliquity
- Has the right amount of water in the crust
- Has steady plate tectonic cycling
- Is a habitable planet where single-celled life actually emerged
- Experienced a critically low number of large impacts during its history
- Experienced a critically low number of transient radiation events from outer space during its history
- Is a planet where plant and animal life successfully evolved from single-celled life
- Is a planet where an intelligent, technology-wielding life form evolved from lower animals
- Is among that subset of planets that have hosted technological life where the technological life has not destroyed itself or otherwise gone extinct.5
How Many Earths?
Gonzalez and Richards assign a 10 percent chance for each of these 19 factors, but they make a strong case that the odds are much slimmer in many instances.
Some of the factors may have an infinitesimally small probability of occurring by happenstance. For instance, perhaps the origin of the first self-reproducing cell is improbable or even impossible given the limited tool kit of materialists (chance + natural laws). The same may hold for the origin of plants and animals. Richards and Gonzalez are skeptical that chemistry and natural selection alone can evolve fundamentally new forms of life, as am I. But for the sake of argument they generously grant even these factors a 1 in 10 chance of occurring on any terrestrial planet where all the other necessary factors are in place.
We are multiplying out fractions here, so the odds get geometrically smaller with each additional factor. Multiply out just the first 13 and we find that the odds of any one star system having a habitable planet is less than one in a trillion, meaning that even with all the billions of stars in the Milky Way, the odds are strongly against there being even one other habitable planet in our galaxy besides Earth. And note, Gonzalez and Richards further lowballed the odds by leaving out several factors. Also keep in mind that for some time now the calculated odds have been getting slimmer and slimmer as new habitability requirements are discovered.
Rx for the Rare-Earth Blues
In light of these observations, what’s an astrobiologist to do who is keen to believe in the likelihood of extra-terrestrial life, and eager for the tax-paying public to believe in his research mission so they will continue supporting government outlays for the research? One option is simply to pretend that these habitability requirements aren’t telling us what they clearly are telling us.
But there is, I would argue, a more rational and intellectually defensible strategy for maintaining a measure of optimism about the possibility of extraterrestrial life. The above are the slender odds of another habitable planet in the Milky Way if the whole affair is a matter of dumb luck. But what if neither Earth nor the universe is a matter of blind chance?
What if the space-time-matter-energy that emerged in the Big Bang was the product of a timeless, immaterial mind, as some Nobel Laureate physicists and astronomers have held? And what if this cosmic architect not only fine-tuned the laws and constants of nature to allow for the possibility of life, but also made sure that habitable planets swarming with life sprang up far more frequently than dumb luck would allow?
Many scientists insist they can’t go there. They hold to the dogma known as methodological materialism. This rule says scientists must only invoke material causes to explain aspects of nature — no intelligent design allowed. It’s a shame. Scientists looking for a sound reason to suppose the Milky Way may be teeming with inhabited planets, or who just want to pursue the question unhindered by a question-begging rule, should ask themselves whether the dogma of methodological materialism has worn out its welcome.
There is, alas, another stumbling block preventing many scientists from opening themselves up to the possibility of intelligent design in the formation of habitable worlds. Many do not want there to be a Creator God. And indeed, their search for advanced extraterrestrial life is a search for a substitute god or father figure, to replace the void left by the God they deny.
Free God, Fallen Aliens
As for proponents of intelligent design, most of us take an open view on the question of whether there are other inhabited planets out there. As for the teachings of Christianity, a prominent stream of orthodox theology holds that God is free in his creative activities. He was free to create only a single inhabited planet among the starry hosts, or to create five of them, or five million. It’s his call.
What about extraterrestrial creatures who are made in the Image of God and also, like us, fallen into sin? C. S. Lewis wrestled with this possibility in his essay “Religion and Rocketry.” He gave a long and nuanced answer there, but also a short one. “It is no very new idea,” he wrote, “that the eternal Son may, for all we know, have been incarnate in other worlds than earth and so saved other races than ours.” These considerations leave the Christian, as with other theists, intellectually unfettered regarding the possibility of other inhabited worlds. Maybe they’re out there. Maybe they’re not. Let’s follow the evidence streaming in over the next few decades and see.
Notes
- Also known as G-type main sequence stars, they are very close to the sun’s mass, temperature, brightness, and spectral type. Most stars are either much more massive or much less massive than G dwarf type stars.
- Gonzalez helped pioneer the idea of a galactic habitable zone, now widely embraced in astrobiology. A habitable star system can’t be too close to the center of the galaxy or too far, for reasons delineated in The Privileged Planet.
- The corotation circle is the circular band around a spiral galaxy’s center where the stars move at about the same speed as the spiral arms.
- Eccentricity in this context is a measure of how non-circular a body’s orbit is. A low eccentricity orbit is one that is close to circular.
- I encourage readers fascinated by this topic to get a copy of the new edition of The Privileged Planet, where the authors explain each factor in detail and answer objections.
This article is cross-posted from Salvo Magazine with permission of the author.
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