In the last decade, more than 3,500 exoplanets have been discovered, most famously by the Kepler Space Telescope. In the next decade, our ability to detect more exoplanets and reveal their atmospheric compositions will be revolutionized by next-generation telescopes like the James Webb Space Telescope (2021) and the Giant Magellan Telescope (2025). Detecting a biomarker like oxygen – an unstable element unlikely to exist in large quantities in the absence of organisms that produce it – on an exoplanet would be a strong indicator that it harbours life, just as any relatively nearby extraterrestrials with suitably advanced technology would be able to detect the existence of life on Earth simply by looking at our atmosphere.

Our world-views will require adjusting after we have looked at the atmospheres of hundreds of planets in the habitable zone of their stars, whether or not we discover extraterrestrial life. In ‘The Copernican Principle, Intelligent Extraterrestrials, and Arguments from Evil’ (Religious Studies), I contend that evidence for extraterrestrial life is evidence against the existence of God according to prominent forms of the problem of evil. If this is correct, the next-generation telescopes may have important implications for traditional monotheism.

If we find atmospheric biomarkers on an extrasolar planet, we won't be able to tell if the life on that planet is intelligent. We will, however, have strong reason to think the universe is teeming with life. Our search for exoplanets has so far encompassed only a small fraction of the Milky Way Galaxy. If we find extrasolar life by looking in a small fraction of the area of a single galaxy, it would be overwhelmingly likely that life is common throughout the 100 billion galaxies in the universe. Even if it is extremely unlikely for a planet with life to develop intelligent life, there would be so many candidate planets with life that we can infer the existence of intelligent extraterrestrial species spread throughout the universe from the observation of a single life-supporting exoplanet.

Let's say this happens: we are confident there are intelligent extraterrestrials, but don't have any more information about them. I argue this would make it less likely that God exists, at least according to prominent versions of the problem of evil. My argument begins with the claim that we can infer some things about intelligent extraterrestrials based solely on our observations of our own situation.

The Copernican Principle, devised and defended by Richard Gott, an astrophysicist at Princeton University, says that we should take ourselves to be randomly selected observers in the absence of information to the contrary. Gott has used the Copernican Principle to predict the longevity of the human species. The basic idea is that we would be atypical observers if we were near the very end or the very beginning of humanity's existence; therefore, we are likely to be somewhere in the middle. More precisely, there is a 95% chance that we are in the middle 95% of humanity's lifespan.

Gott uses the commonly cited figure that modern humans developed 200,000 YBP (years before present) as the value for the dawn of humanity. We could be on the early edge of the 95% range:

On the other hand, we could be on the late edge:

Gott concludes that there is a 95% chance that humans will go extinct between 5,128 and 7.8 million years from now.

I claim we ought to use the Copernican Principle to shape our expectations of where we stand in relation to intelligent extraterrestrials, should they exist. For example, we would be atypical if we were the fastest species, or the slowest, and we probably aren't atypical. Usain Bolt would be unlikely to win the 100 m at the Galactic Olympics, but he probably wouldn't come in last either. Similarly, it's unlikely that we are the cruellest species, or the kindest; the tallest, or the shortest; the most miserable, or the happiest; and so on. We are most probably mediocre.

I once had the opportunity to ask Kurt Vonnegut why he was an atheist rather than an agnostic. ‘Because there's more money in it’, he replied. After the laughter subsided, he became earnest: ‘It's because of what happens to children.’ This is the problem of evil framed in the spirit of Ivan Karamazov. In the Abrahamic religions, God is traditionally supposed to be perfect, without limitation of power, knowledge, or goodness. We would expect such a being, at a minimum, to protect children from harm. After all, as flawed as we are, if just by willing it we could feed all the hungry children in the world, protect children from violence, and cure childhood cancer, we would. If God existed, God could and would do these things. Since these things remain undone, God doesn't exist.

This is a powerful argument. It becomes more powerful if there are intelligent extraterrestrials. By the Copernican Principle, Earth is unlikely to be the planet where children suffer the most. There are likely to be planets where children have it even worse than they do on Earth: more of them suffer, and to a greater degree, and their suffering is less often redeemed by future joy. The worst stories that Ivan Karamazov can tell about the suffering of children on Earth are not the worst stories there are to be told about children in the universe. It is even less plausible that a perfect God would allow the greater suffering of these extraterrestrial children, and so less plausible that such a God exists.

Of course, there would also probably be planets where children lead substantially better lives than children on Earth. The Copernican Principle tells us to infer just as much good stuff as bad stuff. However, according to many versions of the problem of evil, the average amount of good and bad stuff isn't what's relevant, but rather the maximum degree of suffering. As Roderick Chisholm observed, we cannot offset doing wrong by doing right. I cannot murder someone, save someone else's life, and call it even. So too God cannot offset the suffering of a miserable child by creating a joyful child elsewhere in the universe.

I have argued that the problem of evil is strengthened by evidence of intelligent extraterrestrial life. One way to resist this conclusion is to claim that children who lead miserable lives on Earth are rewarded by infinite bliss in Heaven. Evidence that there are extraterrestrial children who suffer even more than children on Earth is of no account, as those extraterrestrial children are also rewarded with eternal life. The finite difference between the suffering of the most miserable human child and the most miserable extraterrestrial child pales next to the infinite reward awaiting in Heaven.

But this response reflects a mistake. As Peter Vallentyne and Shelly Kagan have argued, the mathematics of infinity is not a good model for thinking about value in infinite contexts. Say that God were to offer you a choice between (a) eternal salvation and (b) 100 years of torture followed by eternal salvation. Mathematically, 100 years of torture is irrelevant in the context of infinite bliss, but we all would have a strong preference for (a). For the same reason, we cannot blithely say that differences in finite suffering don't matter for the overall value of a life, even one that encompasses eternal bliss.

Let's consider a child who is miserable in life and then enjoys a blissful, eternal afterlife. This child's total existence is valuable. However, it is not as valuable as it would have been had the child not led such a miserable life. Why should the cost for some (but not all!) children to enter Heaven be so high, when it would have been better for them had they suffered less? This is a difficult question for theists to answer. It is all the more challenging the greater the suffering of children. Since evidence for intelligent extraterrestrials is evidence for children who suffer more than even the most miserable children on Earth, my thesis holds: evidence for intelligent extraterrestrials is evidence against the existence of a perfect God.

This reasoning applies to any situation where people learn there are many more intelligent beings than they had previously supposed. When the Lucayans first encountered European explorers, not only the explorers’ actions, but the simple fact that they existed, was evidence against the existence of a perfect God. We've already learned several facts that increase the chances of extraterrestrial life: outer space is a place where creatures can live; the universe is massively large; planets are ubiquitous. If my conclusion is correct, we are on the brink of more discoveries that could give us evidence against the existence of God.