A particularly exciting area of astronomy right now is the rapid discovery of exoplanets: other worlds in other solar systems. Each new report is inevitably accompanied by media buzz about possible aliens. This speculation is occasionally bolstered by assertions by scientists (physicists or astronomers, never biologists) that the existence of millions of extraterrestrial biospheres is a virtual certainty. And the science fans eat it up. In fact, I often see the inevitability of aliens invoked with a certain smugness: that if you don’t accept it you aren’t actually pro-science. The implication is either (A) if you don’t think life will readily appear under the right abiotic conditions you must secretly be a creationist, or (B) you don’t truly appreciate the enormous number of potentially habitable planets out there.

So let’s do a little math. Take out a deck of cards. Ignore the jokers, and you have 52 unique cards. Give them a quick shuffle. Now, what is the probability that the particular order of cards now in your hand could arise by chance? Basic probability theory tells us that there are 52 factorial (52!) permutations, which is about 8 × 1067. That’s a pretty big number, and hard to wrap your head around. So consider these comparisons: The universe is less than 1018 seconds old. There are about 1023 stars in the universe. And there are an estimated 1022 animals on Earth (not species, but individual animals, down to every last ant). Meaning, if every star had an Earth-like planet full of Earth-like animals, and every animal had their own deck of cards, and they all had been shuffling once per second since the Big Bang, we would have seen ~1063 permutations. Less than one thousandth of the total possibilities. So, the particular order you now hold in your hand would probably never occur. A living cell, meanwhile, is a lot more complex than a deck of cards. Even the simplest free-living microbe contains hundreds of distinct proteins, most of which have more than 52 amino acids. So exactly how likely is the independent origin of life? We honestly have no idea, but it’s far from a foregone conclusion that life has even the billionth of a chance of arising on a planet even when conditions are perfect. Most biologists will agree. Eugene Koonin, a trailblazer in the ancient evolution field, thinks that life is so unlikely, it can only be explained if there are multiple universes, and one of them happened to hit upon the right molecular combination. Once.

A lot of folks are reluctant to admit this because of the creationists. The anti-evolution crowd loves to trot out similar arguments as proof of “irreducible complexity” and therefore an intelligent designer. This divine intervention argument fails because natural selection cuts through irreducible complexity like a sword through a Gordian knot. Say you want to put a deck of cards in a particular order (such as “new deck” order). You shuffle repeatedly, but every time two cards are correctly adjacent, you tape them together. You would need far fewer than 1067 random shuffles to reach your goal. Probably fewer than 100 shuffles, in fact. This is how natural selection builds complicated organisms like orchids and wallabies from a single-celled ancestor. But natural selection only works once a self-replicating entity appears. That had to happen by chance, a perfect molecular arrangement with an astronomically low probability.

Why does this bug me so much? Why do I have to be a wet blanket? Can’t I let people look up at the stars and dream about the unknown, marveling at our vast universe? Well, mostly I feel like folks are in the Louvre with their backs turned to the Mona Lisa, starting at a janitor’s closet and wondering what’s inside. Biologists are not so impressed with the number of stars in the universe. To a non-scientist, 1023 sounds like a big number, but it pales compared to the number of possible gene sequences (over 101000000), let alone the number of possible genomes containing thousands of genes. Most species here on Earth are still undescribed, and each is doubtlessly amazing and bizarre in its own way. Why look for monsters on a distant world, when they can be found lurking in the deep sea, in the rainforest canopy, or attached to the foreleg of the beetle that just landed on your desk? Take the mud dragons (kinorhynchs), for example: tiny, bristly, burrowing, seashore critters with spiny, extendable heads. This is a whole phylum of animals, yet I’d bet they are completely unknown to most of the people who can’t wait for news of galactic neighbors. And the relative popularity of scientific endeavors has practical consequences. The budget for NASA, currently $18 billion, exceeds the budgets of the NSF, NOAA, and USGS combined. We spend more to study dead, distant space than to study our own breathing Earth.

What we have here on this planet is unfathomably precious. We may be the most powerful beings in existence, the stewards of the only planet with intrinsic value across all the galaxies, and we cannot dismiss the towering responsibility we have inherited. Now, I love science fiction. I love books and movies about aliens: Contact, the Hainish Cycle, the Ender Saga. Honestly, the exotic intrigue of weird creatures was a major motivator in my becoming a scientist. And of course, we don’t really know what’s out there. But the next time you find yourself wondering about the otherworldly, take a moment to realize that most, perhaps all, mysterious life forms might be a lot closer than you think.