How Likely Is It That There Is Life On Other Planets?

How likely is it that there is life on other planets besides Earth? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by William Poundstone, Author of "The Doomsday Calculation", on Quora:

Stanley Miller and Harold Urey conducted the great Frankenstein experiment of the 20th century. In 1952 they put water and simple gases—hydrogen, methane, and ammonia—in a flask and subjected them to “lightning” (well, electrical discharges). They produced not life but an assortment of amino acids, the building blocks of proteins in all living things. A similar experiment produced adenine, one of the four bases in DNA’s genetic code.

These findings triggered a paradigm shift. They encouraged the belief that the origin of life is just a matter of routine chemistry. There was nothing miraculous about it.

Accept this, and it follows that life must have originated on every planet with suitable conditions. By the dawn of the space age it was common to think that life, and indeed intelligent life, must be abundant in the universe. Yet this posed a paradox, as physicist Enrico Fermi pointed out. If the universe is brimming with intelligent life, why don’t we see any evidence of it? The mystery has only deepened as we have sent probes to planets and searched the sky for ET radio signals. So far no one has found any trace of extraterrestrial life.

There are two common arguments in favor of the life-is-common theory. Neither stands up to close examination. One is that amino acids and DNA bases are the “building blocks of life.” It’s easy to produce these building blocks in Miller-Urey experiments, so it must also be easy to go from that to living things.

If you’ll excuse a stale analogy: We’ve got a room of monkeys tapping away at keyboards. “Someday the little devils will produceHamlet!” Every now and then we find a monkey has typedAS… orHER… orULU(which is a knife used by Inuit women—such a clever monkey!) “They’re producing words,” we tell ourselves, “the building blocks of language.”

Yet producing words is exponentially more probable than producing sentences or the second season ofBreaking Bad. Similarly there’s a lot of hand-waving in the notion that the ease of producing isolated amino acids and bases implies that these can readily link together, in just the right way, to create functional self-replicating units that are not immediately destroyed by some other molecule. It could be that life required a chemical “miracle” that might not happen for billions of years, in an ocean of molecules. We just don’t know.

A second argument was proposed by astronomer and author Carl Sagan, who spent his career as a proponent of the life-is-common theory. The Earth is a typical planet as far as we know, and life arose on Earth. That’s only one data point, but so far it’s 1 for 1 for life arising. That’s prima facie evidence that life is common.

Francis Crick, of double-helix fame, felt Sagan was wrong about this. Actually, Crick said, the fact that life originated on Earth provides almost no relevant evidence about how common or probable life is elsewhere.

How can that be? There is what statisticians call aselection effect. An opinion poll may be biased if the people who answer aren’t representative of the population. Likewise the Earth may not be representative. We do not live on any old planet, but rather one where life and intelligencedidarise. (Otherwise, we wouldn’t be having this conversation, or this Quora session!) And this would be true, whether life is common or exceedingly rare.

The only thing we can deduce from the fact that life arose on Earth is that life is not so fantastically improbable that it never occurs anywhere in the universe. Beyond that, our experience doesn’t allow us to distinguish between the possibility that life is common, and that life is super-rare.

In recent decades scholars such as physicist Brandon Carter and philosopher Nick Bostrom have championed Crick’s case. They argue that the widespread confidence in the ubiquity of life is misplaced. It’s better to say it’s an open question.

In fact, it’s the best kind of open question for scientists, for it’s one we can expect to answer. There are ambitious plans to send probes to places like Europa or Enceladas, which might conceivably have life under oceans of ice; and even to search for signatures of life-related compounds in the atmospheres of distant planets circling other stars. Finding life anywhere else would lend great support to the case for a universe brimming with life.

But until then, we should admit what we don’t know. We don’t know much of anything about how probable the origin of life is.

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