All These Worlds Are Yours: The Scientific Search for Alien LifeBy Jon WillisYale University Press, 2016 It takes Jon Willis, astronomy professor at the University of Victoria in British Columbia, 170 pages – the length of a Henry James short story, for Pete's sake – to get around to the Drake Equation in his winning debut, All These Worlds Are Yours: The Scientific Search for Alien Life, but we should probably deal with it immediately. In 1961, at a conference in West Virginia, astronomer Frank Drake worked up an exotic-looking equation on a blackboard. Drake had just recently completed a project designed to detect alien radio broadcasts from the region of the star systems Tau Ceti and Epsilon Eridanus, and the blackboard equation he wrote has become a signature parameter for the whole subject of Willis's book, astrobiology.The equation is this: The N is the goal, the dream of every stargazer and science fiction writer in human history: the number of communicating alien societies in the galaxy. And the equation gets to that number by multiplying key variables: R*, the number of stars being born, Fp, the fraction of those stars that have planets orbiting them, Ne, the number of those planets that are habitable by any kind of life, Fl, the fraction of those planets that actually develop life, Fi, the fraction of those planets that then develop intelligent life, Fc, the fraction of those planets that then develop civilizations technologically capable of communicating with other worlds, and finally L, the life-span of that remaining number of civilizations – do they avoid being destroyed (or destroying themselves) long enough to do any communicating.Willis quite rightly calls the equation both deceptively simple and deceptively troubling – as he puts it, the Drake Equation is “a scientific wish list.” And Willis is a true believer:

Personally, I would be (more than) happy to understand the terms of the Drake equation leading to even basic life, let alone quantifying the potential new Twitter followers among the stars. Yet no one criticizes other branches of astrobiology on the grounds that the fraction of habitable worlds that go on to develop life is a complete unknown. In fact, one could admit that our efforts to discover basic life beyond Earth are so exciting because the question is so big and unknown.

It's an optimistic reading, but for all its enviable sense of wonder, All These Worlds Are Yours is inevitably constrained by the unknowns of the Drake Equation, and some of those unknowns are daunting. Not, ironically, the raw numbers – those are clearer and clearer with every passing year. The rate of star birth in the Milky Way galaxy, for instance, is now well known, and for the last twenty years, the number of Earth-like planets detected in orbit around other stars has leapt from two or three to hundreds – just in late August, for instance, one was discovered orbiting Earth's nearest stellar neighbor, Proxima Centauri, a mere 4.2 light years away.No, the problems with the Drake Equation cluster on the imponderables, the guesses: the probability that any of those exoplanets would develop life, the probability that any of those life-forms would evolve technological intelligence, the ability of those intelligences to survive long enough to make themselves known outside their own solar systems … as Drake knew well, there's no way to quantify those variables, for one depressingly simple reason: as far as we know, there is no life, much less any other technological civilization, anywhere else in the universe. On Earth, life is mind-bogglingly profuse: it may well have evolved several times independently, and it fills every tiny cranny of every one of Earth's biomes, from the warm hospitality of tropical jungles to the black, boiling hostility of subterranean volcanic vents. Life is tenaciously abundant on this planet, a marvel of organic complexity thriving and reproducing everywhere. And yet, a century's worth of searching and probing and testing of other worlds has turned up nothing like this vitality anywhere else.And as Willis points out, we know that this propensity for organic life is obvious from a distance. In December of 1990, the NASA space probe Galileo, on its circuitous path to examine its destination planet, Jupiter, passed close enough to Earth to give scientists an elegantly simple idea: could Galileo's onboard instruments actually detect signs of life on Earth?The answer turned out to be, effectively, yes. Earth's atmosphere not only showed the distinctive blue-green color that's a byproduct of photosynthesis, but it also showed ample signs of molecular oxygen, ozone, and methane – tell-tale signs of metabolic life altering the raw basics of hydrogen and helium. The planet also gave off, as Willis puts it, “a strange cacophony of pulsed, narrow-band radio signals,” which is another dead giveaway that something strange is happening down below the cloud cover. An Earth probe coming close enough to a living world would have little trouble detecting life.It's never happened, of course. Voyager I & II, Curiosity, Opportunity, Sojourner, Spirit – these and other intrepid craft over the last fifty years have been launched with a variety of scientific payloads, but one common mission has been so consistent as to be almost forlorn: to search for signs of life. It's the smallest toe in the vast expanse of what lies beyond our world, as Willis knows full well:

Each galaxy contains hundreds of billions of stars. Each one may be accompanied by a planetary system. Beyond the Milky Way, the observable universe may contain hundreds of billions of galaxies. That alone gives a possible 10 ²² planets. But it is the possibility of an infinite universe beyond the cosmological horizon that takes the number of possible habitats for life from the merely numerous to the strictly infinite. But let's remember that such numerical gymnastics brings us no closer to examples of life beyond Earth.

“We must start somewhere,” he writes. “So let's start with the solar system.”The exciting search for exoplanets can sometimes distract from the fact that there are several very promising exoplanets right here in Earth's own solar system. It's true that Earth's sister planets aren't particularly encouraging, Venus being too hot and Mars too cold to support what science currently considers the basic building blocks of life. But they might hold surprises even so (here on Earth, there are microbes that thrive on rocks a mile underground), and there are plenty of other possibilities. Jupiter has several large moons with atmospheres, as does Saturn, whose satellite Enceladus is a water world with large subterranean oceans. Jupiter, Neptune, and Saturn all have volcanically active moons. The solar system is filled with potential havens for enterprising comet-borne bacteria.But as convincingly upbeat as Willis may be throughout his book on the subject of such bacteria, neither he – nor, I'd wager, any of his readers – would be satisfied even with such a paradigm-shifting discovery. Invariably, and quite rightly, whenever books like All These Worlds Are Yours talk about “the search for alien life,” they mean the search for intelligent alien life, for life that can know it's been found and give some kind of answer, for life that might simultaneously be looking for us. For these books, Drake's Fi is always the prime number.And when it comes to that variable, it's not Drake's Equation but the other ideological bookend of exobiology that comes into play: the dour, humorless sibling of Drake's Equation – the Fermi Paradox. Willis doesn't mention it, and little wonder: its stark practicalities would stiff-arm this book and all books like it into the Science Fiction section of the bookstore and keep them there.Named after physicist Enrico Fermi, the paradox asks a simple question: where are the aliens? If the Milky Way has hundreds of billions of stars, millions of them millions of years older than Earth's, and if even a tiny fraction of those stars have life-bearing planets, and if even a fraction of those planets developed sophisticated technology, even the most severe mathematical parsing should result in parking lots full of aliens, or at least museums full of their ancient relics. Even hobbled by the limitation of light speed, this paradox complains, there should be at least hundreds of alien civilizations that proved out the L in the Drake Equation and either came visiting – to conquer, colonize, or just sightsee – or sent mechanical probes to do it for them.Instead, nothing. Entire arrays of gigantic radio telescopes have been probing the night sky for decades, spacecraft have been launched, and yet, as of this writing, there's been no hint of life anywhere, and no hint that life has ever been anywhere in the past. The Drake Equation makes such life all but inevitable; the Fermi Paradox points out its resounding absence; the questing human spirit is caught somewhere in between.All These Worlds Are Yours holds on to the Drake Equation with both hands – Willis is an immensely readable guide to the planets of our solar system (although he overdoes the chumminess with section titles like “Galaxies are Far, Far Away,” or “To Infinity and Beyond,” or “It Came From Outer Space!”), and although he's stern in assessing the chances that such places as Titan or Enceladus could actually host any kind of life, he's clearly thrilled by the pace of astronomical discoveries being made in the 21st century and by the promise of wonder those discoveries hold out for the future:

Perhaps the most frustrating aspect of writing was the realization that the book must end at some point while the accumulation of new knowledge will continue. What major discoveries are just around the corner? What current knowledge will soon be out of date? Which space missions will fly? Which will fail to even get off the drawing board? Under what conditions will contact occur? If I claimed to have definitive knowledge, I would be lying. I do believe, however, that the themes outlined in this book – exploration, curiosity, ingenuity, and open-mindedness – will take us a long way toward our goal.

But always in such geeky arias you hear the keynote: contact. He insists repeatedly in this book that it would be totally unrealistic for anybody to expect such contact to take the form of a fleet of spaceships suddenly appearing in the afternoon sky over British Columbia, but you can tell that nothing would make him happier. The extreme alternative – that the occurrence of complex life, and later of complex, technological civilizations might be so staggeringly rare as to make the universe essentially uninhabited – slaps at the face of the seeking human spirit that's inspired everything from Columbus to Star Trek. So we keep looking and hoping, despite the cold sense of the Fermi Paradox.And if a lowly paramecium is someday eventually discovered in a subterranean Venusian lake, Jon Willis will probably cheer the loudest. Until then, his All These Worlds Are Yours is a prayer for the waiting.____Steve Donoghue is a writer and reader living in Boston. His reviews have appeared in The Washington Post, The National, The Wall Street Journal, The Boston Globe, and The Christian Science Monitor. He is the Managing Editor of Open Letters Monthly, and hosts one of its blogs, Stevereads.