“It’s part of the nature of man to start with romance and build to a reality,” Ray Bradbury poignantly observed in his seminal 1971 conversation with Carl Sagan and Arthur C. Clarke about space exploration. Indeed, much like so many of Bradbury’s romantic predictions eventually became reality, the fictions of science have always extended into visions of the future, among the most persistent of which is that of other habitable worlds onto which humanity can project itself. In Five Billion Years of Solitude: The Search for Life Among the Stars (public library), science writer Lee Billings explores the possibilities of building that romance into a reality. The title refers to the projected longevity of life on earth — life that will be extinguished once the Sun burns out. Billings sets the backdrop:

Life emerged here shortly after the planet itself formed some four and a half billion years ago, and current estimates suggest our world has a good half billion years left until its present biosphere of diverse, complex multicellular life begins an irreversible slide back to microbial simplicity. In all this time, Earth has produced no other beings quite like us, nothing else that so firmly holds the fate of the planet in its hands and possesses the power to shape nature to its whim. We have learned to break free of Earth’s gravitational chains, just as our ancient ancestors learned to leave the sea. We’ve built machines to journey to the Moon, travel the breadth of the solar system, or gaze to the edge of creation. We’ve built others that can gradually cook the planet with greenhouse gases, or rapidly scorch it with thermonuclear fire, bringing a premature end to the world as we know it. There is no guarantee we will use our powers to save ourselves or our slowly dying world and little hope that, if we fail, the Earth could rekindle some new technological civilization in our wake of devastation.

This longview choice of life and death, he argues, falls somewhere between science and spirituality, a new kind of scientific humanism to which the people Billings profiles — from evolutionary climatologists to futuristic cartographers to astronomers in search of extraterrestrial life — have dedicated their lives, driven by the shared belief that “in the fullness of planetary time any human future can only be found far beyond the Earth.” Billings puts it beautifully:

As precious as the Earth is, we can either embrace its solitude and the oblivion that waits at world’s end, or pursue salvation beyond this planetary cradle, somewhere far away above the sky. In our lives we all in some way contribute to this greater choice, either drawing our collective future down to Earth or thrusting it out closer to the stars.

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I won’t pretend to know what our collective choice will be, how exactly we would embark on such an audacious adventure, or what we would ultimately find out there. I am content to merely have faith that we do, in fact, have a choice. Similarly, I can’t suggest that we simply ignore all of our planet’s pressing problems by dreaming of escape to the stars. We must protect and cherish the Earth, and each other, for we may never find any other worlds or beings as welcoming. Even if we did, we as yet have no viable way of traveling to them. Here, now, on this lonely planet, is where all our possible futures must begin, and where I pray they will not end.

One of the most fascinating scientific romantics Billings spotlights is the astrophysicist Greg Laughlin, a 44-year-old professor at UC Santa Cruz. In 2009, shortly after NASA launched its Kepler spacecraft on a mission to hunt for Earth-like planets elsewhere in the cosmos, Laughlin devised “a strange, half-whimsical equation” to calculate the approximate value of such candidate planets and gauge whether they were worthy of scientific investment beyond the expected media hype such discoveries would inevitably produce. Using a number of variables, from a planet’s temperature and mass to the age and type of its star, the equation promised to yield a crude dollar valuation of the respective world. Billings breaks down the math:

Small, rocky worlds in clement orbits around middle-aged, middle-of-the-road stars similar to the Sun merited the highest values, as those planets presumably offered the best chance for harboring complex biospheres that could eventually be detected by future space telescopes. For a planet to be worthy of wide attention, Laughlin opined, it would need to at least break the million-dollar mark. Laughlin drew his economic baselines from simple math, dividing Kepler’s federally funded $600 million price tag by 100, a conservative estimate of how many terrestrial planets the space telescope would discover during its lifetime. If such planets could be considered commodities, the math suggested that the 2009 market price, as determined by U.S. taxpayers, could be set at $6 million per world—a value that could drop over time if small rocky planets began to overflow astronomers’ coffers. If, however, Kepler found a terrestrial world in the middle of a Sun-like star’s habitable zone, Laughlin’s test runs suggested such a planet’s value could exceed $30 million in his equation.

But most inventive of all was that Laughlin extended the formula not only to planets, but also to their home stars — which meant that, in theory, the calculations could be applied to the planets of our very own solar system, measuring their value not in dollars but in photons:

Photons, not dollars, are a planet hunter’s fundamental currency, as they are what allow a planet to be not only detected but also subsequently characterized. Generally speaking, the more photons astronomers can gather from an exoplanetary system, the more they can learn about it. Stars and planets nearer to our solar system are brighter in our skies due to their close proximity, and hence more valuable, providing floods of useful photons where more distant objects would only offer trickles. This facet was why so many of Kepler’s small planets would struggle to reach a valuation of even a million dollars: the Kepler field stars were far away, and thus very dim. The brightest star visible in the solar system by many orders of magnitude is, of course, the Sun, which has the capacity to send local planetary valuations into truly astronomical territory.

If Laughlin based his calculations on the early-twentieth-century belief that Venus’s clouds would reflect the Sun and shield the planet from the scorching solar flux, its scientific value would swell to a whopping $1,500 trillion — that’s one and a half quadrillion dollars. But if Venus’s actual runaway-greenhouse surface temperature were used for the calculation, the planet would be worth a trillionth of one cent. (This gaping disparity of valuations, Laughlin asserted, is akin to the dot-com bubble of the 1990s, a case of companies translating investors’ blind enthusiasm into exorbitant valuations that eventually caused the bubble to burst once their true value was revealed.) When Laughlin applied his equation to our Earth, he arrived at a value of roughly five quadrillion dollars, or about 100 times the global gross domestic product — “a handy approximation of the economic value of humanity’s accumulated technological infrastructure.”

But he didn’t stop there — he also ran the equation for a habitable candidate from Alpha Centauri system, which he valued at $6.5 billion. As Billings wryly remarks, that’s approximately the amount astronomers estimate would be necessary to build the kind of space telescope capable of detecting signs of life on such a world. The most intriguing dynamic, however, is that just like we confer value on art, we confer value on the world itself — the star to such a world would become increasingly brighter if we were to actually voyaged there, eventually becoming a new Sun to our new home. Laughlin tells Billings:

In going there, you have this ability to intrinsically increase value. And that’s an exciting thing because it ultimately provides a profit motive for perhaps going out and making a go of it with these planets. This is saying that something that is several billion dollars on Earth could be, if you go there, a quadrillion-dollar payoff.

Of course, Billings is careful to admonish against the lazy shorthand mainstream media tend to use in headlining such news — it is equal parts lazy and foolish to put an actual price sticker on a planet, especially on our Earth, which is home to other worlds of immeasurable complexity and contains “the aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love…”

Five Billion Years of Solitude goes on to explore the thrilling and terrifying repercussions of what it means to live as “creatures with the intellectual capacity to discover their genesis and the technological capability to design their fate,” and to remind us, above all, that we do have a choice in that design. Complement it with Isaac Asimov’s witty 1969 letter on why we should invest in space exploration.