In The Principles of Philosophy (1642) Descartes lamented: “We do not doubt but that many things exist, or formerly existed and have now ceased to be, which were never seen or known by man, and were never of use to him.” Descartes didn’t know the half of it. As our understanding of the natural world has improved across the past half-millennium there has been a clear trend of dethronement, of blows to the collective self-esteem of Homo sap.
No, our Earth is not at the center of things, only a middling planet among several, all in orbit around the Sun. The Sun itself is a humdrum star, one of billions in our galaxy, which is likewise one of billions of similar objects in the universe — “galaxies like grains of sand” (Aldiss). In recent years some serious physicists have even put forth a “multiverse” theory of creation, in which our very universe is merely one among innumerable others. Along the way there we passed Charles Darwin telling us that we are not transcendent beings, only twigs on the great tree of terrestrial life. It’s been humiliating.
Once scientists became aware of this trend — Lee Billings calls it the Principle of Mediocrity — it influenced their speculations about extraterrestrial life, leading to fantasies of a jungle-covered Venus or canals on Mars. When closer examination of Venus and Mars became possible in the 20th century, however, we learned that those planets are lifeless deserts.
It is still possible that there might be life on planets beyond our Solar System, planets orbiting other stars or floating free in space. It might even be that intelligent life — language, reflective consciousness — exists out there, some subset of it perhaps having attained technological civilization, with telescopes and radio transmitters. The probability for a given star may be tiny, but there is a mighty host of stars.
The first person to take a systematic approach to this topic was Frank Drake, who in 1960 was a young astronomer at a federally-funded radio telescope in West Virginia. Drake got a small grant to listen for possible signals from extrasolar civilizations. Nothing showed up, but Drake’s efforts caught the interest of other scientists, and SETI — the Search for Extraterrestrial Intelligence — was born. The following year, 1961, the National Academy of Sciences convened a conference at which Drake presented his now-famous equation:
N = R×f p×n e×f l×f i×f c×L
That N at the left of the equals sign is the number of technological civilizations in our galaxy that we might be able to detect. The seven numbers on the right, to be multiplied together, are distinct factors determining N.
Taken in order, R is the rate of star formation in our galaxy, now thought to be seven per year. The factor f p is the fraction of stars that have solar systems, while n e is the average number of habitable — by life of any kind — planets per solar system. The other f‘s are the fractions of, respectively, habitable planets that actually produce life, life-bearing planets that produce intelligent life, and intelligent life-forms that produce civilizations we might be able to communicate with. So, for example, if half of all life-bearing planets eventually produce intelligent life, then f i = 0.5. Finally. L is the span of time for which a given technological civilization is “visible” (e.g. by sending out radio waves) to us.
The trouble with the Drake Equation, as was noted at once, is that some of the factors are utterly unknown, to the degree that quite plausible guesses about them yield values of N — the number of other civilizations such as ours currently thriving — ranging from one to a million. At that original 1961 meeting, Drake argued that the first six factors might easily cancel each other out, leading to N = L as an initial approximation. So if “visible” technological civilizations last an average 10,000 years, as Drake surmised, then there are 10,000 of them in the galaxy.
Frank Drake is still with us, and Lee Billings gives over a whole chapter to a 2011 meeting with him. Fifty years of working with SETI have lowered Drake’s expectations. He still thinks that 10,000 is about right for the number of technological civilizations in our galaxy, but believes they will be difficult to detect.
We are far less radio-visible now than formerly, with digital television now carried by coaxial cable and optic fiber. Inhabitants of solar systems sixty light years distant, if there are any, could currently be enjoying early seasons of I Love Lucy, but they may never see The Sopranos. High visibility to radio telescopes may be a passing phase in civilizational development.
L is therefore problematic, and not necessarily a guide to the actual average lifespan of technological civilizations, which of course we all hope is long. Nor has there been great progress with Drake’s other factors since 1961, with a single exception.
Lee Billing’s book describes our present state of knowledge about all the factors in Drake’s Equation, with good necessary background information on astronomy, cosmology, geology, chemistry, and biology. He has hunted down key players and interviewed several of them at length, with some good human-interest seasoning to help the reader connect. We meet, for example, Jim Kasting, “the world’s foremost authority on planetary habitability,” and geologist Mike Arthur, who has interesting things to say about fracking and climate change.
Naturally, though, that one exceptional factor on which science has made real progress is the one to which Billings gives by far the most coverage. This is f p, the fraction of stars that have solar systems.
The 1961 conferees guessed f p to be between one-half and one-fifth, but a guess is all that was; they had no data to work from. The stars (other than our Sun, of course) are so distant, and so bright by comparison with any planets they might own, that we had no way to detect extrasolar planets in 1961. All that changed in 1995, when the first confirmed extrasolar planet was found by a Swiss team. The planet orbits 51 Pegasi, a star fifty light years away. Discoveries thereafter came thick and fast. The number of extrasolar planets known is now close to one thousand.
Both the sociology and the astronomy here are good stories, and Billings gives full coverage to both. For the first decade or so after 1995 the field was dominated by competition between an American team using observatories in California and Hawaii, and a Swiss group based in Geneva, but with collaborators worldwide. Quite a little Space Race went on into the mid-2000s, with some fine rancorous disputes over priority.
The astronomy for finding extrasolar planets rests on two main techniques. In the first, called radial-velocity spectroscopy, the scientists measure minute stretchings and compressions of a star’s optical spectrum — its “signature” — as it is tugged toward or away from us by orbiting planets.
In the second technique we measure the very slight dip in a star’s light output as a planet transits across the star’s disk, eclipsing some of the starlight. This depends on star, planet, and earth being aligned, so it can catch only a small minority of planets. It was deemed sufficiently worth doing, though, for NASA to launch the \$600 million Kepler space telescope in 2009. Kepler has found dozens of extrasolar planets and hundreds of candidates to be confirmed by other means. We now believe that f p is bigger than one-half , probably close to one — that is, almost all stars have some companion planets.
With the finding of exoplanets now routine, the most visionary researchers seek to discover facts about them: to gain information about their atmospheres by spectroscopy, ideally to form actual images of them. This will be really, really difficult, and funding for the research is hard to come by.
Billings nonetheless gives over three chapters to it, introducing us to some striking characters. Most striking of all is MIT’s Sara Seager, who sees a possible escape from the funding limbo via private enterprise: “We really can’t just expect the government to do it for us; we very well may have to do it on our own, and having a robust commercial space industry can only help.” Seager is an advisor to Planetary Resources, a private asteroid-mining venture.
Five Billion Years of Solitude is an excellent survey of its field, only slightly marred by some lapses into Creative Writing. (“He … dragged a hand over his jaw, producing a sound like dry, windblown leaves.” I have tried without success to reproduce this sound. Perhaps I have the wrong kind of jaw.)
Billings himself offers no opinion as to whether we are alone in the universe. I think this is wise; there are far too many unknowns. Agnosticism is the only rational opinion. Drake’s f l, for example, concerns biogenesis, about which we understand very little more that we did in 1953, when the Miller-Urey experiment produced organic compounds by simulating lightning in a primordial atmospheric “soup.” Concerning f i and f c our ignorance is total.
The issue will be decided at last by observations made by researchers working at the furthest edge of astronomical technology with chronically uncertain funding. If our civilization survives, persons alive today will see images of earthlike planets orbiting other stars, like the iconic “blue marble” image of Earth itself sent back from Apollo 17. If the Principle of Mediocrity is still operative, they may even hear the voice of an extraterrestrial civilization. Now that will be worth having waited a few centuries for!