history of thinking about the extra-terrestial question

Growing up in the 70s / 80s, it seemed to me, even as a kid, that speculation on alien life was firmly within the realm of wild fantasy. Nowadays the debate seems much more measured. Of course, comparing what you knew as a kid to what you know as an adult is a fools errand, but it does seems like there’s a more statistical approach taken nowadays - there are more planets in the universe than grains of sand on planet earth is a common anecdote, and people are taking a stab at what percentage of these might be expected to be earthlike. Were people saying things like this in the 60s and 70s?

The Drake equation has structured a lot of thinking in this area, and that came out in 1961. I wonder how widely publicised this sort of approach was in the pre-internet days. Have recent advances in astronomy put the discussion on a much more serious footing, or was it possible to estimate the number of earth-like planets in the milky way, say, in 1961?

I’m trying to get a sense of how the discussion has changed over the past 50 years in terms of the data at hand.

It isn’t possible to estimate the number or distribution of “Earth-like” planets now from posterior knowledge. We haven’t detected a single world that is sufficiently like Earth in composition, size, orientation, and distance from its parent star, much less a sufficient number to create any kind of posterior distribution or any world that demonstrates life-bearing characteristics such as a heavily oxidizing atmosphere.

We can make some reasonable assumptions based upon models of stellar and planetary formation that suggest that our solar system is not atypical and that worlds capable of providing conditions similar enough to Earth to support life as we might imagine it are not rare. We also know that amino acids–the basic building blocks of carbon-based genetic life–are frequently found both in interplanetary space and in the interstellar medium. We don’t know under what conditions life first emerged, but it seems quite possible that whatever the conditions were they may be frequently repeatable. The bottlenecks to complex, multi-cellular, and self-aware life, however, appear to be quite restrictive; it may be that simple life is very common, but more complex forms of life are vanishing rare and separated by such vast distances that they’ll never contact one another.

As for the history of the plausibility of extraterrestrial life, as we have come to understand more about the processes of life, from Friedrich Wöhler to J.B.S. Haldane and Francis Crick, that while while life is special in the sense of being self-organizing and thermodynamically self-regulating, there is nothing particularly novel about the mechanics behind life. The same physical and chemical reactions that drive inorganic and simple organic chemistry also apply to the far more complex polymers and proteins that form the structures of organisms. While we don’t have any empirical data about extraterrestrial life, we can firmly assert that the conditions that would support and at least provisionally allow for the abiogenesis of life ought to be not infrequent in the Universe.


I’m reading the book Lonely Planets: The Comparative Philosophy of Alien Life right now, and while I think sometimes the author is pushing a bit hard for his own interpretation, the first two sections offer an in-depth history of scientific and philosophical thought about the existence of alien life, as well as the history of the actual scientific search for alien life.

At least in the 70s, they did (I was born in 1965). And I also knew about the Drake equation back then. Pre internet, we used more books and pop-science publications.

However, we had much more flying soccers in our sky (and accordingly debates about them), and, it seems to me, much less serious belief in life somewhere in the Universe (even though it was the time of the project SETI, so enquiring minds were definitely present). Another difference is that it seems to me that for some reason we have now more hope of finding life in the Solar System itself than we used to. Maybe caused by the discovery of the extremophile organisms on Earth and by a better knowledge of unthought of potential candidates for life, like satellites of the giant planets.

Note also that back then, we didn’t even know how common planets could be in other stellar systems. The detection of the first extra-solar planet is quite recent and I was enthusiastic about it.

The only real difference that I see, having been a fan of space since Sputnik, is that until very recently the default assumption was that the universe was full of solar systems much like our own. And if most stars (except for multiple star systems, which were thought to make orbits unstable) had earthlike planets, then most stars also had life. This was the popular assumption because of decades of science fiction, which can be traced pretty directly back to The War of the Worlds even though others preceded Wells. However, even scientists took this view.

That was the reason Fermi asked, well, if there are all these civilizations out there, why haven’t we seen them? That was back in 1950, after the 1948 craze for UFOs made it a national topic for discussion. (Despite what many people think today, there had been many previous series of sightings of strange objects in the sky, dating back before the Wright Brothers.) Fermi’s question wouldn’t have resonated unless the opinion of most scientists was that the galaxy was full of life. (Nobody reasonable thought that extra-galactic life would be visiting.)

The actual discovery of extra-solar planets defies the conventional wisdom of how planetary systems should look. And many of them are in multiple-star systems, too. But as Stranger says, we’re in such early days of discovery that we’re just beginning to touch on planets that could be considered earthlike and few, if any, of thought are in positions that are comparable to earth’s favorable location from the Sun. I feel that any extrapolation of the possibilities of life from these early findings is just bad science or bad science writing, although of course you can easily find them. Some of them answer Fermi by stating that there are practically no planets that could harbor life like ours. Maybe, although that’s like predicting the lack of major rivers in the New World based on Columbus.

The 50s were a major period in serious popular science (an oxymoron that really isn’t one) because so many rocket veterans had seen the advances made in WWII and created a deliberate media push to make the concept of space travel the natural coming thing. Von Braun worked with Disney, for Pete’s sake. Willy Ley wrote a series of brilliant books. Chesley Bonestell’s paintings looked realer than life. People took space much more seriously then than today. And it didn’t seem as hostile, as barren, or as unconquerably large.

But the entire history of thinking about alien life has gone in these swings and cycles since ancient times. You can’t generalize about the past anymore than you can generalize about planets. The field is too big.

Actually, at least some of the planets the Kepler mission has found are Earthlike in at least size and distance, and probably composition. I don’t think we know anything about the atmosphere of any of them, though, nor axial tilt.

I was born in 1958. When I was a kid, it was thought that water was an extremely rare commodity on the solar system and thus probably the the universe. It was also thought the water is a prerequisite for life.

Now, it seems that water is all almost everywhere. And that is one of the reasons that there is more confidence among some scientists that life exists elsewhere.

I doubt even one in a thousand people were aware of the Drake Equation until Cosmos was televised in the late '70s.

I don’t think any of the planets discovered thus far by the Kepler mission are both somewhat Earthlike in mass and composition, and at a habitable distance from their parent star. Kepler-10b, for instance, is the first planet that is assuredly an nickel-iron core, rocky surface world around a G4V star, but its period is less than a day, making it far to close to sustain any kind of carbon-based life or hold liquid water. This doesn’t mean that terrestrial worlds in a habitable zone don’t exist; only that at this time we lack the technology to discern them at interstellar distances. While a database of other types of planets helps us refine our models of system formation and empirical expectation of the composition of other star systems, we have no direct evidence of worlds that would be suitable for Earth-type life.

Of course, we should not limit ourselves to conceiving of life only as it would develop and thrive in a terrestrial enviroinment. It is entirely possible that self-organizing, self-replicating systems could exist in environments we could consider extreme, and in mediums other that complex carbon-based polymers. We would have no real idea what to look for (other than general signs of organization and thermodynamic regulation) and thus, no way to make any kind of quantitative assessment of the probability or distribution of such life, but considering the possibility allows us to greatly increase our expectation that it may existi.