Although he doesn’t post nearly as often as some of us would like, Caleb Scharf’s Life, Unbounded site is always worth reading. Scharf, author of the textbook Extrasolar Planets and Astrobiology (University Science Books, 2008) is the director of the Columbia University Astrobiology Center. As such, he’s positioned to offer valuable insights into our investigations of the forms life might take on other worlds. Not long ago he wrote a fascinating post for Scientific American on a statistical approach to astrobiology, a timely idea as we discuss ongoing missions like Kepler and proposed space telescopes like WFIRST.
Natural Selection on a Galactic Scale
Scharf’s latest is a quick take on panspermia, the idea that viable organisms may be exchanged between planets as various early impacts spread debris through a planetary system. We know that surface material moves continually between the rocky moons and planets of our own system, and we’ve also come to understand that microbial organisms of great hardiness might survive such extreme journeys, even though they involve millions of years of exposure to interplanetary and even interstellar space. Life may indeed be seeded on a galactic level.
But if this is the case, what about the role of natural selection? Scharf writes:
Although it’s a complex problem, it seems likely that life driven by cosmic dispersal will end up being completely dominated by the super-hardy, spore-forming, radiation resistant, rock-eating (endolithic) type of critters. There will be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes – super efficient, super persistent, and ubiquitous – the galactic top dogs.
All of this would fit with what we see on Earth, for we know about numerous organisms in extreme environments here that do indeed survive under conditions most living things would consider hostile. Scharf’s point, though, is that if panspermia is true on a galactic level, then tough organisms like these should be just about everywhere. As our robotic probes grow in sophistication, they should start finding life’s tenacious foothold throughout the Solar System, from the ancient seabeds of Mars to the smog-choked surface of Titan. A galactic panspermia would know no favorites, and it has had billions of years to work.
Galactic panspermia, in other words, is going to make itself apparent in the not distant future. If we find that this is not the case, that life doesn’t pop up just about everywhere we look, then the case for panspermia at this level is vastly weakened, although we can still see a role for panspermia between planets. The larger question of life around other stars, in that case, will remain as intractable as it does today, and will require our most advanced instrumentation to detect in the form of atmospheric biomarkers on Earth-like planets near enough to study.
From Sagan to Drake
All of which reminds me of a recent interview with Seth Shostak. Asked about Carl Sagan’s estimate that there might be one million intelligent species in the galaxy and Frank Drake’s speculation that the number was closer to 10,000, Shostak comes down on the side of Drake, noting that if 3 percent of the solar systems in the Milky Way have Earth-like planets, then 10 billion such planets must exist. Assume just one in a million to have intelligent life and you still end up with 10,000 civilizations. Kepler will let us tighten these numbers within a few years.
Shostak takes note of the debates he has had with Rare Earth author Peter Ward, who argues in his book with Donald Brownlee that intelligent life must be scarce due to the huge number of factors — Jupiter-like planets, a nearby moon, the tight parameters of habitable zones — that would make it possible. Shostak:
I’m not at all convinced that moons are needed to support life. Without the moon, the tides would be different and the poles would migrate every so often. But that wouldn’t wipe out life. Regarding gravity, there are already two planets with earthlike gravity in our solar system, and even Mars could probably have supported life earlier in its history (and maybe even today). And we are now fairly certain that Jupiter sized planets are commonplace – we have already located hundreds of them. So I just don’t find the argument that complex life must be rare in the galaxy to be compelling.
The encouraging thing about these discussions is that we are dealing with issues about which we will have preliminary answers within a matter of years. Kepler will be able to give us statistical answers about the prevalence of Earth-like worlds in the galaxy, and using Scharf’s reasoning, we can draw extrapolations on the likelihood of panspermia based on what we find in our own system fairly soon, just as long as it takes to get complex robotics to environments like Europa.
True terrestrial planet hunter spacecraft — the kind that can make spectroscopic analyses of exoplanet atmospheres on worlds this small — are at least a decade and perhaps much more away depending on funding issues, but they represent logical extrapolations of near-term technology. In 25 years, we will have not just a philosophical view of life in the universe but a practical knowledge based on data that can tell us whether we’re likely to be alone or simply one among many galactic species.