In a happy synchronicity, the same week that I attended a lecture by Rare Earth co-author Don Brownlee, two posts on the same topic showed up on a couple of blogs I read. First was Kepler and the Rare Earth hypothesis from Chris Rowan's Highly Allochthonous, followed by A Habitable Zone by any other name… from Johnson R. Haas at The Planetologist. Go read their posts for much more coherent thoughts on this topic than I'll have.
The lecture was part of the M.E.L. Oakes Undergraduate Lecture Series, named for a physics professor who retired from the University in 2004. Dr. Oakes was very committed to undergraduate education (he was my professor for Waves and Optics) and when he retired, his colleagues established this lecture series in his honor.
Here are some of my notes and thoughts from Brownlee's lecture.
First off, a disclaimer of sorts - I haven't read the book "Rare Earth" yet (although it's on my Amazon wish list!). Brownlee co-authored the book with Peter Ward, a paleontologist who specializes in research into mass extinctions (particularly the K-T boundary event, if I remember correctly).
The lecture was more of an overview of the questions that need to be asked to determine whether or not we are a "rare earth". Since I haven't read the book yet, I don't know if they address how all of these odds stack up and determine just how rare we are. Although judging by the rest of the title "Why Complex Life Is Uncommon in the Universe", I think I know the answer.
I am one of those people who grew up with Star Wars, Star Trek and science fiction in general and I *want* to believe that the galaxy (or universe at large) is teeming with life, but I have to concede that the notion of complex life or civilization, might be pretty rare. Simple life may be far more common (even in our own solar system), but there is a long way from a bacterium to a dinosaur or Homo sapiens.
The two main questions Brownlee started with were "How typical is Earth?" and "What is typical?". There are two ends of the spectrum - either there are earth-like planets all over the place or we're just about the only one. These are the extremes though, and like with so many other things, the truth is probably some where in between. He brought up the additional question of why rarity is important - and I loved his answer. Rare means no Star Wars cantina. If the distance between planets supporting complex and intelligent life is too great, there will be no opportunity for contact and/or study.
Microbial life may be more frequent but complex animal life may be more rare. On earth microbes formed almost as soon as they could after the late heavy bombardment. Animal life took a lot longer to arise and is more picky about its environment and is much more easy to extinguish.
Another question he raised was "What is Earth-like?" and again supplied a memorable answer - like porn "you know it when you see it". He covered a lot of the common issues that we look at when trying to search for Earth-like planets - such as the habitable zone (distance from parent star), type of parent star (a hot, blue star will die before life can take hold) and type of orbit (more circular means fewer extremes in seasons). But one thing he brought up that I haven't thought of as often is the factor of time. Not just the time that it takes for complex life to evolve, but also the time that it takes to get complex elements from the death of the first few generations of stars after the Big Bang. There is also the time that during the sun's lifetime that earth is in a comfortable spot (for us, at least).
With the discoveries of other solar systems since 1995, we're also beginning to get an idea of how rare our solar system itself is, not just Earth. We've discovered the "Hot Jupiters", and we have to consider how the migration of a gas giant from the outer solar system (like in ours) to the inner solar system would affect any of the smaller, inner, rocky bodies. The new Kepler mission (which is addressed in the blog posts I linked to at the top), will start to give us some good ideas of what the planetary population is like in our neighborhood of the Milky Way.
Other factors in our "rarity" to consider are: our Moon, which creates tilt stability for the Earth (moderating our seasons), our position in the Galaxy (not too close to the center), Jupiter's position in the solar system (helping to protect us from some impacts). There are lots of factors that can change on Earth itself that would affect life: the salinity of the oceans, plate tectonics, mass of the Earth (gravity), right amount of carbon, the rock cycle and its effect on carbon dioxide. And there are the "wild card" factors, such as impacts or nearby supernovae.
When you start to look at all of these factors and how subtle changes in them might change the outcome of life on earth (as we know it), you can see where people might get the notion of a "rare earth". There are a lot of "fudge factors" which start to add up.
Brownlee also addressed challenges that our species (or considering the amount of time for some of these - whatever the intelligent species on Earth is if it isn't us) will face in the future. In the near-term there are issues of global warming and impacts, but these are things that can be dealt with through technology. We know that there are other intelligent species on earth, such as "Flipper" and "Lassie", but they can't save us from impacts. [This reminds me of a quote I once heard that the reason the dinosaurs don't still exist is because they didn't have a space program.]
There are also some long-term factors that we have no direct control over, such as the inevitable death of the sun. We can adapt to the changes for a while but in the end, Earth will either be a cinder or completely swallowed by the Sun after it has swelled into a red giant. [Phil Plait covered this nicely in a chapter in Death From the Skies.] Even before we get to that point, the movement of the continents will change ocean cycles, which will in turn change weather and climate. Again, we can't do anything to stop plate tectonics, but we can possibly adapt to the resulting changes.
Ultimately, whether we can prevent the impacts, adapt to changing climates or escape the earth for other worlds, we still have our ambassadors to the stars - the Pioneer and Voyager spacecrafts.
All told, this was a nice overview of the topic and it made me want to read the book. So many of the factors he touched on could have been entire talks of their own!