April 7, 2020

Review: Rare Earth by Peter Ward & David Brownlee.

A controversial classic!

It is perhaps one of the greatest scientific questions of our time. How common are we? Is our existence here in time and space a widespread occurrence in the cosmos, or are we so unique that we are effectively alone? The topic of this week’s review represents a landmark paradigm shift and is an often quoted book that I’ve always wanted to get around to reading and reviewing. Rare Earth: Why Complex Life is Uncommon in the Universe by David Ward & Peter Brownlee posits that animal life in general and intelligence such as our own in particular is a rare, perhaps a singular event in our corner of the galaxy. Published in 2000, its interesting to see how the science of the day stacks up to current thinking. For example, in 2000 a handful of exoplanets were known; almost all were “hot Jupiters,” and the prospects for terrestrial planets looked slim. To date, 779 extra-solar planets have been discovered using a variety of methods, providing researchers with enough data to classify and characterize various types of planetary systems.

It should be noted that the authors do point out that while animal life may be a tough hurdle, simple bacterial life may be common in the cosmos. Our own story lends some credence to this supposition; once conditions in the early history of life on Earth stabilized about 3.5 billion years ago, simple life arose readily. For almost 90% of the span of life on Earth, however, life remained at the simple one-celled stage. It seems that at least in our own case, going to complex multi-celled life was the hard part; but yet in less than a billion years, the explosion of plant and animal life led to dogs, cats, humans, Ipads, etc. How common this tale is remains to be seen. Certainly, the discovery of bacterial life past or present within our own solar system may lend weight to the first half of the Rare Earth hypothesis.
Among the factors that the authors site as conducive to life as we know it;

-An orbit around a single relatively stable star that maintains a steady output for many billions of years, long enough for life to develop;

-A stable orbit within the habitable zone of said star, a place where water can exist in liquid state;

-Condensation from a proto-solar nebula with a high “metallicity” (remember, to an astronomer, the universe is hydrogen, helium, & metal!) full of lots of great but scarce raw materials such as carbon, silicon, nitrogen, etc.

-A single large Moon that acts to stabilize the tilt of the Earth;

-A large “goal tending” planet like Jupiter that deflects a good portion of the life extinguishing comets that come our way.

-A stable position in the galactic habitable zone, not too close to the radiation-riddled core and not in the outer metal poor ‘burbs. A good distance from any life extinguishing supernovae or gamma-ray bursters helps too, a sort of “may you live in mediocre times” curse/blessing.

-Active tectonic plates allowing for subduction and sequestration via a rock and carbon cycle.

To this end, the authors add some interesting twists to the famous Drake Equation, allowing for the events that brought us here in the mix. Certainly, if some of the scenarios such as the formation of our Moon are mandatory, chances for life are slim. One only has to look at the caveats offered by our neighboring worlds of Venus and Mars to see how different the Earth could be.

Still, a nagging hunch pulls at the back of our brain as we read Rare Earth… just how viable is a statistic of one? Are all of these happy accidents mandatory, or can life, once it’s started, make due even under drastically different conditions? One could also point out that these conditions aren’t exactly stable or permanent; the tilt of the Earth’s axis does vary, the output of the Sun is increasing and fluctuates with time, etc. It would be great to have a better understanding of the minimum and maximum criterion for life as it relates to these events. Carbon is probably crucial; no other element forms such long complex chains, although silicon is sometimes also cited as a possible alternative. Water also makes a great ‘universal solvent…” but might oxygen be poisonous to some forms of life? Would we recognize life drastically different from us if we saw it? I’m reminded of Arthur C. Clarke’s Report on Planet Three, where a Martian scientist gives a long and convincing discussion on why life on a hostile planet such as Earth couldn’t be possible.

Read Rare Earth as a very timely and still largely pertinent discussion on one of the biggest questions of our time. I would also recommend James Kasting’s How to Build a Habitable Planet as a great look at how the Earth came to be. Either conclusion has stunning implications; of course, most of us root for sentience and a cosmos teeming with diverse life with Klingons and Vulcans bickering about treaties in a Galactic Federation… but if we are truly  ”it” in our tiny niche of time and space, doesn’t that make us and the Earth all that much more precious and unique, a jewel worth safeguarding and preserving?


  1. Torbjörn Larsson, OM says:

    Wow! And I was just gloating the other week when the Habitable Exoplanets Catalog promoted a 2nd Gliese 581 planet to habitability that having 2 systems with 2 habitables (Earth/Mars; Gliese 581 g/d) would make it “Common Gliese” instead of “Rare Earth”. I imagined that the Rare Earth contenders would face into oblivion. [Disclaimer: Not an astrobiologist, but studying it when I have time.]

    In other words, I don’t think Rare Earth has weathered well. Almost all factors that they rather propose in a rather arbitrary bayesian model fashion has been show erroneous:

    - Corot & Kepler shows many planets, many stable orbits, many terrestrials and many habitables.
    - Binaries have planets; metallicity doesn’t affect number of terrestrials (but make giants slightly rarer as the disk scatters faster).
    - A large Moon isn’t necessary for long periods of little tilt, an error in the then models that was found out this year.
    - Jupiter has been found to _increase_ impact rates to the inner planets AFAIK.
    - Sun has about the average metallicity of Milky Way, but we happen to be far out in the hood.
    - While Earth is marginal for plate tectonics and need water, it is a runt as terrestrials goes. SuperEarths are modeled as having magnetic fields and plate tectonics from convective cores up to at least twice Earth radius.

    And so on.

    The recent finds that Earth, Moon and Mars mantle initial water content were similar, and that a better model for the planetary disk predicts the relative dryness for terrestrials around the habitable zone removed the last stumbling block IMO. Only water content is finetuned, a factor ~ 5 less and we wouldn’t have oceans, a factor of ~ 5 more and we wouldn’t have continents for habitability as we know it.

    Complex multicellularity is predicted from Lane’s energy theory on eukaryotes: you need an endosymbiosis with a bacteria for making simple multiple “energy plants” giving us ~ 10^5 the energy density of bacteria for protein turnover and so large genomes.

    You don’t necessarily need oxygen, because it was recently found that 0.1 – 1 mm Loricifera can live in anoxic environment, using hydrogen in their evolved mitochondria. But the essential endosymbiosis happened right after the atmosphere was oxygenated, it is believed that the 3 domains (Bacteria, Archaea and Eukaryotes) started to diversify then, and we don’t see eukaryotes without mitochondria.

    Endosymbiosis has been observed many times, even between bacteria, even if it hasn’t resulted in new “energy plants”. Maybe there is an ecological lock in effect, similar to how we no longer see abiogenesis.

    The real question is IMO if you want to study the last factors of the Drake equation beyond simple or complex life, for language capable technological species. (I.e. not just apes or dolphins or stem fishes (!), all found to be using technology of sorts, but something reminding of us.) But the full Drake equation is too little constrained to be of use as of yet. I don’t think that area is “Rare Earth” territory as such.

    PS: Oxygen _is_ a poisonous waste product, no life would start in an oxygen environment which breaks down instead of promote organics. It is still messing with metabolism such as photosynthesis and ATP production et cetera. Oxygen stress likely initiated the domain diversification. Early life originated the worst environmental catastrophe that changed Earth for ever, we can never achieve anything like it even if we tried.


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