Written by a pair of professors at the University of Washington, Rare Earth is a polemic for the view that complex life, both animals and higher planets, is rare in the Milky Way Galaxy and perhaps even in the Universe. Thus, the authors contend that there could be many, perhaps millions, of planets scattered throughout the Galaxy on which single-celled microorganisms thrive. On only a handful of planets, perhaps only one, would the air be filled with flying creatures and the ground covered with creepy or crawly things.
Cast in terms of the Drake equation, the authors believe that fl, the probability that life will originate on a habitable planet, could approach unity (1). However, they argue that fh, the fraction of planets that are habitable, and fi, the probability that intelligent life will develop on a planet on which life has emerged, are individually or both probably quite close to 0. Thus, the total number of technological civilizations in the Galaxy would be small, perhaps only one. (However, they also mis-characterize the Drake equation as assuming that "once life originates on a planet, it evolves toward ever higher complexity." As the Drake equation contains a factor fi there is clearly no assumption in the Drake equation itself that life "evolves toward ever higher complexity.")
The authors bring to bear a number of different aspects of the Earth's astrophysical, geological, and biological history, some of them fairly recently appreciated, to support their argument. Among these aspects are
A Galactic habitable zone, the possibility that habitable planets can only exist in a restricted region of the Galaxy,
A temporal habitable zone, based on the fact that a star's luminosity gradually increases over time meaning that in 1 billion years or so the Sun will boil away the Earth's oceans,
The presence of a comet-slaying giant planet to reduce considerably the number of comets that strike a habitable planet, like Jupiter in our solar system,
The "snowball" Earth hypothesis, that on at least two occasions the Earth's oceans may have largely or entirely frozen over, and
The time between the first appearance of life on this planet and the Cambrian explosion when the diversity of life exploded and animal life first appeared.
It has been this reviewer's experience that reading well-written apologies of a position with which one disagrees can often make one sharpen one's arguments. Rare Earth cannot be put in this category. I found it not only to be not convincing, but not particularly thought provoking and in some places sloppy almost to the point of being wrong.
Consider just a few examples. The authors suggest a Galactic habitable zone. They contend that too close to the Galaxy's center a planet would be fried by the intense radiation at the center, too far from the center and Earth-like planets might never form because the clouds of dust and gas at the edge of the Galaxy do not contain enough "metals" to form terrestrial planets. In principle they are correct. (I would add that too close to the Galaxy's center terrestrial planets might never form or would be ejected quickly because of the frequent close passages of other stars.) In practice, what is the size of this Galactic habitable zone? They argue for an apparently quite small zone, though they provide no firm quantitative estimate. I would certainly agree that the inner 300 light years, maybe the inner 1000 light years, of the Galaxy are probably not too hospitable. They list a number of possible hazards that life toward the Galactic center might face---ionizing radiation (but how strong would the radiation have to be before the atmosphere of a planet would not protect life on its surface?), strongly magnetized neutron stars (which are how frequent?), and supernovae. Their primary concern seems to be with supernovae, but they also seem to neglect one favorable aspect of the inner regions of the Galaxy. The interstellar gas density in the inner regions of the Galaxy is higher than near the Sun (and many supernovae occur in or near quite dense molecular clouds). This higher gas density should also decrease the effects of nearby supernovae on planets with life. Moreover, it's also worth noting that the Sun is currently inside a cavity of hot gas known as the Local Bubble. This cavity may have been formed by one or more supernovae within the (astronomically) recent past. Obviously, life on Earth survived. Similarly, they provide no outer radius for their Galactic habitable zone, but assert that there must be one based on the general decrease of "metals" as one goes outward from the Galactic center. It is true that there is such a general decrease, but there is also considerable scatter about the general trend. Thus, the outer radius is probably far more "fuzzy" than they describe it.
On the topic of the Galaxy's habitable zone, the authors seem to imply that the Sun has little or no interaction with the Galaxy's spiral arms and that the inter-arm regions of a spiral galaxy have a lower stellar density than inside the spiral arms. Neither is correct. It is true that the Sun is not now located in a spiral arm (or if it is, it is located in a "spur"). However, the Sun orbits the Galactic center, taking about 250 million years to do so. The Galaxy's spiral arms do not rotate with the stars. The Sun therefore probably passes through at least one spiral arm every orbit. Over its lifetime the Sun has made approximately 20 orbits, plenty of time to pass through multiple spiral arms. Indeed the authors seem to be unaware of a proposal that massive extinctions in the Earth's past are caused by passage of the Sun through a spiral arm. (The proposed mechanism is that the gravitational effects of the large dust and gas clouds in a spiral arm could cause an increase in the number of comets falling into the inner solar system and thereby striking the Earth.) Nor are the inter-arm regions are devoid of stars, as the authors seem to imply. Indeed, if one looks at a spiral galaxy in the red light produced by the numerous low-mass stars (like the Sun), the spiral pattern can nearly disappear. Spiral galaxies appear spiral largely because of the enormous quantities of light produced by the hot, relatively rare, and short-lived hot stars.
The authors also do a poor job of arguing that there may not be many Earth-like planets. They note that there have been many planets discovered recently, but all are larger than Jupiter. They note, correctly, that there have been considerable selection effects against the detection of smaller-mass planets (around main-sequence stars). They end up concluding that terrestrial planets may therefore be rare. (Not only that, elsewhere they suggest that Jovian planets might rare!) Indeed, it is the prevailing opinion that the stars currently known to be orbited by a Jovian planet probably do not have terrestrial planets. (The reason is that these Jovian planets are thought not to form in the current location but to have migrated closer to their star from their formation location which was similar to Jupiter's current distance from the Sun. In the migration process any terrestrial planets would have been ejected from the system or scattered into the star. Of course nobody expected to find these "hot Jupiters" in the first place so maybe many of those stars are also orbited by terrestrial planets?)
What about those stars, the majority of those surveyed thus far, that are not orbited by "hot Jupiters"? Are they orbited only by "cold Jupiters" that did not migrate inward? or might they also have terrestrial mass companions? The following line of reasoning is, to this reviewer, equally or more plausible as the one suggested by the authors. The first planets ever detected convincingly are around the pulsar PSR B1257+12. They are terrestrial mass. Their existence suggests that planets can form in a variety of environments. Therefore we should expect planets to be nearly ubiquitous. Moreover, given the range of masses in various systems---PSR B1257+12's planets range in mass from lunar mass to greater than Earth mass, the Sun's planets range in mass from Jupiter to Mercury and Pluto, even the masses of the various "hot Jupiters" range over a factor of ten---we would expect that other planetary systems might have a wide range of masses, too. Thus, terrestrial planets might be plentiful.
(After the publication of _Rare Earth_, the discovery of Saturn-mass planets was announced. From the census of planets around main-sequence stars, there is the hint that lower-mass planets are in fact more numerous than higher mass planets. I do not criticize the authors for not knowing the future, but I do think these results suggest they are wrong.)
These are a few examples of how the authors' points seem muddled or poorly thought out. There are a host of other examples. They present the "snowball Earth" hypothesis as near fact even though it is by no means widely accepted by geologists. Even if we accept that the Earth went through a snowball phase, was it essential to life's later diversification? or did it slow down life's inevitable diversification? and on how many other Earth-like planets would a snowball phase occur? (Of course the answer to all of the above is, we do not know.) They seem to overestimate both the fraction of stars in open star clusters and the likelihood of a close encounter between stars in an open cluster which could disrupt planetary orbits. They seem to spend an inordinate amount of time discussing stars in globular clusters, even though only a miniscule fraction of stars in the Milky Way Galaxy are in globular clusters and there are a number of reasons to suspect that planets might not be frequent in globular clusters. They discuss the possible importance of lunar tides in the ocean without ever noting that the solar tides are not that much weaker (only a factor of three).
In many places the presentation also seems muddled. I am not sure if this is the fault of the editor or the authors. However, if there is an entire chapter on the "snowball Earth" hypothesis, why are we asked to conduct a thought experiment in at least two other sections of the book on the consequences of the Earth's oceans freezing over? Why not just refer to the appropriate chapter? Similarly, if we are told that Jupiter is more than 300 times the mass of the Earth (p. 235), do we really need to be told less than three pages later that Jupiter's mass is 318 Earth masses (p. 238)?
Finally, the tone of the book, in many places, struck me like a book report. Perhaps it is because I manage to keep abreast of developments in fields outside of astronomy by reading the first sections of the journal Science. (Indeed, this is a good strategy for other interested in recent developments in astronomy, planetary science, geology, and biology. Subscribe to a journal like Science or Nature and read the short updates that fill the first few sections of the journal.) Yet I somehow have the sense that having read sections of Science over the past few years, I too could have written this book.
In summary, if you are already of the persuasion that animal life and/or technical civilizations are rare in the Galaxy, reading this book probably will provide you with warm fuzzies. Your opinion will be seconded, but you probably won't gain much. If you are not of this persuasion, you probably have better things to do with your time than read this book. If you really feel you must, wait until your local library obtains a copy, then stroll down there some afternoon when you have nothing better to do.
(After I finished this review, I read the review by C. P. McKay in the 2000 April 28 issue of Science. McKay is one of the premier astrobiologists. His review seems lukewarm. He describes the authors as "[making] the case (if not always convincingly) that the situation on our Earth is optimal for the devlopment of complex life." Later he also writes that "we have only one example of life" and that the "assessment of [the] probability" for the development of life "is uncertain at best." He concludes that "theories of life and evolution" should guide us but not constrain us"---they may be wrong. In this spirit Rare Earth provides a sobering [...] perspective in just how difficult it might be for complex life [...] to arise.")
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