On November 12, 2002, Dr. John Chambers of the NASA Ames Research Center gave a seminar to the Astrobiology Group at the University of Washington. The audience of about 100 listened with rapt attention as Chambers described results from a computer study of how planetary systems form. The goal of his research was to answer a deceptively simple question: How often would newly forming planetary systems produce Earth-like planets, given a star the size of our own sun? By "Earthlike" Chambers meant a rocky planet with water on its surface, orbiting within a star's "habitable zone." This not-too-hot and not-too-cold inner region, relatively close to the star, supports the presence of liquid water on a planet surface for hundreds of million of years—the time-span probably necessary for the evolution of life. To answer the question of just how many Earth-like planets might be spawned in such a planetary system, Chambers had spent thousands of hours running highly sophisticated modeling programs through arrays of powerful computers.
The results presented at the meeting were startling. The simulations showed that rocky planets orbiting at the "right" distances from the central star are easily formed, but they can end up with a wide range of water content. The planet-building materials in a habitable zone include dry materials that form locally, as well as water-bearing materials that originate further from the star and have to be scattered inward, mostly in the form of comets. Without water-bearing comet impacts, Earth-wannabes would just stay wannabes— they would never contain any water.
The model showed that the inbound delivery of water worked best in planetary systems where the intermediate planets, in the position of our giants Jupiter and Saturn, were far smaller. In solar systems such as our own, the efficiency of water being conveyed to the surface of an inner, Earth-like planet is relatively small. Yet in systems where the intermediate planets were much smaller—perhaps Uranus- or Neptune-sized—water delivery was relatively frequent. But then another problem arises: in such a system, the rate of waterbearing comet impacts is great; the rate of asteroid impacts, however, is also so great that any evolving life might soon be obliterated. And oddly, it is not only the asteroid impacts, with their fireballs, dust storms, meteor showers, and "nuclear winters," that cause a problem. An excess of water-bearing impacts can amount, in effect, to too much of a good thing: too much water produces planets entirely covered with water, and such an environment is not conducive to the rich evolution seen on our planet. Earth seems to be quite a gem—a rocky planet where not only can liquid water exist for long periods of time (thanks to Earth's distance from the sun as well as its possession of a tectonic "thermostat" that regulates its temperature), but where water can be found as a heathy oceanful—not too little and not too much. Our planet seems to reside in a benign region of the Galaxy, where comet and asteroid bombardment is tolerable and habitable-zone planets can commonly grow to Earth size. Such real estate in our galaxy—perhaps in any galaxy—is prime for life. And rare as well.
We, the authors of Rare Earth, were in the audience that November day. One of us raised his hand and asked the question: What does this finding mean for the number of Earth-like planets there might be—planets with not only water and bacterial life, but with complex multi-cellular life? Chambers scratched his head. Well, he allowed, it would certainly make them rare.
There was one other aspect of the lecture that struck us. Chambers matter-of-factly spoke of the necessity of planets having plate tectonics to be habitable, and of the effect of mass extinctions. We know that plate tectonics provides a method of maintaining some sort of planetary thermostat that keeps planets at a constant temperature for billions of years. We know, too, that mass extinctions can end life on a planet abruptly, at any time, and that the number of mass extinctions might be linked to astronomical factors, such as the position of a planet in its galaxy. Prior to the publication of the first edition of Rare Earth in January 2000, neither of these concepts had publicly appeared in discussions of planetary habitability. Now they do, as a matter of course, and this has been a great satisfaction to us. Our hypothesis that bacteria-like life might be quite common in the Universe, but complex life quite rare, may or may not be correct. But the fact that we've been able to bring new lines of evidence into the debate, evidence that was once controversial but is now quite mainstream, has been extremely gratifying.
With its initial publication, Rare Earth struck chords among a wide community. Because it took a rather novel position about the frequency of complex life, the discussion spurred by the book often left the realm of scientific discourse, where we'd intended it to take place, and entered the arenas of religion, ethics, and science fiction. Science has progressed since the publication, yet nothing we have read or discovered in the years since has caused us to change our minds. One of the most remarkable developments has been the continual discovery of new planets orbiting other stars (the count is now over 100). While this shows that planets are common, it also shows how complex and varied planetary systems are, and how difficult it is to make a stable Earth-like planet. Most of the extra-solar planets that have been discovered are giant planets in orbits that preclude the possibility of water-covered Earths with long-term stability.
This edition, then, is changed only in the removal of several egregious and sometimes hilarious typos and errors. We stand by our initial assessment and are proud to see that Rare Earth continues to spawn heated debate even as it makes its way into textbooks as accepted dogma.
Peter D. Ward, Donald Brownlee Seattle, February 2003
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