Many works of science fiction, for example those of David Brin, Larry Niven, and Poul Anderson, assume that our galaxy is populated by many intelligent alien species that we will eventually contact for cooperation, competition, or perhaps combat. Isaac Asimov, on the other hand, assumed in his Foundation Series that humanity would spread out into the empty galaxy and colonize it without encountering other intelligent species.
Rare Earth, a science book written by two of my University of Washington colleagues, geologist Peter Ward and astronomer Don Brownlee (W&B) depicts our galaxy as a lonely place dotted with hostile lifeless planets orbiting stars that are mostly too hot, too cold, too unstable, or too short-lived to sustain the development of complex life. W&B argue that while bacterial life may be very common in the universe, complex multi-cellular and animal life forms must be rare, and intelligent life very rare indeed. This column is not a book review. Instead, I want to summarize the ideas of Ward and Brownlee and what they call the Rare Earth Hypothesis.
In the 1970s astronomers Frank Drake and Carl Sagan devised a method, now called the Drake Equation, for estimating the number of advanced civilizations that might be present in our galaxy. In 1974, Sagan used the method to estimate that a million advanced civilizations might exist in the Milky Way galaxy alone. However, many discoveries in both astronomy and geology have been made since Sagan did that estimate, and these lead to a more pessimistic conclusion.
As Goldilocks learned in the house of the Three Bears, it's very important that conditions are not too far toward either extreme, but "just right". W&B argue that for the origins of complex life, these just right conditions are rare. They provide a long list of situations peculiar to the Earth that make it suitable for the evolution of higher life forms. Near the end of their book they present a revision of the Drake equation, a chain of probabilities that they call the Rare Earth Equation. It gives N, the number of Earth-like planets having complex life forms, as:
N = N*× fp × fpm × ne × ng × fi × fc × fl × fm × fj × fme
Here N* is the number of stars in the Milky Way galaxy, fp is the fraction of stars with planets, fpm is the fraction of planets that are metal-rich, ne is the average number of planets in the star's habitable zone, ng is the number of stars in the galactic habitable zone, fi is the fraction of habitable planets where life does arise, fc is the fraction of planets where complex metazoans arise, fl is the fraction of the total lifetime of the planet that is marked by the presence of complex metazoans, fm is the fraction of planets with a large moon, fj is the fraction of solar systems with Jupiter-size planets, and fme is the fraction of planets with a critically low number of extinction events. In the rest of this column, I want to discuss the terms in this Rare Earth equation one by one, and then discuss some of the other factors not in the equation that may turn out to be important.
N*(stars) - Estimating the number of stars in the Milky Way galaxy is tricky, because we don't know our galaxy's mass very well, and there is little information about the population of very small stars. N* is roughly 500 billion stars of all classes.
fp(planets) - In 1995 astronomers discovered the first planets orbiting other stars. Since then more and more planets have been discovered. However, it is not clear what fraction of the stars in the galaxy actually have sizable planets.
fpm(metal) - One interesting correlation stands out among the existing observations of extra-solar planets. All of the observed planets orbit metal-rich stars. This suggests that planets, or at least planets large enough to have been observed so far, may not be all that common and may be peculiar to the subset of stars that are rich in metals.
ne(habitable zone) - The orbit of Earth happens to be "just right", falling in a narrow habitable zone of orbit distances in which a planet not only has liquid water now, but also had liquid water several billion years ago when the Sun was cooler and life first formed. W&B mention a 1993 estimate indicating that if Earth's orbit was 5% smaller or 15% larger it would not be in this habitable zone. This zone shrinks for more massive stars because of their more rapid evolution and for less massive stars because the zone of liquid water itself in narrower. Thus the average number of planets in habitable zones, averaged over all stars in the galaxy may be very small indeed.
ng(galactic zone) - The Solar System is about 25,000 light years from the galactic center, roughly a third of the distance from the center to the outside edge. This position is fortunate. Stars too close in have too may close neighbors that disturb the system's orbits, too much fireworks from neighbor supernovas, and too much radiation that comes from the galactic center. Stars too far out are too deficient in the heavy elements cooked up in supernovas near the galactic center.
fi(life) - W&B suggest that the fraction of habitable planets where life does arise, at least in the form of bacteria, may be large. Geological evidence suggests that bacteria were present on the Earth as early as planetary conditions made it possible for them to exist. This view is also supported by observation of living bacteria from rock extracted from very deep wells and mines. The controversial claim that bacteria fossils may have been observed in meteorites of Martian origin found in the Antarctic, if true, also supports this idea.
fc(complex metazoans) - W&B argue that the fraction of planets with bacterial life where complex metazoans arise may be very small. They base this view on the observation that for four fifths of the time since life first appeared on the Earth, some 2.5 billion years ago, there was only bacterial life. They also point out that the Cambrian Explosion, when complex metazoans first appeared, was preceded by some extraordinary climactic and geological events that may have triggered it.
fl(planet lifetime) - Even if complex metazoans arise, their development, as indicated above, may take a long time. Finding complex life on another planet depends on the size of this time window.
fm(large moon) - Except for the Earth's moon, the satellites of the Solar System have only a tiny fraction of the mass of their primary. Mercury and Venus have no satellites at all. The moons of Mars, Phobos and Demios, are small rocks with masses of only 27 and 5 billionths of the mass of Mars. Even the rather large moons of Jupiter and Saturn have masses of only a few parts in 105 of their planet's mass. Our Moon, on the other hand, has 1.2% of Earth's mass. This raises the question of how Earth could have acquired such a large satellite. The prevailing explanation is that a random collision occurred between the just-forming Earth and a Mars-size object, with Earth capturing most of the mass from that collision while about 1% of the debris coalesced into our Moon. Such an event is very unlikely, which suggests that most Earth-size planets will not have moons with anything approaching 1% of the planet's mass.
An important consequence of our giant Moon is that it stabilizes the 23º tilt of the Earth's rotational axis with respect to its orbital plane. Geological evidence indicates that over many millions of years the tilt of the Earth's axis has stayed within a few degrees of its present value. Recent calculations have shown that without the Moon, the gravitational effects of Jupiter and the Sun would have caused the Earth's tilt to wander chaotically over a wide range, producing enormous changes in climate and a hostile environment for the development of complex life.
fj(Jupiter) - W&B argue that if Jupiter (300 times more massive that Earth) were removed from the Solar System, the frequency of comet and asteroid impacts on the Earth would increase by a factor of about 10,000. A major asteroid strike capable of significant extinction of species is estimated to occur in an average time interval of about 100 million years. If Jupiter were not present or was in a significantly different orbit, this interval might increase to one strike every 10,000 years, impeding the development of complex life.
fme(extinctions) - Since the evolution of bacterial life on Earth some 2.5 billion years ago, there have been no extinction events large enough to sterilize the planet. W&B argue that this critically low number of extinction events may be unusual. The fossil record shows that there have been some very severe events, the most recent of which was the asteroid strike 65 million years ago that killed the dinosaurs as well as much of the life in the oceans. W&B argue that a very stable planetary system is required, which nearly circular orbits for all of the outer planets, for this condition to exist. An accident such as the gravitational perturbation of a passing star could easily destroy this delicate stability.
In addition to the factors in their Rare Earth equation, W&B mention other factors peculiar to Earth that may have affected the development of complex life. Here is a partial list.
Mars: W&B argue that the fossil record shows that the cooling Earth developed bacterial life as soon as conditions permitted. They suggest that this may be because the bacteria first developed on Mars, which cooled earlier, and that perhaps Earth was then seeded with these bacteria carried by meteorites reaching Earth after having been ejected from Mars by asteroid impacts. The low gravity of Mars makes this more likely, and it is estimated that 10% of meteors ejected from Mars may impact Earth. A system lacking a Mars-like planetary companion might have been slower to develop bacterial life.
Snowball Earth: Recent evidence from geology indicates that twice in the Earth's history the planet became very cold and ice covered, even near the equator. The first of these Snowball Earth eras occurred 2.5 billion years ago and the second about 550 million years ago. The timing of these eras seems significant. Although bacteria first appeared on Earth about 3.8 billion years ago, there was a dramatic change in the single-cell life in the Earth's oceans about 2.5 billion years ago. Organisms of the domains Archaea and Eucarya first appeared then, and it is the Eucarya from which all animal life is descended. The Cambrian Explosion, when complex multi-cellular creatures appeared on the planet, occurred about 550 million years ago. Thus, two key events in the evolution of animal life on Earth seem to have each followed a Snowball Earth era of planetary deep-freeze.
Plate Tectonics: The Earth is hot inside because of the decay of radioactive isotopes, primarily potassium-40, in its interior. This heating drives plate tectonics, the upwelling of material from ocean rifts, its subduction at the edges of continents, and the drifting of continents. It has recently been realized that this action has a moderating influence on temperature excursions in the Earth's climate. When the temperature rises too high, plate tectonics cause the capture of carbon dioxide into rocks, reducing the greenhouse effect and producing cooling. When the temperature falls, the opposite happens and less carbon dioxide goes into rocks, producing warming. A planet without plate tectonics would lack this temperature regulation mechanism. Without a stable temperature, the evolution of complex life becomes more unlikely.
The conclusion of W&B, which I have only been able to sketch here, is that Earth is a rare place where many random factors have conspired to create a nearly ideal site for the evolution of complex life. Putting plausible numerical estimates into the Rare Earth equation (not done in the book) suggests that there are only a few planets with complex life per galaxy. Thus, it's possible that Earth is the only planet in our galaxy with intelligent life.
Should SF writers therefore stop writing stories with intelligent aliens? Of course not! Ward and Brownlee may have missed some factor that makes complex life more likely. And in any case, the fictional examination of aliens teaches us much about ourselves.
AV Columns On-line: Electronic versions of more than ninety past "The Alternate View" columns by John G. Cramer are available on-line on WorldWideWeb at the URL: http://www.npl.washington.edu/AV.
The Rare Earth Hypothesis:
Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe, (Copernicus, New York, 2000, ISBN: 0-387-98701-0)