The rare earth hypothesis is the contrary of the principle of mediocrity, advocated by Carl Sagan and Frank Drake, among others. It argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances. The term “Rare Earth” comes from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both from the University of Washington.
According to the Rare Earth hypothesis, the emergence of complex life requires a number of fortuitous circumstances such as a galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, the size of the planet, the advantage of a large satellite, conditions needed to assure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of “evolutionary pumps” such as massive glaciation and rare bolide impacts, and whatever led to the still mysterious Cambrian explosion of animal phyla. The emergence of intelligent life may have required yet other rare events that are still unknown.
In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of light years. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox: “If extraterrestrial aliens are common, why aren’t they obvious”.
- Proper distance from the star. If a planet orbits its sun too closely or too far away, liquid water would not exist. There isn’t much margin for error here: a change of 5 to 15 percent in Earth’s distance from the Sun would lead to the freezing, or boiling, of all water on Earth.
- Proper distance from the center of the galaxy. The density of stars near the center of the galaxy is so high, that the amount of cosmic radiation in that area would prevent the development of life.
- A star of a proper mass. A too-massive star would emit too much ultra-violet energy, preventing the development of life. A star that is too small would require the planet to be closer to it (in order to maintain liquid water). But such a close distance would result in tidal locking (where one face of the planet constantly faces the star, and the other always remains dark — as with the moon in its orbit around Earth). In this case one side becomes too hot, the other too cold, and the planet’s atmosphere escapes.
- A proper mass. A planet that is too small will not be able to maintain any atmosphere. A planet that is too massive would attract a larger number of asteroids, increasing the chances of life-destroying cataclysms.
- Oceans. The ability to maintain liquid water does not automatically imply that there will be any on the planet’s surface. It looks like Earth acquired its own water from asteroids made of ice that crashed here billions of years ago. On the other hand, too much water (i.e., a planet with little or no land) will lead to an unstable atmosphere, unfit for maintaining life.
- A constant energy output from the star. If the star’s energy output suddenly decreases, even for a relatively short while, all the water on the planet would freeze. This situation is irreversible, since when the star resumes its normal energy output, the planet’s now-white surface will reflect most of this energy, and the ice will never melt. Conversely, if the stars energy output increases for a short while, all the oceans will evaporate and the result would be an irreversible greenhouse-effect, preventing the oceans from reforming.
- Successful evolution. Even if all of these conditions hold, and simple life evolves (which probably happens even if some of these conditions aren’t met), this still does not imply that the result is animal (multi-cellular) life. The evolution of life on Earth included some surprising leaps; two worth mentioning are the move from simple, single-cellular life to cells which contain internal organs, and the appearance of calcium-based skeletons. It appears like the first of these leaps took more time than the evolution from complex single-celled life to full-blown humans.
- Avoiding disasters. Any number of disasters can lead to the complete extinction of all life on a planet. This include the supernova of a nearby star; a massive asteroid impact (like the one that probably caused the extinction of dinosaurs, and 70% of all other life-forms at the time); drastic changes of climate; and so on.
There are also a few attributes that seem, at first, to be completely unrelated to life and not required for its development. Ward and Brownlee argue strongly for the importance of the following attributes:
- 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.
- Bolid impacts. The impact of a sufficiently massive asteroid or comet can act as an evolutionary pump. The evolution of complex life requires long periods of tranquility. Frequent impacts from large bolides, while not incompatible with the emergence and survival of microbes, make it unlikely that complex life will emerge and survive. Rare bolide impacts, however, while making many forms of complex life extinct, on balance appear to act as evolutionary pumps. A small number of mass-extinction events may be required to give evolution the chance to develop radical new responses to the challenges of the environment, rather than remain trapped in a suboptimal local maximum. By “suboptimal” is meant “the likelihood that human-like intelligence will eventually emerge is not at a maximum.” A case in point is the asteroid impact that created the Chicxulub Crater, believed to have triggered the Cretaceous-Tertiary extinction event, when an estimated 70% of extant metazoans species, including all dinosaurs, became extinct. By removing dinosaurs from all niches they happened to fill, the K-T extinction opened the way for mammals to become large and take their place.
- The existence of a Jupiter-like planet in the system. Apparently, Jupiter’s large mass attracted many of the asteroids that would have otherwise hit Earth. Could life evolve in a system with no Jovian planet? On the other hand, too many Jovian planets, or one that is too large, could lead to a non-stable solar system, sending the smaller planets into the central sun or ejecting them into the cold of space.
- The existence of a large, nearby moon. Apparently Luna, Earth’s moon, is atypically large and close. Both of Mars’s moons, for example, are minor rocks by comparison. What does this have to do with life? Well, it turns out that Luna kept (and still keeps) Earth’s tilt stable. Without Luna, the tilt would have changed drastically over time, and no stable climate could exist. If the tilt would have stabilized on a too-large or too-small value, the results could also be disastrous; Earth’s tilt is “just right”.
- Plate tectonics. Surprisingly enough, it seems like plate tectonics are required for maintaining a stable atmosphere. When the temperature of Earth interior 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.