Rare Earth hypothesis

The rare Earth hypothesis states that complex life on any planet other than Earth is highly improbable, as it requires just the right combination of favorable conditions. These conditions include location in the galaxy; type of star and distance from the star; planetary mass, tilt, and orbit; size and orbit of a large moon; amount and kind of atmosphere and oceans; plate tectonics; and magnetic iron core.

Origin of the Rare Earth Hypothesis

The rare Earth hypothesis was proposed by paleontologist Peter Ward and astronomer Donald Brownlee, both of the University of Washington, in their book Rare Earth: Why Complex Life Is Uncommon in the Universe (2000). Contrary to prevailing opinion, the two scientists proposed that complex life is extremely rare in the universe, although they conceded that single-cell life might be quite common. Thus, the rare Earth hypothesis combined two predictions: the commonness of microbial life and the rarity of complex life in the universe, leading to the conclusion that Earth might be the only planet in the Milky Way galaxy with complex life.

The hypothesis challenged the accepted view of most of the scientific community that Earth was not special or unique, but was just one of billions of other similar planets in the galaxy of some 200 billion stars. This view is called the principle of mediocrity, or the Copernican principle, and it suggests that complex life is to be expected throughout the universe. A famous prediction by popular astronomer and astrophysicist Carl Sagan in the 1960s, and based on a proposal by astronomer and astrophysicist Frank Drake, concluded that one million intelligent civilizations exist in the galaxy.

The rare Earth hypothesis recognizes that microbial life exists throughout the earth under nearly all conditions, and may even be common on planets quite different from Earth. This view was reinforced with the discovery in the 1970s of new microbes called extremophiles, which live under extreme conditions of temperature and pressure in deep-sea hydrothermal vents and in volcanic hot springs and geysers. However, complex life appears to require much more limited environments, in which can be found liquid water, minimal radiation, and a favorable atmosphere. These conditions greatly restrict the location, size, and type of planet that can support complex life. Analysis of these requirements leads to a low probability that complex life as known on Earth exists elsewhere in the universe.

The rare Earth hypothesis provides a solution to the Fermi paradox, in which Italian-born American physicist Enrico Fermi's answer to the question of intelligent life on other planets was “Where are they?” He assumed that if intelligent life was common on other planets, some of them would have developed technology over a few million of the last few billion years and would have made contact with Earth by now.

In the following sections, some life-supporting requirements are reviewed in terms of astronomical conditions of location in the galaxy, geological conditions of size and kind of planet, and biological conditions imposed by the nature of evolutionary processes.

Astronomical Requirements for Life

According to the rare Earth hypothesis, the earth is a “Goldilocks planet” with just the right conditions for life: It exists in the right kind of galaxy and in the right location in that galaxy, it has the right kind of sun and is located the right distance from that sun, and it is the right kind of planet with the right kind of moon. Only spiral galaxies contain stars with enough of the heavier elements needed for life. These are second- or third-generation stars that result from supernova explosions, which are the only known processes for producing elements heavier than lithium. Elliptical galaxies, small galaxies, and globular clusters contain mostly first-generation stars with few heavy elements; most of these stars of a suitable mass have evolved into giant stars that are too hot for Earth-like, rocky, inner planets to retain liquid water and sustain complex life.

Even within spiral galaxies containing metal-rich stars (heavy elements), only a narrow galactic habitable zone exists with the right conditions for life. Near the galactic center and inside the spiral arms, energetic processes produce too much radiation for life to develop; in the outer edges of a galaxy most stars are metal-poor. Thus the habitable zone in a spiral galaxy is a narrow ring around the center of the galaxy, which must be nearly circular for a star to maintain the right distance from the center as it orbits around the galaxy. Only about 5 to 10 percent of stars reside in this habitable zone, limiting life-supporting stars in a spiral galaxy to about ten or twenty billion stars.

Most stars are too hot or too cold to allow for liquid water on any planets these stars might accompany. Hot stars have shorter lives and eventually expand to become red giants that would envelop any rocky inner planets. Cooler stars might provide sufficient warmth for nearby planets, but these planets would be subject to dangerous radiation from solar flares and would tend to have one side gravitationally locked toward the star, making that side too hot and the other side too cold. Various estimates suggest that less than 10 percent of stars are of a suitable size and temperature for supporting life. Furthermore, any rocky planets orbiting a suitable sun-like star must be at the right distance from the star in a planetary habitable zone.

Estimates for the habitable zone for Earth indicate that a 5 percent decrease in orbital radius would be too hot, like Venus, and a 15 percent increase would be too cold, like Mars. Stable orbits are also required, which would preclude most multiple-star systems or highly elliptical orbits that allow for too much interaction with neighboring planets. Most of these astronomical requirements for life were not taken into account in the original optimistic estimates by Drake and Sagan of the existence of one million intelligent civilizations in the galaxy.

Geological Requirements for Life

Several characteristics of the earth are unique among the planets and provide its life-supporting capability. These characteristics include being the right size, supporting liquid water, having a favorable atmosphere, having surface chemicals, undergoing plate tectonics, having extended continents, having a strong magnetism, and having a large moon. The size and mass of the earth are critical in retaining its atmosphere and oceans. A smaller mass would not have sufficient gravity to hold water vapor long enough to form the oceans. A larger mass would trap larger amounts of heavier gases such as carbon dioxide and retain too much water to allow for continents to rise above the oceans. A denser atmosphere would have produced a severe greenhouse effect and high temperatures, boiling away the oceans, as seen on Venus, whose atmosphere is one hundred times denser than that of Earth. A thin atmosphere would cause freezing temperatures, as seen on Mars, which has an atmosphere one hundred times less dense than that of Earth.

Many of the more volatile elements needed for life did not readily condense from the solar disk except in the cooler regions farther from the sun, where the giant gas planets formed. Some of this material was returned to Earth's surface by the gravity of the giant gas planets in a heavy bombardment of comets and asteroids, seeding the earth with carbon, water, and other materials needed for life.

There is some evidence from computer simulations to suggest that Jupiter and Saturn may have played a different role (after this initial heavy bombardment decreased) in helping to trap the larger and more energetic comets and asteroids before they could reach the earth, reducing the frequency of giant impacts and the extinctions of life they produce. In either case, it appears that giant gas planets such as Jupiter are needed at the right location relative to an Earth-like planet (but not so close as to destabilize its orbit) to provide chemicals needed for life and to protect from mass extinctions.

The rare Earth hypothesis also recognizes the important role of plate tectonics in supporting life and the unusual conditions on Earth that made it possible. Plate tectonics is the process that forms the continents through the slow motions of large crustal plates. Without the continents, Earth would be covered with water and could support only marine life.

Plate tectonics also helps to regulate long-term climate by acting like a global thermostat. Carbon dioxide in the atmosphere helps to raise temperatures through the greenhouse effect. As temperatures increase, weathering increases and removes carbon dioxide from the atmosphere into the oceans, lowering temperatures. The carbon dioxide becomes trapped in limestone deposits, which are eventually recycled by the subduction of oceanic plates beneath continental plates and released into the atmosphere by volcanic eruptions. This raises the temperature and begins the process again. Earth is the only known planet with just the right combination of crustal thickness, oceans, core heating, and internal heat convection to produce plate tectonics and its resulting continent building and climate regulating results.

Earth is also unique among the known rocky planets with its large moon and large magnetic field, which protects the earth by deflecting high-energy particles in cosmic rays and solar wind. Without this protection, early life forms would have had little chance to survive, and Earth's atmosphere would have been slowly depleted. The dynamo effect that produces Earth's magnetism requires strong heat convection, which is enhanced by plate tectonics. Earth's large moon also appears to have contributed to a strong magnetic field and several other beneficial results. Computer simulations support the giant impactor theory for the formation of the moon. This theory also maintains that there was a substantial increase in Earth's iron core and internal heat, both of which are required for a strong dynamo effect.

The giant impact also increased the earth's size, spin, and tilt, making them more favorable for life. The impact probably thinned the earth's crust as well, making plate tectonics more likely. The moon also stabilizes the tilt of the earth, giving it regular seasons over long periods of time that provide better support for the development of life.

Biological Requirements for Life

Several other unusual biological features of planet Earth made the evolution of life possible, including early conditions that nurtured complex molecules, the right kind of atmosphere for plants and animals, the emergence of photosynthesis, the right amount of oxygen, and certain trigger events that stimulated evolutionary diversity and opened new niches for complex life. The giant impactor theory of the moon implies that the moon was once closer and that it produced huge Earth tides, sweeping far inland and enriching the oceans with minerals needed for life. These tides would also produce tidal pools that could have concentrated nutrients by evaporation for emerging life forms. Tidal cycling of wetting and evaporation in these intertidal pools might have provided the kind of environment in which proto-nucleic acid fragments could begin to associate and assemble molecular strands, leading to the origin of life.

Computer simulations of the giant impact formation of the moon suggest that the impact removed much of the primordial atmosphere of the earth, thus avoiding a greenhouse effect like that on Venus. As the earth cooled after the giant impact, a new atmosphere was produced through outgassing and comet collisions. Eventually, a new crust formed and water vapor condensed to form oceans, which then began to absorb carbon dioxide. The reformulated atmosphere on Earth after the collision, and subsequent water condensation, was thin enough to prevent a runaway greenhouse effect and was sufficiently transparent to eventually allow photosynthesis to occur (with its associated production of oxygen). Plate tectonics increased the rate of oxygen production by enhancing biological activity through the recycling of nutrients such as phosphates and nitrates.

Rare-Earth proponents also argue that evolutionary diversity was greatly enhanced by unusual events on Earth that are highly unpredictable and perhaps unique to Earth in their magnitude and timing. Two snowball Earth periods of nearly global glaciations coincided and probably triggered mass extinctions that were followed by explosions of new species. Single-celled (prokaryotic) life on Earth began about 3.8 billion years ago with little development until approximately 2.4 billion years ago, when falling temperatures led to the first snowball Earth; this was followed by the emergence of the first nuclear-celled (eukaryotic) life. Through the next billion years came many new multicellular species.

The second snowball Earth occurred between 800 and 600 million years ago, followed by the Cambrian explosion of new species and the great diversification of animal phyla, with no known phyla appearing since. Apparently, the great stress of environmental changes produced new niches and stimulated rapid evolutionary changes. Similar mass extinctions followed by new evolutionary developments have been associated with giant impacts, such as the one that appears to have ended the age of dinosaurs 65 million years ago. If these events had involved larger colliding objects, they could have sterilized the earth and ended all complex life.

The Improbability of Complex Life Beyond Earth

The rare Earth hypothesis greatly constrains the probability of life in the universe. The Drake equation that predicted one million intelligent civilizations in the galaxy requires many changes to its original fractional factors and many new factors. These factors include the fraction of stars in the galactic habitable zone and the fraction of those stars with planets; the fraction of rocky planets, of planets in habitable zones that develop microbial life, and of planets where complex life evolves; the fraction of the lifespan of a planet during which complex life is present; the fraction of planets with a large moon; and the fraction of those planets with large gas planets in the right location, and of planets with a critically low number of extinction events. If each of these nine fractions averaged one-tenth, the probability of complex life in the galaxy would be reduced by one billion, even without other rare-Earth factors such as plate tectonics and snowball Earth events. Thus the rare Earth hypothesis implies that there is less than one chance in one thousand that another planet in the galaxy can support complex life, and even less for one with intelligent civilizations.

Although the requirements for simple microbial life are fairly lenient and may be common on many planets or moons, requirements for complex life are much more stringent. During nearly four billion years of life on Earth, complex life has existed for only about one billion of those years. Many biologists agree with the rare Earth hypothesis that the evolutionary path from microbial life to complex life, and especially to human life, was so highly improbable that it is unlikely to occur again, even given a “Goldilocks” environment like that of Earth. The biologists also emphasize the many adaptive disadvantages in the high metabolism required for a large brain and the long gestation period and childhood for humans. Other unusual characteristics such as hand-eye coordination and vocal apparatus allowing for speech and cooperative efforts also seem highly improbable. If the rare Earth hypothesis is correct, then Earth is not just a mediocre planet in a far corner of the galaxy, but a very special place indeed.

Principal Terms

complex life: multicellular organisms with integrated organ systems comprising animals and plants

dynamo theory: an attempt to explain the magnetic field of some celestial objects in terms of rotation, heat convection, and electrical conduction in a fluid, metallic inner core

escape velocity: the minimum speed required for an object to escape from the gravity of the earth or other celestial object

extremophiles: microbial forms of life that live in extreme environments above or below the temperature range for liquid water

giant impactor theory: a lunar origin theory suggesting that a Mars-size planetesimal collided with the earth, blasting enough debris into orbit to form Earth's moon

greenhouse gases: atmospheric gases such as water vapor, carbon dioxide, and methane that trap heat by absorption of solar radiation, causing an increase in temperature

habitable zone: the region around a star where a planet with an atmosphere similar to that of the earth would have a surface temperature that permits liquid water

plate tectonics: the geological processes involving the large-scale motions of the earth's crust that account for continent formation, mountain building, and continental drift

snowball Earth: conditions that prevailed in the earlier periods of Earth's history when the planet was covered with ice, perhaps extending to the equator

solar wind: the stream of high-energy charged particles flowing out from the upper atmosphere of the sun, consisting mostly of protons and electrons

subduction: a plate tectonics process in which an oceanic plate is forced under an adjacent continental plate, often resulting in volcanic eruptions

Bibliography

Dartnell, Lewis. Life in the Universe: A Beginner's Guide. Oxford, England: One World, 2007. This book describes the processes and possibilities of evolution and how it came to occur on Earth, and speculates on various possibilities for the evolution of life elsewhere in the universe.

Davies, Paul. The Eerie Silence: Searching for Ourselves in the Universe. Boston: Houghton Mifflin Harcourt, 2010. The author responds to issues raised by the rare Earth hypothesis, suggesting new approaches in the search for extraterrestrial intelligence and the significance of the possibility that humans are alone in the universe.

Gonzalez, Guillermo, and Jay Richards. The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery. Washington, D.C.: Regnery, 2004. This book presents a comprehensive review of evidence for a rare Earth, with the added insight that the place and time of humans in the universe appear to be uniquely adapted for observing and understanding that universe.

Morris, Simon Conway. Life's Solution. New York: Cambridge University Press, 2003. The author, a leading paleontologist who believes that evolution converges toward intelligent life, is a proponent of the rare Earth hypothesis. He discusses the hypothesis in Chapter 5.

Ward, Peter, and Donald Brownlee. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Copernicus Books, 2000. This groundbreaking book describes many unusual life-sustaining features of the earth in a readable format. The authors were the first scientists to use the term “rare Earth hypothesis.”

Webb, Stephen. If the Universe Is Teeming with Aliens …Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life. New York: Copernicus Books, 2002. Webb considers the possibility that aliens are already among humans, or that they have not yet communicated with humans. In Chapter 5, Webb reviews evidence from the rare Earth hypothesis and concludes that aliens probably do not exist.