A hypothesis by paleontologist Peter Ward, and astronomer Donald Brownlee in response to the Fermi Paradox, and in attempt to explain why although microbial life could be abundant, complex tellurian life forms (multiecellular fungi, plants, and animals) might be rare in the universe. Combined with the assumption that an earth-like planet is a prerequisite for the development of advanced life, the theory offers an explanation for the current lack of evidence of extraterrestrial civilizations.
In their book Rare Earth: Why Complex Life Is Uncommon in the Universe, Ward and Brownlee explain the Rare Earth Hypothesis in detail, and use a modified Drake Equation to argue that the existence of a planet that closely exhibits certain earth-like conditions must be an extremely rare event in the universe.
Parent Star - Terrestrial planets don't just grow on trees, they take time to form. And a 4.5 billion year old planet, like our earth, is definitely not an exception. First, the planet must be formed around the right kind of star. That parent star should be a main sequence star, rich enough in metals to allow it to form terrestrial planets, within the spectral type F7 to K1 (our sun is a G2 star), with a circular orbit, old enough to allow complex life to develop, and located in a safe place in the galaxy away from the path of dangerous stellar debris.
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 complex life. At the edge of the galaxy though, stars tend to be metal defecient.
Stellar mass. A star too-massive 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 to achieve more habitable conditions. But such a close distance would result in tidal locking where one face of the planet constantly faces the star, and one side always remains remains lighted while the other is in the shadow. In this case one side scorches, one side freezes, and the planet's atmosphere escapes.
Stellar energy. The star's energy should be enough to be of use to developing life but not too much as to eradicate life altogether. Also important is a constant energy output from the star as sudden burst of energy and sudden energy fluctuation can be deadly to complex life.
- Home Planet - Obviously, the planet should be habitable, that is, it has to at least possess properties that complex life could tolerate. According to Ward and Brownlee, a habitable planet should be terrestrial, at the right distance from the sun (to allow liquid water to exist) at a circular orbit (to avoid planetary instability), and large enough to possess a protective atmosphere.
Distance from the star. If a planet orbits its sun too closely or too far away, liquid water would not exist. Otherwise, the planet must have enough liquid water substitutes that would prove effective (such as liquid ammonia or perfluorocarbon).
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 with its high gravity, increasing the chances of life-destroying cataclysms.
Presence of an ocean. An ocean, be it an ocean of liquid methane or water, will directly affect the climates and weather conditions in the planet, as well as the life forms developing in it. Apprently, the ability to maintain an ocean does not automatically imply that there will be any on the planet's surface: from gathered evidence, earth acquired its own water from asteroids made of ice that crashed into it billions of years ago (very large chunks of ice hit Venus and Mars too although the circumstances in those planets prevented oceans from forming or remaining). On the other hand, too much water will lead to an unstable atmosphere, unfit for maintaining life.
Plate tectonics. plate tectonics are known to cause earthquakes, tidal waves, volcanic eruptions, and continental shifts. Quite surprisingly, plate tectonics are required for maintaining a stable atmosphere. Plate tectonics play an important role in a complex feedback system that prevents too many greenhouse gases from existing in the atmosphere. No other planet in earth's solar system (except maybe for Jupiter's moon Europa) is known to have plate tectonics.
Presence of large moon. Earth's moon, is atypically large, and close. Both of Mars's moons, for example, are minor rocks by comparison. A moon this big (relative to its planet) can keep the planet's axis stable. In the absence of such a moon unstable climatic patterns could prevent the formation of complex life.
A relatively massive satellite also increases the chances of survival for complex organisms because it act as an asteroid shield. Successfully hitting the more massive object in a binary system where the two bodies are as closely matched in size as the Earth and Moon is quite difficult. Most incoming impactors will either be deflected entirely or will hit the less massive object; hitting the more massive one requires just the right incoming velocity and angle. A planet with a large moon will therefore be somewhat protected from impacts.
Note: According to The Giant Impact Theory, The Earth's moon is created when a Mars-sized body hit Earth early in its formation. The impact also caused plate tectonics to develop, and when the cores the original planet Earth and the impacting body merged, a super-massive core formed with a powerful magnetic field.
Local Star System - The planet's star system should be in a peaceful stellar neighborhood, away from mature red giants (which could go supernova and destroy all complex life) and dense interstellar clouds. Gas giants should form in the outer regions of the star system or they would destroy any terrestrial planets trying to form closer around the parent star.
Impact Frequency. Life has to be given a chance to evolve. Frequent large asteroid impacts could arrest the development of advanced life. Although life itself is very unlikely to be wiped out, more complex and more evolved organisms are more delicate and easily rendered extinct. However, evolution is a two edged sword: it stagnates when given the chance. Once we have an ecosystem is present and all habitats are filled, evolution slows down considerably. Thus a small number of mass-extinction events are required to give evolution the chance to use the complexity it has. Therefore, while large asteroid impacts are necessary, they can also be dangerous when they occur too frequently.
Presence of Jupiter-like planet. Apparently, Jupiter's large mass attracted many of the asteroids that would have otherwise hit Earth. 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. Too close and a gas giant could also crush a terrestrial planet: an earth-sized world once revolved in an orbit between Mars and Jupiter where the Asteroid Belt now exists. So the Jupiter-like planet should be far enough to allow the terrestrial planet to form, but near enough to capture large asteroids and comets which could collide to the home planet and threaten advanced life forms.
Even if all of these conditions hold, and simple life evolves (which probably happens even if some of these conditions are not met), this still does not imply that the result is multi-cellular life. The evolution of life on Earth included some surprising leaps. Two worth mentioning are the transformation from simple, single-cellular life to organisms which contain internal organs (as in complex animals), and the appearance of calcium-based skeletons (in vertebrates and mollusks) and protective layers composed of complex carbohydrates (in higher plants, advanced fungi, and all insects). Once the gap from simple life to complex life is bridged, the trend for animals is to develop more complex plants and animals following each massive-extinction.
The most controversial part of the hypothesis is the assumption that an Earth-like planet is a prerequisite for the development of advanced life, and that life everywhere in the universe is similar in many ways to life on Earth. Even Jack Cohen, biologist and science fiction author, believes that this assumption is too restrictive and unimaginative and is based on a circular argument. With Ian Stewart, a mathematician and also a science fiction author, Cohen argues that it is possible to make a scientific and rational study of the possibility of life forms that are so different from life on Earth so as to render them unrecognizable in the first instance.
- Peter Ward and Donald Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books. January 2000.
- Jack Cohen and Ian Stewart. Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. 2002.
- Jack Cohen and Ian Stewart. What Does a Martian Look Like: The Science of Extraterrestrial Life. Ebury Press. 2004.