Rare Earth hypothesis
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The rare Earth hypothesis is a response to the Fermi paradox which explains why we might expect a planet such as Earth to be very rare. Combined with the additional assumption that an Earth-like planet is a prerequisite for the development of advanced life, this offers an explanation for the current lack of evidence of extraterrestrial civilisations.
The rare Earth hypothesis is explained in detail in the book Rare Earth: Why Complex Life Is Uncommon in the Universe by palaeontologist Peter Ward and astronomer Donald Brownlee. Ward and Brownlee use an extended Drake equation to argue that the existence of a planet that duplicates certain characteristics of the Earth must be an extremely rare event in the Universe.
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Conditions
The star
Making a planet like Earth and having it turn out "right" after 4.5 billion years is no easy task. First, it must be formed around a metal-enriched star. Those stars which are metal-deficient can never have planets other than gas giants -- there simply is not the material in the star's surrounding nebula to form terrestrial planets. So, this excludes the outer part of the galaxy. On the other hand, if a star is too enriched, any planets become very large, accrete gas envelopes and hold them with their extreme gravity, and run away into gas giants again.
The star must also be in a circular galactic orbit; an orbit that takes it too near an energetic galactic core will expose it to hard radiation. Our star has to be in the suburbs of the galaxy, but it cannot be in the city or the countryside.
Once we have a star with the correct metallicity, we need to make sure it can have a habitable planet. A hot star such as Sirius or Vega has a wide habitable zone, but there are two problems with that: The first problem is that the habitable zone is so far away from the star that rocky planets are likely to form closer in. This does not rule out life on a gas giant's moons, however. Hot stars also emit much more ultraviolet radiation which would significantly ionize any planetary atmosphere. The second problem, related to getting advanced life, is that a hot star doesn't last very long. After a billion years it is ready to become a red giant. This may not leave enough time for advanced life to evolve.
The situation is not much better with a cool star. The habitable zone would be close to the star and narrow, reducing our chances of getting a planet in there. Close to a cool star, solar flares would bathe the planet in radiation and ionize the atmosphere just like a hot star would. Hard X-rays would also be more intense.
It turns out that the "just right" kind of star ranges from F7 to K1 (see stellar classification). These are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 5% of the stars in our galaxy. More than 90% of stars are red dwarfs which are very likely unsuitable. The massive and powerful F6 to O stars are no good either.
Interaction with other bodies
Once a planet forms within the habitable zone, a Mars-sized body might be required to impact it (as postulated by the Giant impact theory). Without this impact plate tectonics can not develop because the continental crust covers the entire planet and there is no room for oceanic crust. The impact may also result in a large moon to stabilize the axis, and the cores of the original planet and the impacting body merging to form an over-massive core could produce a powerful magnetic field to protect against solar radiation. Recent work by Edward Belbruno and J. Richard Gott has suggested that a suitable body could form in a planet's trojan points (L4 or L5) potentially making this a less improbable event.
A relatively massive satellite also increases the chances of survival for complex organisms because it acts 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 (although occasional impacts may be necessary, as evolution theory suggests that mass extinctions can catalyze the development of further complexity). The presence of a large gas giant such as Jupiter is also required to gravitationally eject the remains from planet formation into the Kuiper belt and Oort cloud.
Impact frequency and evolution
Life has to be given a chance to evolve. Frequent large asteroid impacts may prevent the development of advanced life. Life itself is very unlikely to be wiped out but more complex and more evolved organisms are also more delicate and easily rendered extinct. The Evolutionary theory of Punctuated equilibrium argues that:
- Once a planet has an ecosystem with all habitats filled, the rate of evolutionary change drops considerably.
- The period within which evolution fills all niches (reaching equilibrum) is relatively short on Earth, in relation to geological time.
The fossil record is thought to show that a stable ecology has been reached on Earth several times, first just after the Cambrian Explosion. A small number of mass-extinction events may be required to give evolution the chance to explore radical new approaches to the challenges of the environment rather than becoming stuck in a suboptimal local maxima (suboptimal to maximum likelihood of evolving human-like intelligence). The K-T_extinction, for example, removed dinosaurs from the ecology and allowed other types of animals (such as mammals) to fill their niches in new ways.
So apparently just the right values for hundreds of variables are required to be able to support 'advanced' life on an Earth-like planet. The Universe is tremendously large, much larger than a human mind can even begin to comprehend, so the chances are that other Earth-like planets exist somewhere. The chances, however, are remote enough that the other Earths are likely separated by many thousands of light years and unable to communicate with each other due to being separated by large amounts of both time and space. Earth-like planets would need to be quite rare in an entire galaxy to explain the lack of extraterrestrial colonization in this way.
Criticism
The most controversial part of the rare Earth hypothesis is the assumption that an Earth-like planet is a prerequisite for the development of advanced life. Some biologists, such as Jack Cohen, believe that this assumption is too restrictive and unimaginative and is based on a circular argument (see carbon chauvinism). For a detailed critique of the rare Earth hypothesis see Jack Cohen and Ian Stewart's book Evolving the Alien: The Science of Extraterrestrial Life.
Other issues with the Rare Earth theory have also fallen under attack:
- Much of its evidence is contested — for example, the giant impact theory has good support but is far from universally accepted.
- It relies on the improbability of its evidence, when much of it merely seems improbable. Taking into account the size of the universe, the extremely long time spans of astronomical time, and alternate ways for similar circumstances to arise, there may be a much larger number of Earth-like planets than this evidence suggests.
- It ignores the ability of intelligent life to adapt their environment. One intelligent space-faring race might be able to colonize many otherwise uninhabitable planets for very long periods of time (though they would have needed an inhabitable planet from which to arise).
See also
- The mediocrity principle is the antithesis of rare Earth hypothesis
References
- Peter Ward and Donald Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books. January 2000. ISBN 0387987010.
- Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. February 2002. ISBN 0091879272.
External links
- Reviews of Rare Earth:
- Athena Andreadis (http://www.setileague.org/reviews/rarearth.htm), PhD in molecular biology
- Kendrick Frazier (http://www.findarticles.com/p/articles/mi_m2843/is_6_25/ai_79794362), Skeptical Inquirer editor
- Tal Cohen (http://www.forum2.org/tal/books/rare.html), PhD student in computer science