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 civilizations.

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.

Contents

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Conditions

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Position in the galaxy

The star must be in a galactic orbit with a period that generally keeps it out of the galaxy's spiral arms, which have more frequent supernovae, which create radiation hazards. If the orbit is eccentric (egg-shaped), it will pass through the arms.

The star must also orbit well away from the core. An orbit that takes it too near an energetic galactic core will expose it to hard radiation.

The Sun is in a nearly perfect circular orbit with a period of 226 million years, in a narrow ring of orbits whose periods nearly match the rotational period of the galaxy.

The Sun's galactic orbit is so perfect that it has remained outside the galactic arms for more than 18 orbits. Our star has to be in the suburbs of the galaxy; it cannot be in the city or the countryside.

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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-rich 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, perhaps any planets become very large, accrete gas envelopes and hold them with their extreme gravity, and run away into gas giants again.

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 (see Aurelia).

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.

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Interaction with other bodies

Without plate tectonics, carbon and water are not recycled from rock, which limits the biosphere's lifetime to a few million years. One cause of plate tectonics is a large moon. So, once a planet forms within the habitable zone, it may still need to form as a double planet. For example, in the Earth's case, a Mars-sized body might be required to impact it (as postulated by the Giant impact theory). Without this impact, plate tectonics might not be able to develop because the continental crust would cover the entire planet, leaving no room for oceanic crust; it is currently not known whether the organization of the large scale mantle convection needed to drive plate tectonics could develop even in the absence of crustal inhomogeneity.

A large moon might also be needed to stabilize the axis, which is otherwise chaotic, resulting in extreme weather.

A magnetosphere also may be required to protect the biosphere from radiation. To get a magnetic field, a massive conductive core is needed to form the dynamo. In the Earth, the cores of the original planet and an impacting body are thought to have merged to form an over-massive core that produces a powerful magnetic field to protect against solar radiation.

Recent work by Edward Belbruno and J. Richard Gott has suggested that a suitable impact body could form in a planet's trojan points (L4 or L5) potentially making this a less improbable event.

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 and thenceforth act as a partial asteroid shield; however, the gas giant must not be too close to the planet upon which life is developing, unless it has that planet as one of its moons, and the gas giant must not be too close to another gas giant. Either misplacement of the gas giant(s) could disrupt the orbit of a potential life-bearing planet -- in the first case directly, and in the second case by mutual disruption of the orbits of two gas giants resulting in one of them crossing the orbit of the potential life-bearing planet.

A relatively massive satellite also acts as a minor asteroid shield. Nevertheless, occasional asteroid impacts may be necessary, as evolution theory suggests that mass extinctions can catalyze the development of further complexity.

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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:

  1. Once a planet has an ecosystem with all habitats filled, the rate of evolutionary change drops considerably.
  2. The period within which evolution fills all niches (reaching equilibrium) 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 maximum (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.

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, and it is possible that other Earth-like planets exist somewhere. However, if they exist, Earth-like planets are likely separated by many thousands of light years and unable to communicate with each other due to distance. Earth-like planets would need to be quite rare in an entire galaxy to explain the lack of extraterrestrial colonization in this way.

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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 Alternative biochemistry). 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 habitable planet from which to arise).
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See also

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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.
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External links

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