Rare Earth hypothesis Guide, Meaning , Facts, Information and Description
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.
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 start 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 start 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, comprising less than a half of a percent of all stars, while 99% of stars are red dwarfs which are completely unsuitable. The other half a percent goes from F6 to O -- the massive and powerful stars that are no good either.
Once a planet forms within the habitable zone, a Mars-sized body must impact it at high speed and an oblique angle (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 must also result in a large moon to stabilise the axis and orbit, and the cores of the original planet and the impacting body must merge to form an over-massive core with a powerful magnetic field.
Successfully hitting the more massive object in a binary system where the two bodies are as equal as the Earth and Moon are is quite difficult. The impactor will either be deflected entirely or will hit the less massive object. A large moon is therefore an asteroid shield. 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.
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. Evolution, though, is a two edged sword. It stagnates if given the chance. Once we have an ecosystem and all habitats are filled, then evolution slows down massively. This is thought to have happened on Earth several times, first just after the Cambrian Explosion. So a small number of mass-extinction events are required to give evolution the chance to use the complexity it has rather than allowing it to be "just good enough." The dinosaurs, for example, had 250 million years to develop large brains and a space program to deflect the Chicxulub impactor. They did not. Evolution seems to have stagnated yet again in the late Triassic and just didn't get anywhere until the K-T extinction event, which allowed mammals to rise into the gaps in habitat left by the dinosaurs.
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.
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 .
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