common, and second, that our solar system is typical. According to Bruce Jakosky (1998: 238), computer simulation of planetary formation indicates that ‘Terrestrial-like planets appear to be a natural consequence or by-product of the formation of stars, and they should be widespread throughout the Galaxy.’
To date, about one quarter of the young stars that can be observed have protoplanetary disks around them. On these terms we can expect that ex-solar planets will be widespread. If stars and planets are thus formed together, at the rate of 10 planets for each star, there would be a minimum of 400 billion planets in the galaxy. Nevertheless, f p is still very much in the realm of theory; there is no compelling proof that many of the rotating clouds have evolved into planets. Confirmatory empirical evidence would appear to be moving in support of the belief that planets are the rule, rather than the exception (see Chapter 3). Studies of young stars (about 1 million years old) in the Orion Nebula have been undertaken by means of the Hubble telescope. Photographs have revealed a number of dusty disks which, according to the prevailing theories of planetary formation, could become planetary systems in the Orion Nebula, so it would appear protoplanetary disks are the rule. Photographs from the Hubble telescope also support theories regarding the stability of protoplanetary disks, and hence, theories which suggest that planetary systems are formed by the slow accretion of dust particles which stick to each other for over millions of years until they become planets (Reichhardt, 1996).
Although searches for ex-solar planets are usually conducted in the vicinity of stars, which conforms with the standard model of planetary formation, there have been several claims regarding the possibility that there are millions of nomadic planets in the Milky Way Galaxy alone (Muir, 2000: 14). A British team of astronomers, using the infrared telescope on Mount Kea, Hawaii, discovered quite a number of objects about five to thirteen times as heavy as Jupiter which do not orbit a star. This survey confirmed similar results by a Japanese team of astronomers in 1998. Although these claims might push up the number of planets in the universe, current theory suggests that these nomadic gas giants, with low levels of carbon and oxygen, are unlikely to be inhabited.
ne: the number of planets that could support life
This, again, is an appeal to theoretical possibility. Chemical surveys suggest that there is nothing unique about Earth material and that all the matter in the solar system and beyond has a common origin. By extension of the theory the same building blocks which are necessary for life will be found throughout the universe. This, however, only specifies the necessary, not the sufficient, conditions for life, as other intervening factors may prevent the actual emergence of life. The essential chemicals for living structures may be in abundance but the environment might not be favourable. It would require a planet with liquid water; the presence of elements necessary for life to use either in metabolism or reproduction; a source of energy that is available to biota, and an environment
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that is sufficiently stable for biota to survive and evolve. Extreme temperatures would inhibit many forms of life, although in the past few years several micro-organisms have been discovered on Earth which can exist in extremely inhospitable conditions. Nevertheless, most forms of life will require a stable temperature: a planet too far away from the Sun, like Mars, will freeze in a stillborn death. If it is too close, like Venus, it will suffer from a runaway greenhouse effect. Strictly speaking, ne is limited to the number of planets which are found in a habitable zone. Even on the most optimistic estimates the number assigned to ne would be restricted to planets with an orbit similar to that of the Earth around the Sun, with a similar ecosphere capable of sustaining life, and even then the estimate would remain a theoretical possibility. Appeals are sometimes made to a ‘habitable zone’ within the Milky Way Galaxy, which is some 1,500 light years wide and 30,000 light years from the centre, which contains over a billion stars, many of which could provide habitable stellar ecospheres. According to Oliver and Billingham (1973), the authors of Project Cyclops, there are probably in the order of 1010 potentially habitable planets in the galaxy.
In any discussion regarding potential sites where life could develop, it is important to resist extreme forms of biological determinism, which entails the view that given the right conditions life will inevitably emerge. This belief is not grounded in any known scientific laws. The point about searching for planets where life could develop, and the attendant analogies with life on Earth, is that such sites are more plausible candidates for the search. In this respect discoveries of potentially habitable sites are supportive of a campaign for exploration, not as evidence of possible life.
Nevertheless, a significant increase in the number of potential sites might be derived from a consideration of satellites of large gas giants like Jupiter, which generates a considerable source of heat and could be regarded as a mini solar system. There may also be numerous nomadic dark planets derived from matter which has been expelled from a stellar system in its early stages of development. For example, when a new planetary system forms, the gravitational effects of the large planets could catapult Earth-sized planets out of the system. If they have internal sources of heat and hydrogen-rich atmospheres, they could produce life or sustain life that has developed elsewhere. Late-twentieth-century discoveries of terrestrial forms of life deep underground or beneath frozen lakes revealed that life can exist without sunlight. Of course the detection of dark planets would be difficult, and the only conceivable means would be to observe any dimming of light should they pass in front of a star.
f l: the number of planets that have actually developed life
So far the factors in the Drake equation call for guesses, but appeals to fairly well-established theories provide them with a degree of plausibility. More work may be undertaken to strengthen the theory, more reliable data may contribute
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