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Biology, n. science of physical life, dealing with the morphology, physiology, origin and distribution of animals and plants.

Exo-, prefix, Greek, outside.

Scientists define exobiology as the branch of science dealing with the morphology, physiology, origin and distribution of extra-terrestrial life, although of course this definition assumes as a basic prerequisite that such life exists! Owing to the nature of the study and Mankind's limited knowledge, this science is necessarily highly speculative, especially in the areas of morphology and physiology. This article therefore concentrates on the origin and distribution of extra-terrestrial life, these being the aspects of the subject which are more closed and well-defined.

Formation of the Earth

We owe our existence to a cataclysmic event which occurred in our galaxy before the beginning of formation of the Solar System. Many of the very elements from which we are formed were themselves manufactured deep inside a large star where much of the original hydrogen and helium had been burned and a more complex chain of nuclear reactions had begun than the proton-proton chain. These more complex reactions involved the fusion of elements such as helium and lithium to form heavier elements like carbon; later on, the star used carbon too as a nuclear fuel and the escalating chain of nuclear reactions continued until all 92 naturally-occurring elements which we know today were formed. Eventually, the star suffered a cataclysmic explosion as a supernova, ejecting most of the material of the star, together with matter from other supernovae, to a region of the Orion arm of the galaxy where a small nebula (a cloud of gas and dust) was condensing, eventually to form a star which we now call the Sun.

As the nebula condensed, several smaller condensations, like eddies in a river, started to form at distances from the central condensation. Once the central condensation reached sufficient density, it started to radiate weakly in infra-red wavelengths. The smaller aggregates (in common with the central condensate) inherited the angular momentum of the cloud of gas and dust and began to rotate. The smaller aggregates began attracting and collecting material in the cloud which lay close to their orbits, due to gravitational attraction. After perhaps a few million years, the central condensate contracted and increased in temperature sufficiently to begin thermonuclear fusion in its centre. The smaller aggregates also condensed to form the planets that we know today. The larger bodies of the outer condensates, which formed the outer planets Jupiter, Saturn, Uranus and Neptune retained much of the hydrogen from the original nebula. Conversely, the smaller bodies of the inner condensates, which formed the terrestrial planets Mercury, Venus, Earth and Mars were able to retain only the heavier elements such as carbon and oxygen. This lead to the differentiation in the Solar System between the terrestrial planets and gas giants that we observe today.


Abiogenesis is the hypothesis by which life can arise from inanimate matter. The mechanism is thought to be as follows.

As the infant Earth condensed, it heated up, but later, once it reached a stable density, it cooled again. As it cooled, simple molecules such as CH4 (methane), NH3 (ammonia) and H20 (water) formed; the water first formed as steam, and eventually cooled the molten Earth, probably after some millions of years, and the methane and ammonia formed what is termed a reducing atmosphere, i.e. one containing hydrogen compounds.

In 1953, S L Miller at the University of Chicago conducted an experiment in which he passed an electric current through a mixture of methane, ammonia and hydrogen over water. After about a week, he analysed the mixture and found it to contain several amino acids and fatty acids, thus showing that the production of complex organic compounds (without life) in this way was possible. Such compounds are very important since fatty acids are building blocks of fats, and amino acids can form long chain molecules and eventually proteins. Scientists know that there was much lightning in the atmosphere of Earth billions of years ago, together with plenty of light energy from the Sun and probably some geothermal energy. In fact, Miller's experiment was originally designed to test the theory of Oparin and others that life could have arisen in such a manner. However, even amino acids are a long way from life, so what was the next step on the way to living organisms?

Under the right conditions it is possible for large molecules such as proteins to group themselves into aggregates known as coacervates. A kind of skin formed of some of the molecules holds together the coacervate, and the latter has some properties similar to those of living organisms. For instance, a coacervate has the ability to split into two separate bodies, although the daughter groups are not necessarily very similar to the parent.

Scientists believe that the right conditions for coacervates may have existed some four billion years ago. At this time, the various proteins, carbohydrates and nucleic acids (chemicals responsible for the duplication of characteristics from generation to generation) formed what is often termed an organic soup covering much of the Earth. It is possible that coacervates formed in this early soup, and they may even have formed simple enzymes (biological catalysts which can speed up chemical reactions). These chemicals grew in complexity and eventually synthesised their own nucleic acids.

Thus, by about 3.5 billion years ago, simple life had truly evolved, fulfilling the following criteria:

  1. It had a means of replicating itself exactly.
  2. Enzymes were available with which to obtain energy from chemical reactions. The method by which this can occur is called anaerobic (without oxygen) respiration.

In anaerobic respiration, complex sugars such as glucose are broken down into simpler molecules. The energy obtained from this reaction is stored in the form of chemical bonds in a substance called adenosine tri-phosphate (ATP). A much simplified equation for the overall reaction is:

C6H12O6 =  2CH3.CHOH.COOH (lactic acid) + energy.

The processes of anaerobic respiration do not release as much energy as does the aerobic respiration which occurs in our own bodies. The early atmosphere of the Earth was a reducing one without oxygen, which is a requirement for aerobic respiration. At this time, the life-forms present on Earth were very simple ones known as procaryotes. In these bacteria-like animals the nucleic acids, responsible for genetic coding, were to be found in the cytoplasm of the cell, and not separated from it in the nucleus, as they are in mankind today.

After this situation had persisted for hundreds of millions of years, the primitive animals were beginning to exhaust the organic soup which they used to provide their energy. The original organic soup had been synthesised from ammonia, methane and hydrogen, and these chemicals were gradually being exhausted and replaced by lactic acid formed during anaerobic respiration. The bacteria which existed at the time had no means of producing more food.

It was at this point that the first plants arose. The first plants were simple, unicellular organisms, very similar to the blue-green algae of today, which had evolved from the original proto-bionts (first organisms). The important mechanism which the first plants developed was photosynthesis, which provides the means by which plants use light energy from the Sun to build up complex molecules from simple inorganic chemicals. The following equation summarises photosynthesis:

6CO2 + 6H2O = C6H12O6 + 6O2.

Note that an end product of photosynthesis is oxygen, which later allowed animals and plants to develop the process of aerobic respiration, which is more efficient than anaerobic respiration.

Another development which probably occurred some time later was the evolution of organisms which could utilise the oxygen produced by the first plants. Such organisms drove a reaction which is essentially the opposite of photosynthesis, i.e. it decomposed complex molecules into smaller ones, as in aerobic respiration, except that the end products are carbon dioxide and water rather than lactic acid.

Life In The Universe

A sad spectacle. If they be inhabited, what a scope for misery and folly. If they be not inhabited, what a waste of space.

Thomas Carlyle

More is known about conditions in the Solar System than those outside, and hence by necessity a large part of the discussion of life in the universe is devoted to the Sun's family. This is a pity, since our little corner of the Universe is very diminutive and the possibilities of it harbouring interesting life forms are thus limited!

The following discussion is concerned mainly with the planets Venus, Mars and Jupiter for the following reasons:


Venus has often, inappropriately, been called Earth's twin. However, the only similarities between the two are in mass and volume. Venus is an inferior planet with a diameter of 12,400 km orbiting the Sun at a distance of 0.72 AU every 225 days.

From knowledge of the distance of Venus from the Sun, it is possible to calculate the flux of radiant solar energy incident on its surface. It turns out that although the solar radiation should cause the surface temperature on Venus to be hot, it should be within the range where life can exist. However, Venus has a dense atmosphere (atmospheric pressure at the surface is well over 100 bar) composed mainly of carbon dioxide, acting in a similar way to a greenhouse. The greenhouse effect traps most of the heat and creates a surface temperature up to 500°C. Thus Venus would appear to present a most inhospitable environment, in stark contrast to the pre-space age science fiction stories of idyllic seascapes and abundant plant life gracing the surface of the planet.

On the other hand, conditions in the upper atmosphere of Venus may be less severe. Infra-red observations show that temperatures in this region could be much lower than at the surface. One could imagine microbes or even larger animals swimming in the upper layers of the atmosphere, buoyed by the dense atmosphere below.

A much-observed phenomenon on Venus is the Ashen Light. This is a faint, ghostly luminance that appears on the part of Venus not illuminated by the Sun. It was once thought that the cause could be light from numerous cities inhabited by civilised Venusians. But in terms of our current knowledge of the planet, this seems unlikely! However, is it not possible that a simple form of life existing in Venus' atmosphere could be responsible for the phenomenon? After all, there are bacteria on Earth which emit enough light to enable them to be photographed. Personally, however, I believe that a more mundane explanation probably exists.


Mars orbits the Sun at a distance of 1.53 AU with a period of 687 days. Its period of axial rotation, 24 hours 37 minutes, is close to that of the Earth. Mars is much smaller than the Earth: the equatorial diameters of the bodies are respectively 6779 km and 12,757 km.

Surface conditions on Mars are quite different to those on Venus; Mars has a very thin atmosphere and low surface temperature. On a hot day at the equator, the temperature on the surface may reach 30°C, but this falls rapidly after sunset as the thin atmosphere is inefficient at trapping heat.

The 1976 Viking landers failed to produce definitive evidence of life on Mars and the issue is still open for debate. Most scientists believe that nothing more intelligent than microbes or possibly lichens (symbiotic organisms which are extremely hardy) exist on Mars. However, Carl Sagan has suggested (see Other Worlds by Bantam Books) that large animals (macrobes) would be best suited to the Martian environment because their relatively small ratios of surface area:volume would help them to retain heat. However, he did not explain how they would resist the ultra-violet radiation which is not absorbed by the thin Martian atmosphere.

In 1877, the Italian astronomer Giovanni Schiapperelli observed several faint, straight lines which he called canali on the surface of Mars. Inevitably, the Italian word was mis-translated into English as canals rather than channels, implying artificial origin. More and more observers reported seeing canals, and the belief grew that they represented heroic efforts of a Martian civilisation to irrigate the equatorial areas of the planet with water from the polar caps. The myth was only finally abandoned in the 1960s when the Mariner probes executed fly-bys of the planet and returned detailed photographs which showed that the canals do not exist.

However, the idea of a Martian civilisation which died out because of some catastrophe has stayed with us. It is possible that, less than one million years ago, the atmospheric pressure on Mars was similar to that of the Earth (one bar). A catastrophe involving loss of the atmosphere would also have resulted in loss of all liquid water from the surface as, at the current atmospheric pressure of 6-7 millibars, water exists only is a vapour.

Asaph Hall at the Washington Observatory discovered Mars' two tiny moons, Phobos and Diemos in 1887. Phobos, the inner moon, makes an orbit of the planet every 7.5 hours and hence appears to rise in the west and set in the east. It has been suggested that the moons are artificial bodies put into orbit around Mars by an ancient civilisation. However, detailed photographs of Phobos and Diemos, taken by space probes, show no evidence of artificiality. And what reason would a Martian civilisation have for disguising an artificial satellite?


It has been said that the Solar System consists of the Sun, Jupiter and debris! Jupiter is more massive than all the other planets put together, and if hollow it could accommodate over 700 Earths. It orbits the Sun every 11.9 years at a distance of 5.22 AU. Owing to the planet's high escape velocity (nearly 60 km/s), it has retained much of its original hydrogen atmosphere. There is also much ammonia and methane present, as would be expected from the abundance of hydrogen. Hence it is likely that areas of Jupiter's atmosphere are similar to the primordial atmosphere of Earth. The many and varied delicate colours which are visible in the cloud tops of Jupiter are thought to be due to coloured salts of potassium, iron and other elements dissolved in ammonia or in combination with ammonia ice.

Jupiter emits more energy than it receives from the Sun. It is also the strongest radio source in the Solar System after the Sun. These facts suggest that the planet has a warm interior, and scientists believe that although the temperature of space in the vicinity of Jupiter is only circa 135 K, the temperature deep in the atmosphere may be much higher. It may therefore be possible for abiogenesis to occur in the Jovian atmosphere much as it is thought to have done in the atmosphere of Earth some four billion years ago.

The most prominent feature in the cloud tops of Jupiter is the Great Red Spot (GRS). Various theories have been propounded to explain it. According to one theory the GRS is the top of a stagnant column of gas called a Taylor column. According to another the GRS is a solid hydrogen "iceberg" floating near the top of the atmosphere: this theory has the advantage of explaining why the GRS appears to dim and almost disappear from time to time as temporary episodes when it submerges beneath the surface of the atmosphere. According to yet another theory the GRS is the location of an enormous storm caused by convection currents and its pink hue is caused by organic molecules synthesised in the lower atmosphere rising to the surface.

The belief that organic molecules are to be found in the atmosphere of Jupiter is widely held; less commonly held is the belief that life forms may exist in the atmosphere of the planet. However, it has been suggested that life could evolve to utilise ammonia in place of water: ammonia remains liquid at a much lower temperature than water and hence would be a much better solvent than water at the low temperatures prevailing in the Jovian atmosphere.

Two problems which Jovian life forms would face are the high pressure and high gravity. These would result in any life likely taking the form of small, flat, fish-like animals. Since Jupiter has no definite surface, there could be no land animals as on Earth. Large animals (if they exist) might be a strange mix of sea-creatures and birds with either very strong skeletons or no skeletons, and probably well-developed muscles.

The Galaxy

Any discussion of life in the galaxy must be speculative since we know little about the conditions that prevail other than in our local neighbourhood! This section addresses the possible distribution of life within the galaxy, rather than the form that such life might take.

Not all stars provide a local environment suitable to support life. Some stars are young and have not been in existence long enough to enable the formation of planets which support life. Others, the red giants, are grotesquely enlarged stars which may have swallowed any planets which once orbited around them. White dwarf stars are unsuitable since they represent the next stage in the evolution of a "typical" star following the red giant stage. Very dim or very bright stars might be unsuitable to sustain life. Variable and multiple stars are also unsuitable in general to support life, as a planet orbiting such a star is likely to experience large changes in radiation levels.

Astronomers can use spectrosopic observations to estimate the rate at which a body rotates. They have found that most stars rotate slowly, at speeds similar to that of the Sun, and thus have relatively small angular momenta. The presence of planets orbiting a star and contributing to the angular momentum of the system as a whole could explain the relatively small angular moment of a star itself. The consensus of opinion in astronomy at present is that most stars have associated planetary systems for at least part of their lifetimes. But not every star which has planets will have a planet which is at the right distance for conditions to be conducive to life.

Similarly, the mass of the planet will dictate to a considerable extent the conditions that prevail upon it - for example a massive planet is likely to hold onto a dense atmosphere containing compounds such as methane and ammonia.

Following the American astronomer Frank Drake, it is possible to estimate the proportions associated with various factors necessary for life, multiply them together, and then multiply by the number of stars in the galaxy to estimate the number of life-bearing planets which the latter contains. A guesstimate based on this approach is as follows:

Quantity Estimate
Number of stars in galaxy 100,000,000,000
Proportion of stars suitable in terms of age, brightness, etc. to support life on planetary systems 5%
Proportion of stars with planetary systems 50%
Average number of potential life-bearing planets per planetary system (estimate based on Solar System, where only one out of eight major planets is known to host life) 0.125

Combining the above factors gives the estimated number of life-bearing planets in the galaxy as approximately 300 million. However, there is, of course, great uncertainty about the values of the factors involved.

In any case, even 300 million may not seem large when compared to the size of the galaxy. If we approximate the latter as a cylinder of diameter 100,000 ly (light years) and height 50,000 ly, its volume is 4*1014 ly3. This yields an estimate of one life-bearing planet every 1.3 million ly3. So perhaps the galaxy is not so densely populated after all!

It is possible to enhance the above estimate of the number of life-bearing planets in the galaxy to estimate the number of planets supporting intelligent life. Two additional factors need to be considered:

  1. The proportion of planets supporting life which give rise to intelligent life.
  2. The fraction of a planet's existence for which intelligent life will persist.

One estimate of the first factor is as follows: intelligence is just another facet of animals which should, in time, develop from basic life forms. The proportion of planets supporting life which give rise to intelligent life is therefore circa 100%. It is exceedingly difficult to estimate the second factor. The Earth came into existence around 4.5 billion years ago, and human intelligence has been in existence for approximately one million years. What is unclear is how long such an intelligent phase of terrestrial life will persist. It may be that Mankind will effectively destroy itself in a nuclear war in the near future (on an astronomical timescale) in which case the factor is circa 1/4500. Or, Mankind may be wise and live harmoniously on Earth until the Sun becomes too hot for comfort (or possibly much longer); in this case it would be appropriate to estimate the factor as 0.5. At present, it is simply impossible to estimate how long intelligent life will last. Assuming, as a wild guess, the value 5% for the second factor, produces an estimate of 16 million planets in the galaxy supporting intelligent life, with an average of one such planet every 25 million ly3!

Current Attempts At Contacting Extra-Terrestrial Civilisations

Present attempts to contact extra-terrestrials are severely limited. We cannot yet travel to the stars (even though our current space travellers are erroneously termed astronauts!) Potential communication by optical means is limited, since even our parent star, the Sun, is not bright enough to be visible at the distance of the nearest likely extra-terrestrial civilisation. Mankind has already attempted to make contact by radio communication, but so far without success.

Signals broadcast from a transmitter can be detected at much greater distances if they are beamed at the intended recipient rather than broadcast widely into space. So in order to have the best chance of contacting an extra-terrestrial civilisation, it is necessary to know approximately where in the sky it is located. Above are listed some of the factors which could influence, positively or negatively, the development of life. Applying these criteria to the stars closest to the Earth results in identification of only a few which are considered likely to host planetary systems capable of supporting life; in fact, only the following three out of the 20 stars closest to Earth: Epsilon Eridani, Epsilon Indi and Tau Ceti.

In 1960, Drake started monitoring the radio emission from Tau Ceti and Epsilon Eridani using the 26 m radio telescope at Green Bank, West Virginia. Officially, the work was named Project Ozma, but unofficially it was nicknamed Project Little Green Men! Drake analysed the emissions but found no evidence of regularity to suggest the presence of intelligent life. Since then to the time of writing, no noteworthy attempts have been made to transmit or receive signals over interstellar distances.

It is possible that projects similar to Ozma will be undertaken in the future, and the content of possible transmissions has been a subject of great discussion. A potential radio transmission to a suspected extra-terrestrial civilisation could be similar to the following, suggested by Drake. A series of pulses would be sent to represent 551 squares each of which is either black or white. The only way to arrange 551 squares into a grid is as 19 rows of 29 columns or 29 rows of 19 columns. Assuming that the recipient of the message gets this far, there are still formidable difficulties to be overcome in interpreting the message, as the following illustrates, where the intended meaning of the message is displayed alongside a potential alternative interpretation in square brackets.


A notional television picture consists of 19 lines, each consisting of 29 cells. It evidently shows a being [robot?] who, as the Pythagorean diagram [some form of native robot bird?] in the upper right indicates is intelligent and numerate. He is pointing to the innermost of three planets, presumably his home, orbiting a star [he is pointing to the last drop dripping from a prickly fruit: he is dying of starvation]. The marker to the right probably indicates his height and the symbol 1011 [?] may be a binary number - assuming that this is the case it probably reads to the left [right?] and is equivalent to the decimal representation 13. This indicates that the height of the being is perhaps equivalent to 13 wavelengths of the frequency of transmission [the height of the robot is perhaps equal to 13 diameters of the prickly fruit].

What are the chances that Mankind will find a friendly civilisation which wishes to establish communications? There are many unknowns. For example, a civilisation might become bored with sending signals to other stars if they did so for a long time without receiving a response. Or perhaps there is a rule among advanced civilisations in the galaxy which forbids civilisations from contacting those which are not sufficiently "mature".

We might guess that an intelligent civilisation might attempt for only a hundredth or a thousandth of its lifetime to make contact with other civilisations. Applying this reasoning to the previous estimate of the number of planets in the galaxy supporting intelligent life, we may guess that there are no more than perhaps 50,000 civilisations in the galaxy attempting to contact other civilisations. Expressed another way, the average distance between nearest pairs of broadcasting extra-terrestrials would be approximately 2000 ly. For mankind, this means that the practical issues of contacting an extra-terrestrial by radio communications are formidable!

Interstellar Spaceflight

Our preoccupation with Heaven, somewhere out there in outer space... is a kind of homing instinct. We are drawn to where we come from.

Eric Hoffer

Whether or not Hoffer's statement is true, Man at the moment is a rather homebound animal! He is tied to his local area of the Solar System by the lack of a means to venture across the colossal distances of interstellar space. Although in time the technology of space propulsion systems may improve dramatically over what is available today, nevertheless the speed of light will remain a fundamental barrier to travelling the cosmos.

The nearest star (after the Sun), Alpha Centauri, lies at a distance of 4.3 ly and the nearest star with a planet thought capable of hosting intelligent life lies at a distance of 1250 ly. If a spaceship using current chemical propulsion technology were to set off on a journey to such a planet, it would take approximately 3.5 million years to arrive! Obviously such a journey is completely impossible, and a totally different propulsion technology is required. One possibility is fusion generators, a technology which is being developed today and which is likely to be in use a few years hence. In time, perhaps, mankind may be able to develop propulsion systems utilising matter-antimatter annihilation, to obtain 100% efficient conversion of fuel into energy.

However, even with fusion generators, matter-antimatter annihilation and other possible futuristic propulsion systems, journeys to find intelligent life may still be impossible because of the fundamental barrier of the speed of light (but see below). One approach to overcome this problem would be to populate a huge spaceship with people of all ages and skills and send it off into interstellar space like a great ark. The craft would be like a huge city in space, big enough to be self sufficient, to house large numbers of people, with some form of "farms" to produce food as well as large quantities of fuel to produce energy to sustain life aboard. The engineering difficulties associated with such extremely large structures in space can only be guessed at today. The crew would consist of entire families, and generation after generation would take its turn in the tasks of running the ship and directing its journey through the cosmos. But there might be fundamental problems: for example, though the first generation crew might be willing and enthusiastic for the journey, would the same be true of subsequent generations? Over a period of many generations, the inhabitants (crew) of the spaceship might forget about the original purpose of the mission and become wanderers in space. Many science fiction stories have been woven around such a theme!

The Special Theory of Relativity may provide another mechanism. Under the theory, a process known as time dilation occurs at speeds approaching those of light; time as measured by an observer travelling on board a spacecraft moving at a significant fraction of the speed of light would appear to pass more slowly than that measured by a stationary observer. As the traveller's velocity approaches that of light, the effect becomes more pronounced. If we could make spaceships travel at close to the speed of light, journeys could be accomplished more quickly (in terms of the crew of the spaceship). In this way, if a person made a journey at 98% of light speed, which took 20 years by his clock, he would find that when he returned 100 years had elapsed on Earth.

More esoteric possibilities may exist. Wormholes, exotic objects associated with black holes, may provide immediate linkage between distant parts of the Universe. However, at present such objects are purely speculative, and in any case, how a traveller could enter a wormhole without first being stretched like elastic and then pulverized out of existence has not yet been answered!

Mark Howe