The question as to whether life is prevalent in the universe is often asked, not only by laymen but also by astronomers. One of the best, most closely and logically set out approaches to this question is that by Prof Stuart Clark in his very interesting and well-written book "Life on other Worlds and how to find it", published by ‘Springer-Praxis. He refers to the fact that Fred Hoyle had shown in his book "The Intelligent Universe" that the probability that life appeared on Earth is vanishingly small. But Clark rightly points out that once certain steps in the chain of life had been taken, further steps would follow as a matter of course. If this were not so, life would be a very rare accident and we could consider ourselves tricky to be alive!

The argument runs as follows: If life is prevalent in the universe, why haven’t intelligent beings on nearby planets made contact with us?  Maybe our instruments are too crude to pick up the very faint signals coming from planets tens of light years distant. Maybe we are older and therefore more advanced than our near neighbours so that they are not yet able to send signals or to receive signals from us. Besides, a message from a planet circulating around Alpha Centauri, our nearest stellar neighbour would take 4,3 years by radio. If we replied immediately on reception of the message, another 4,3 years would elapse before "they" at Alpha Centauri received our rely, i.e. 8,6 years - you could hardly speak of a two-way conversation - not to speak of the time lapses in the cases of other stars further away. Would you care to listen? The immensity of space and the fact that nothing, not even radio signals, can travel faster than the speed of light, place severe limitations on our being able to communicate with aliens when our radio industry is still less than one hundred years old. This seems to be an insurmountable problem, for the present. Clark states that he is unhappy in accepting that life is an incredibly low probability event and therefore he cannot accept the religious explanation that an omnipotent "Guiding Hand" made an impossible event come true. Therefore the probability of life coming abouton a planet which has all the necessary conditions, must be higher than it seems.

Clark therefore takes the stance that life is an "emergent phenomenon" which is a phenomenon that cannot be predicted from a knowledge of its components, the cells and DNA molecules, etc, which go to make up life. Life may just happen when a sufficiently complicated and correctly orientated system of components begins to work in unison.

At that stage one must postulate an emergence, for example, the atoms which make up your body are not separately alive, but jointly, and in unison, they constitute cells and organs which together produce the living being, which is you and you are alive. He may add to Clark’s exposition the case of hydrogen and oxygen. There is certainty about the existence of hydrogen, the most prevalent substance in the universe; and oxygen is made in copious amounts in the nuclei of stars. When these two elements come together they combine explosively to form water, the most important constituent of the living cell. Water is therefore very plentiful in the universe and in the Solar System. Clark cites the wetness of liquids, a property which the constituent molecules do not possess. Each ant in a colony is an insignificant insect, but in their thousands they display a remarkable emergent group intelligence. Packs of dogs and nations of humans also display emergent properties. Another way of saying this is that the whole is greater than the sum of the parts. Somewhere between the simple, inanimate laws of physics and chemistry and the complex behaviour of biological systems, life emerges - it is thus emergent, and bridges the gaps between the sciences.

When certain complex states are built up, life emerges as a certainty. Self organisation is another property that stems from the attainment of complex systems. Take the case of water spiraling down the plug hole. If you disturb the flow, it rapidly reinstates the spiral motion. We can sum this up by saying that life possesses the property of decreasing entropy whereas theuniverse at large displays increasing entropy, ie the disorganisation in the universe is constantly increasing. We can say that the universe is like a broad, deep steady-flowing river. All the water is steadily flowing downhill; but here and there, at the edges there are eddy currents where the water tends to circulate in a backward direction as if to decrease entropy. Life is therefore an inescapable property of the Universe. Where the conditions are just right it is a truism to state that if an event can happen, it will happen. What are the right conditions? Firstly there must be a steady supply of energy. The parent star must not be too massive nor too light so that the temperature on the planet will be just right so that, Secondly water can exist in it's liquid phase. The region around a star where this condition prevails is called the ecosphere. If the ecosphere is too narrow, because the star is too light or because it is too cool, there will be a lesser probability of finding a planet in the ecosphere, e.g. Mars is just outside and Venus just inside the Sun’s ecosphere.

Nearer to the Sun than the ecosphere water would have turned to steam which would have been blown away by the solar wind. Outside the ecosphere water freezes to ice - the moons of the outer planets are all mostly covered with ice. In the ecosphere water is found as gas (vapour), liquid and solid (ice). Now, the astonishing thing about water is that when it cools down, it shrinks as other substances do, but not all the way to freezing point. It reaches its densest at 4°C above freezing point. Ice therefore forms on top of a lake, while the water below remains at an equable 4°C. Organisms in the water below the ice can therefore survive. If water got denser all the way to its freezing point, ice would sink to the bottom and lakes and seas and oceans would be solid ice with a thin layer on top melting theday and then freezing over at night - what a skating rink! Jupiter’s moon Europa was found by the Voyager space probes to be covered with a layer of ice 70 km thick floating on a global ocean of water 30 km thick. The space probe Galileo corroborated this finding.

Now to get back to the conditions for life to form: The third condition is that the planet must have an atmosphere so that water on the surface does not evaporate away into space, but remains liquid. The atmosphere of the Earth may have been very different in the past when life was first formed. We shall see about that in the next article. Fourthly the axis of rotation of the planet should be between 20 and 30 degrees from the vertical on the ecliptic so that the variations between the seasons are not too great. Four of Sun’s planets, Earth, Mars, Saturn and Neptune have inclinations of their axes in this range. Fifthly the mass of the planet must fall in the range 0,4 Earth mass and 2,35 Earth mass in order to, develop a "correct" atmosphere. Sixthly, the period of rotation of the planet must be somewhat less than 96 hours, otherwise the day temperature will rise too high and the night temperature will fall too low. Seventhly, the planet must see to it that the mass of its star is not too great, actually, not more than 1,3 solar masses otherwise its period of residence in the main sequence of the Hertzsprung diagram will be too short, bearing in mind that the Sun has been on the main sequence for five thousand million years. Eighthly, the planet should rather not form around one of a binary star system to avoid temperature complications and to avoid instability of its orbit.

HOW THEN HAS THE EARTH FORMED?

(See the article next month or watch this space!)

Jan Eben van Zyl