Carbon is the fourth most abundant atom in the cosmos and is a substance of enormous versatility.   The process of the building up of molecules containing long chains and rings of carbon atoms together with hydrogen, oxygen and nitrogen starts early, even at the freezing temperatures of the interstellar dust clouds.  Some prebiotic molecules such as carbon monoxide, hydrogen cyanide, ethyl alcohol, amines and amino acids have been detected in the Orion Nebula and similar nebulae where planetary systems appear to be forming. 

Ever since the Renaissance in Europe many members of the scientific community have suspected that apart from our Sun, some other stars may be accompanied by planets. As early as 1698 Christiaan Huygens attested his belief in the existence of other worlds inhabited by plants and animals in his book 'The Celestial Worlds'.   In 1600 ' Giordanno Bruno was burnt at the stake as a heretic.  One of his heresies was to postulate that the stars were suns and some of them were accompanied by planets on which there were people living.  

Already there is evidence that nearby stars are accompanied by large planets as massive as or more massive than Jupiter.  These large planets have been detected by the wobble they produce in the star during the course of their orbit.  It requires the use of extraordinarily sensitive measurements by what are called ASTROMETRIC techniques.  They are called Extra-solar Planets and are dealt with in another section of this web site.  It is believed that there are many smaller terrestrial type planets and some of these may be abodes of life.  The Darwin project and other future advances within the next decade are planned and it is possible that spectroscopic evidence of ozone, carbon dioxide, methane and water may be found on some of these other worlds.

In considering the possibility of life on planets of other star systems it is necessary to consider both the planet and its star. Mathematical studies on planetary evolution and the habitable zones around stars suggest that advanced life may be restricted to planets similar to the Earth orbiting stars similar to the Sun.

The Star

It is generally considered that life-sustaining stars will lie in a range between 80% and 120% of a solar mass.   It has been suggested that planets orbiting very close to small red dwarf stars could play host to life (see New Scientist 27 January 2001 pages 28-31).

However there would seem to be a number of limitations.

1)      If the star is variable and subject to violent flares this may make evolution very difficult.  Recently compiled evidence suggests that some stars of a similar size and composition to our Sun are subject to erratic and violent flare activity.  This may cut down considerably the number of suitable planets where life could comfortably evolve.

2)      If the stellar system is very deficient in higher elements large terrestrial planets may be unlikely to form (by higher elements we mean elements above helium in the periodic table such as oxygen, carbon, nitrogen etc which astronomers persist in calling ‘metals’).

 3)     If the stellar system is too near the galactic centre conditions may be too violent and variable for life to develop.

The Planet

1)  any planet suitable for the evolution of advanced life forms would have to have a mass high enough to retain an atmosphere.  This depends upon the gravitational force at the surface.

2) It would need to have an adequate water supply and be within the right temperature range.  It is widely and almost universally believed that biochemical life requires water in the liquid phase.  The only other candidate is liquid ammonia NH_3.   This will not be discussed here and for a number of reasons it is most improbable that worlds exist where there are large amounts of ammonia and very little water.

3) a planet must orbit its Sun in a relatively narrow 'continuously habitable life zone'.  Had the Earth been only a few percent closer to the Sun it may have become a furnace like Venus early in its history.  If it had been a few percent further out like Mars, it may have become frozen in a permanent ice age.

Thus life may be restricted to fairly large sized terrestrial planets orbiting stable stars in ‘Goldilocks Orbits’.    Although this cuts down the number of places where life may evolve there are still a very large number of possible star systems where life as we know it may exist.  Also there may be a number of planets such as Europa where large quantities of liquid water may be present but where the energy to keep the water in the liquid phase is derived from tidal or other forces.

4) There is another problem that may arise in a planet where the angle of inclination of the axis to the ecliptic plane is rapidly undergoing wild variations.  Such a situation would cause devastating changes in the weather conditions and may make it more difficult for life (or at any rate highly evolved life) to develop.   In this connection the possession of a large moon may be of great help in maintaining fairly stable conditions.  If a terrestrial planet in a suitable orbit (‘Goldilocks orbit”) also needs a large moon then this would cut down considerably the number of planets where advanced life would be likely to develop.

It seems likely that simple bacterial-type life may be relatively common in other ‘solar’ systems but highly evolved life such as animals and plants may be less common.  Nevertheless one day perhaps we shall have some sort of contact with other intelligent beings who share with us the stars.  It is interesting to speculate what form the chemistry of life would be likely to take on other planets

Alien Proteins

If a similar situation exists on another planet like our own we may expect a variation on the theme.  For example, there could be a  different repertoire of amino acids making up the proteins.  

Mirror Image Molecules

 One of the mysteries of molecular biology is that there is a bias in the shape of the molecules that make up living matter. In the second half of the nineteenth century Louis Pasteur and Johannes Siliceous, discovered that lactic acid, a simple molecule that is found in living organisms, existed in two optically active forms.   It soon became apparent to them and other workers that most other compounds that were involved in biochemical processes were similar to the lactic acids.  In most respects, the substances were chemically similar.  It was however found that in general only one of them was biologically active. It took the scientific world many years to discover the true reasons for the phenomenon, which is called optical isomerism or chirality.   In 1875, two young chemists, van't Hoff and Le Bel independently offered a satisfactory explanation for the existence of the pairs of optical isomers.

They realised that in order to fully show the way in which carbon atoms bond to other atoms it is necessary to draw a diagram showing the orientation of the atoms in three dimensional space.

They discovered the simple fact that if a carbon atom is connected by four single covalent bonds to four different atoms or groups of atoms A, B, C, and D then two different arrangements in space are possible.  This results in the existence of two forms of the molecule which are called optical isomers, which are mirror images of one another.

In particular van't Hoff postulated that the four valencies of a carbon atom are directed towards the corners of a tetrahedron with the carbon atom at its centre.   Geometrical considerations demand that compounds in which a central carbon atom is bonded to four groups, all of which are different to each other, must exist in two forms.  These are known as the D-and L-forms or right and left handed forms.  Such compounds are called CHIRAL compounds from the Greek word meaning hand.  The molecules of the two chiral forms are mirror images of one another and the existence of chiral molecules is of great importance in biochemistry. 

 With the exception of glycine all the amino acids found in proteins can exist in two optically active isomeric forms.   They are called the D- and the L- forms. 

Many scientists now believe that life on Earth may have evolved from pre-biotic molecules originally formed in the gas clouds and incorporate into comets and meteorites. The presence of amino acids has now been found in the Murchison meteorite. A sample of the amino acid alanine from the meteorite showed a higher ¹³C/¹⁴C ratio than is normally found on Earth thus indicating its extra-terrestrial origin.  This proves that amino acids can originate outside the Earth.  Although both isomers are present, the 'left-handed' variety predominated just as it does in earthly organisms.

Recent work on the Orion Nebula has detected the presence of circularly polarised ultra violet radiation.  In some regions of the gas cloud it is polarised clockwise and in others anti-clockwise.

Chemical experiments have shown that the decomposition of a 50/50 mixture of an optically active substance by circularly polarised ultraviolet light is affected by the direction of the polarisation.   The rate of decomposition of the two forms vary, according to the direction of the polarisation.   This would suggest that in some regions the concentration of the L-form would predominate whilst in other regions the D-form would predominate. Although, only L- amino acids such as L-alanine are used to build up into proteins on our planet, there seems to be no apparent reason why one form should be preferred over the other.   There might well be biochemical systems on other planets where only the D-forms of amino acids are used in the synthesis of proteins.  The fate of a hypothetical Earthman or Earthwoman, after travelling through a wormhole to such a mirror world, would indeed be a sad one.  However good the food seemed to be they would soon die of starvation since the mirror image 'food' would be of no use whatsoever!

Energy Sources

Assuming the theories of planetary formation to be more or less correct it would seem that all terrestrial planets would have volcanic activity.   Planets with seas or oceans would be likely to have underwater volcanoes.  It would seem highly probable that chemotrophic organisms would form colonies around volcanic vents just as they do on Earth.

 The most obvious source of energy is from starlight.  At some stage in the development of life on Earth, organisms similar to blue green algae developed a way of trapping and utilising sunlight.  On Earth, photosynthesis has developed using a complex enzyme system that includes a group of compounds called chlorophylls to trap the solar energy.  It seems reasonable to assume that similar systems will have evolved on other planets.  It could well be that the choice of light-trapping catalysts may be different to chlorophyll.  It could be that such a compound may not be green in colour. There may indeed be planets where the predominant colour of the plants is blue, yellow, red or orange depending on the colour of the pigment that plays the main role in the photosynthetic processes.

Another point with regard to biochemistry on Earth is that most energy transfer changes in biochemical systems rely on the use of a system in involving the donation of energy from a molecule of adenosine triphosphate (ATP) which is converted to adenosine diphosphate (ADP) when it parts with its energy..  It would be interesting to know if other life forms used a different energy transfer system.   In economic analogy would be that different nations use different currencies - the American Dollar, the Euro, The Pound sterling.

Alien Genetic Codes

All organisms found on Earth appear to use the same genetic code..   One of the most intriguing questions is whether or not organisms, which have evolved on other planets, use the same code, a similar code or something completely different.

Genetic processes are likely to be similar in principle to ours.  There must be large and stable molecules capable of holding a huge biochemical library.  DNA on Earth is a remarkably stable macromolecule capable of holding an enormous amount of information.  Because of its stability it is able to hold and pass on its message to the next generation.

Nevertheless, because it can occasionally become slightly changed mutations can occur.   By natural selection many mutations will be less successful than their the parent organism or, in sexual reproduction, the parent organisms but a few will be more successful so life will not only change but become more and more complicated and versatile as time goes by.  It is possible that there are a number of possible macromolecules similar to RNA and DNA.  They may well have different sub-units and different sugars.  It would be interesting to conjecture on the nature of the messages encapsulated in the genetic molecules of an alien life form. By a process of natural-selection, a similar type of coding to our own may well develop elsewhere, although different sub-units may be used in the actual DNA-like molecules. If panspermia turns out to be true after all then the genetic code could even be similar on other inhabited planets.

Panspermia

Following the discovery of organic molecules in space, particularly ones such as formaldehyde and the amino acid glycine, the old theory of Panspermia has been revived.  At first sight, it seems an extremely improbable set of ideas.    It was first put forward in a coherent form by the Swedish chemist Svandte Arrhenius.  He postulated that primitive life forms were present in interstellar space and were carried on comets and meteorites.  When these bodies collided with a planet whose environment was suitable they seeded it with life which then went on to evolve and produce the higher organisms.  If such a theory were correct, he argued organisms would have to be able to survive great cold.  To support his ideas Arrhenius quoted in his book 'Worlds in the Making', published in 1908, the results of experiments carried out with bacteria at the Jenner Institute in London.  Bacterial spores were subjected to a temperature of -252 degrees Celsius using liquid hydrogen for 20 hours.  They still maintained the power to germinate.

Evidence as to the ability of bacteria to survive extremely harsh environments was discovered after the Moon landing by one of the early robot space vehicles.  In April 1967 Surveyor 3 landed on the Moon. Over two years later in November 1969, members of the Apollo 12 crew retrieved it.  On return to Earth the TV camera was examined in quarantine and found to contain living bacteria of the species Streptococcus mitis.  The bacteria seemed to have survived two years of exposure to the harsh lunar conditions at almost zero pressure and at temperatures which varied between plus 100 and minus 150 degrees Celsius. 

In the 1960s by Hoyle and Wickramasinghe again began to support the 'heretical idea' of 'Panspermia'.  They argued that bacteria and viruses could probably survive the cold of interstellar space in the vast molecular gas and dust clouds that gave rise to the birth of stars. Opponents of the theory argued that, even if bacteria survived in interplanetary space near a star, they would burn up on entering the atmosphere of a suitable planet.  However, experiments showed that bacterial spores could survive temperatures of 700 degrees Celsius.  Very small particles the size of bacterial cells would only be heated to temperatures of around 500 degrees Celsius on entry into the Earth's atmosphere. Also this temperature would only be maintained for a very short time.  Hoyle and Wickramasinghe went even further in their book 'Our place in the Cosmos' and suggested that bacteria and viruses are still reaching the Earth from outer space.

            Whether panspermia is true or not, it would seem that organic molecules, such as amino acids, do occur in space and have reached the Earth on comets and meteorite.  They must still be doing so to a much lesser extent.

The Search for Civilisation

            There may be many planets on which primitive life exists.  On the other hand, to provide a stable habitat for highly advanced forms of life more stringent requirements may be necessary.  Planets on which advanced technological civilisations similar to ours have formed might be relatively rare.

Whether it may ever be possible to visit planets orbiting other stars depends upon one important consideration.  That is whether the laws of physics permit us to do so.  If the speed of light is a totally limiting factor and there is no other way of overcoming the gigantic distances between the stars then, however brilliant our technology becomes, we will never be able to visit civilisations existing in other star systems. 

On the other hand if the universe is stranger than we have hitherto imagined, then future generations may indeed discover a means of travelling in some way, as yet unenvisaged at 'warp speed' through wormholes in space-time' to planets orbiting other suns.

Even if interstellar travel proves impossible, we may be able to communicate with relatively nearby civilisations by radio and even television. Judging by the number of sun-like stars there is a strong possibility that there are planets with advanced civilisations near enough for this to be possible.  If this ever comes about it will certainly answer many of our questions about the nature of life.  It will indeed alter our whole philosophy even more than the revolution of Copernicus if we learn for certain that we are not alone in the Universe.

Numerous attempts have been and are still being made to try and detect signals from other civilisations through projects such as SETI (the Search for Extra-terrestrial Intelligence).   This is discussed in the section of this web-site dealing with SETI.

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