Space is infinite and yet of all the galaxies in the universe – approximately 100 billion – so far as we understand, life has only been confirmed on one planet, Earth, also referred to as Sol 3 – third planet from the star sol, in the Milky Way galaxy.
The Oxford English dictionary’s definition of ‘life’ is;
‘The condition which distinguishes active animal and plant from inorganic matter including the capability for growth, functional activity and continual change preceding death’
In order for higher forms of life to exist a breathable atmosphere is necessary. Earth’s atmosphere consist of most importantly, 21% Oxygen along with 78% Nitrogen, and a vital (for the growth of plants) 0.2% Carbon Dioxide.
Earth is home to approximately 7 billion people and growing rapidly. Water covers approximately 71% of the earth’s surface area and provides a medium which allows a great number of forms of life to thrive.
Life in the form of bacteria even exists in the hot sulphur-rich waters resulting from volcanic activity.
The higher forms of life which have developed on earth, however can only thrive in a relatively - narrow band of temperature, pressure and humidity etc so we can say that our evolution was dependant on the accidental establishment of the prevailing climate. Even so, there are areas of earth which are difficult or impossible for us to live in, long-term.
For life to have developed on other planets it is necessary for similar conditions to have developed on some other planet or for a very different form of life to have evolved.
There are approximately 100 billion galaxies besides the Milky Way, so statistically it is unlikely that all planets within them are incapable of supporting some form of life.
Our scientific methods for looking for evidence of life on other planets have developed greatly over the last 150 years.
In 1871, Giovanni Schiaparelli using an optical telescope saw what he described as canals on Mars and assumed that this signified some form of civilisation. Later and better telescopes showed that the canals were in fact natural features. Since that time radio telescopes and off planet telescopes such as Hubble have allowed us to pentrate much deeper into space but still without detecting any evidence of life although Earth-like planets have been detected.
The aspect of communication with other intelligent beings brings in a further difficulty of timespan. Humans have only been capable of communicating with space for the last 50 years. Space probes have been launched in the hope that some time in the next few millions of years another intelligent species will intercept and understand them.
They may also have made efforts to contact extra terrestrial intelligence, but bearing in mind the vast distances and times involved the likelihood of communication being established is extremely small.
No species lasts forever! If you take into consideration two intelligent races millions of light years apart there is very little chance of any two way contact being established. So we may not be alone but the chance of finding out whether we are is extreamly slim.
Don’t hold your breath!