What is the probability of extra-terrestrial life in the universe ?

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One of the most fundamental questions in science: Does life exist on other worlds?


The 'Wow!' Signal

What is the probability of extra-terrestrial life in the universe?

Where and How should we Look?

How would we communicate with them?

What would be the significance of discovering them?

What is the probability of extra-terrestrial life in the universe ?

Is intelligent life common or unique? We have to ask (and answer) what environmental conditions are necessary for the formation and long term evolution of intelligent life. For our purposes here we use planetary scientist Christopher McKay's definition of intelligence as 'the ability to build a radio telescope'.

Why should life be unique to our solar system? Our solar system appears to have formed through natural physical processes. At its center is a star that is typical of many other stars in our galaxy. And our milky way galaxy appears to be just an ordinary spiral galaxy amongst numerous others. So if life forms and evolves in such an ordinary arrangement as this, then surely life itself must be nothing out of the ordinary. Surely life is not as rare or as difficult to kick-start as some scientists suggest.

Microbial life appeared fairly quickly on earth - 3.8 billion years ago (700 million years after the planet formed). Of course is may have originated elsewhere and hitched a ride on the back of a comet. But it took almost another 3 billion years for multi-cellular (organisms consisting of more than one cell but with differentiated cells that can perform specialised functions, or function-specific cells) life to appear - about 1 billion years ago. Animal life did not appear until about 550 million years ago during the Cambrian explosion.

Is the universe fundamentally hostile or favourable to intelligent life? In 1961 Frank Drake announced that extraterrestrial civilizations must be numerous and the famous 'Drake equation' came into existence. This equation was later fine-tuned by Drake and Carl Sagan. They estimated that there could be up to a million intelligent civilizations in our own Galaxy alone. These estimates were based on such things as the number of stars with planetary systems, the number of planets per star that might develop life, the fraction of these planets where intelligent life evolves. But are these estimates in any way accurate?

If the authors of "Rare Earth: Why Complex Life is Uncommon in the Universe" are correct, then intelligent life is unique to earth, a sort of cosmic fluke. Paleontologist Peter Ward and Astronomer Donald Brownlee suggest we might well be alone in this universe. While they suggest that single-celled microbial life such as bacteria might be prevalent throughout the universe, the conditions necessary for the evolution of higher life forms (like ourselves) are so complex and rely on such a long chain of events many of which happened by chance, that they are unlikely to arise elsewhere.

But not all Scientists agree. Evolving from microbes to intelligent life need not be such a long drawn out process everywhere, as it was on earth. While the tendency of evolution is to push organisms towards greater complexity, Earth's history shows periods of rapid evolutionary changes followed by periods where very little changed. But by removing these long periods of inactivity the process of evolution to intelligent life could happen more rapidly and there is no evidence to suggest that these periods of inactivity are necessary for the process - instead, they just happened by chance.

Factors limiting evolution on earth were;

But other planets may have had more free oxygen in their early life and have been more fortunate or equally as fortunate with cataclysmic events.

We must also note that the sun is only 5 billion years old whereas our galaxy is about 13 billion years old. So earlier star generations with their planetary systems and eventually intelligent life must surely have evolved billion of years ago within our own galaxy. With a few billion years head start on us surely their technology would be advanced enough to have explored and colonised the galaxy and be aware of our presence. Maybe they are aware! Maybe they are amongst us! Maybe they did visit us in the past (remember Von Daniken)! Or maybe life is after all unique to earth!

It's probably reasonable to assume that microbial life is common since the 'ingredients' for life familiar to us, namely, organic material, water, energy, rocky planetary bases, lots of time, are widespread. A 'channel' or 'method' does exist to go from microbial life to intelligent life as humans have proved, so surely this can happen elsewhere! Then again why were the dinosaurs not the first 'radio-telescope builders' if all the necessary ingredients were available to them along with 150 million years as the 'numero uno' animal!

We also have to ask could something we would recognise or define as intelligent life develop using 'ingredients' different to what created us, and in environments we would consider hostile. We do not fully understand the process which kick-started life on earth so we cannot rule out other types of processes elsewhere! Let's not miss opportunities by focusing on 'life as we know it'!