The question of whether life exists elsewhere in this universe is as baffling as it is romantic. Over the years Hollywood has come up with some really entertaining movies portraying aliens from other planets. From hideous three eyes monsters to cute friendly creatures, we have seen them all on the silver screen. Who can forget the cute little extra terrestrial befriended by a young boy in Steven Spielberg’s famous movie “E.T. the Extra-Terrestrial” or for that matter the hideous aliens who want to exterminate mankind and occupy earth in the movies “Battleship” and “Independence day”. When we think of aliens our imagination knows no bounds.
The idea of aliens is not a modern concept. Even some of the oldest religious texts allude to aliens arriving from other planets in fire lorries. And then what about the number of sightings of unidentified flying objects that we hear about everyday? As many as a staggering forty million UFO sightings have been reported. But this does not cut any ice with scientists.
Ninety nine percent of these UFO sightings have been dismissed as atmospheric phenomena, sightings of aircraft under extraordinary circumstances and bright planets. Admittedly a few cases seem authentic but scientists still do not place much credence on them.
The topic of aliens is rather vast to be discussed in a short article of this kind. Hence I am going to discuss only the scientific opinions behind the idea of life in other parts of the universe. First, let us consider the question of what we need for life to originate somewhere else in the universe. When I say life, I am talking of life as we see it in earth. Or in other words if it is possible for life to originate in some other planet, the planet has to be earth-like. There are a vast number of factors that make life possible on earth but chief among these is the presence of the water or H2O in liquid form in abundant quantities.
Our planet earth is at such a distance from our host star the sun, that it makes the earth’s temperature conducive to sustaining water in its liquid form. Other than having a temperature that can sustain water, the atmosphere of the earth is just right in several other ways. It can trap heat and the ozone layer shields us from all harmful radiation from the sun. The atmosphere contains important gases like oxygen, nitrogen and carbon dioxide which help life forms to run the chemical reactions needed for their life processes. The atmosphere can trap light and help in photosynthesis.
So what is the most important factor that makes all this possible on earth? It is the distance of the earth from the host star: the sun. So we can safely conclude that in every planetary system that exists in this universe, life can originate if the distance of any of the planets from the host star is just right, making the planet earth-like. This distance is commonly known as the habitable zone and varies from planetary system to planetary system. Now let us look at the possibility of this actually happening. At first glance it appears like there is every chance that this should happen.
The universe is simply vast. It is vast beyond the realms of our imagination. The diameter of the observable universe is estimated at 92 billion light years. There are billions of stars and several planetary systems have been observed. So the first impression says that there is bound to be life elsewhere on this universe. But then, as the famous Italian physicist Enrico Fermi quipped “But then, where is everybody?”. This famous statement is known as the Fermi paradox and it simply goes to emphasize the fact that if life is as likely elsewhere in this universe as it seems, then why is it that we have not been able to observe it anywhere except in our own planet?
Fermi realized that if life were to be as abundant as the evidence suggests, then surely we would have been in contact with alien life forms by now. This leads directly to another important question. What would it take an alien civilization to make contact with us earthlings? The probability of alien life existing and being capable of making contact with other civilizations is dealt with by the famous Drake equation first proposed by the astronomer Frank Drake in 1961. The Drake equation considers seven factors.
These are
f1 – The rate of formation of sun-like stars in the galaxy. This factor only considers the galaxy and not the entire universe. The reason is that receiving radio signals beyond the galaxy is extremely unlikely because of the greater distances involved. The rate of formation is important because we have to consider the lifetime of a civilization.
f2 – The fraction of formed stars that have planets. Only a fraction of the stars that get formed have planetary systems which are essential as life can exist only in the planets.
f3 – The average number of earth-like planets per star. This relates to the planets being in the habitable zone or the right distance from the host star and are earth-like because only such a planet would provide an environment conducive to formation of life.
f4 – The fraction of those planets in which life actually emerges. This is taken into consideration because the emergence of life may be thwarted by so many factors even in earth-like planets.
f5 – The fraction of these planets with life in which intelligence evolves. Of these planets only some planets would evolve intelligence. A planet full of microbes cannot communicate with other civilizations.
f6 – Of these planets the number of planets in which technology advances enough to make radio communication possible. The intelligence that evolves should be able to make adequate technological advances to make actual communication with other civilizations possible.
f7 – The average lifetime of such a communicating planet. The planet should live long enough for the entire process from origin of life through making technological advances to actually evolving technology to make communication possible. Several planets die a premature death.
The expected number of communicating civilizations is given as the product of the above seven factors. The equation is :
The problem with this equation is that at least for now, astronomers do not have firm figures for any of the seven factors. They only have rough estimates. For example consider f7. In 1961 Drake proposed 10,000 years for this factor. Carl Sagan on the other hand was not so optimistic. He felt that civilizations were inherently war mongers and could destroy themselves with nuclear bombs and the chances of lasting 10,000 years were very little. Michel Shermer on the other hand felt that human civilizations were typically unstable and would collapse within a few hundred years.
But all said and done astronomers to date have not been able to fill in the appropriate figure for each one of these factors and come up with a definite estimate for N. But one thing we know. Except for a few minor incidents, some scientific, like the detection of a single signal and some bizarre cases of unexplained UFO sightings no aliens have ever been recorded as actually making contact with us earthlings. Till such time as someone actually knocks on our doors and announces themselves or our technology makes sufficient advances to explore galaxies we simply will not know if life exists elsewhere in this universe and we will have to bear this eerie and deafening silence.
I would like to conclude this article by pointing out some of the moons and planets in our own solar system in which there may be some chance of detecting life. Among the planets it is believed that water existed in Mars sometime in the past and can even now be occasionally detected. Ceres the dwarf planet in the asteroid belt has a thin water vapor atmosphere. There may be a chance that Jupiter’s moon Europa has a liquid water ocean below its surface. Saturn’s moon Titan and Enceladus have been speculated to have possible habitats supportive of life.