சொந்தக் கருத்துக்கள்

ARE WE ALONE? 

(An inquiry into the search for extraterrestrial intelligence)

Are we alone?  Are we the only living being in this vast universe? This question ringing in the minds of many modern astronomers and physicists. But this question is very old. The beliefs that other worlds exist and they may have life like us prevail since ancient times. The Hindu mythology pictures that all heavenly bodies like Moon, Sun, Venus, Mars etc are ruled by immortal Gods. The ancient Babylonians went one step ahead and believed that the heavenly bodies and their rulers control the lives of all men. The Egyptians, Mayans, Phoenicians and every old civilization think that powerful Gods with magical powers rule the Stars, planets and the people of earth.

The Greek peoples are the fist peoples who realized that planets are similar to earth and stars are for away objects, without planet like properties. Many Greek philosophers discard the idea that planets are abodes of Gods. Instead they think peoples like us may live there. Fourth century Greek philosopher Metrodoros of Chios said that   “To consider the earth as the only populated world in infinite space is as absurd as to assert that in a vast plain only one stalk of grain will grow”. As the centuries rolled, philosophers in many parts of the globe consider the possibilities of peoples in the planets of solar system.

After the invention of telescope, great personalities like Galelio Gallei, Tyco Brahe, Newton, Percival Lowell and many others turn their telescopes towards the heavens. They discovered many unknown facts about stars and planets. At this juncture, during the fag end of 19^th century, Giovanni Schiaparlli reported that the planet mars is criss-crossed by canals and predicted that a high-tech society living in Mars builded canals to maintain uniform water supply for irrigation purposes. Every one turns towards Mars. Argument and counter argument about the idea flamed across the observatories around the world. This is the time at which Science fiction writers like H.G. Wells and Jules Verne began to write about civilization in other planets. This makes even ordinary peoples to talk about extra terrestrials.

 From the data collected by the space crafts Mariner (1964), Viking (1976), and pathfinder (1997), we come to know that no-body is out there in Mars. The reported Martian canals are nothing but dark-floored craters or irregular dark patches linked unconsciously or by chance by the earth based observers. Recent studies on rock samples from mars shows that microorganisms might be lived in past in mars. Some believe that few microorganisms may still survive in unseen corners of Mars.

Man after knowing that no one is there in the inner solar system, sent out probes to explore the outer arena. Probes shows that all the Jovian planets are gaseous balloon like and only their satellites have solid crust. Data’s collected by Voyager and Pioneer shows only the largest satellites of Saturn - the Titan, and Jupiter’s satellite Europa could have probability of evolution of life in the millenium to come. The Titan’s surface is thought to be covered with organic molecules and liquid oceans of methane. Europa seems to have a mobile surface consisting of plates of ice that shift around on top of a layer of liquid or slushy water. Cassini Mission to Jupiter and Saturn can throw some light on the Titan and Europa’s evolutionary stage. Further landers and orbiters that are scheduled to be launched in the first decade of the twenty-first century can shed light on micro-organic life in Mars and evolutionary stages of other planets and satellites.

The space exploration since 1957 to till date shows that no other intelligent beings are there in the solar system except man. Now our area widens we begin search for a companion among the stars. How to proceed the search for extraterrestrial intelligence. The preliminary requirements in the search for life in space is to look for extra solar planetary systems associated with the nearby stars and investigate if some of these planets present environment conducive to the origin and evolution of life.

In the search of other planetary systems, the target stars of greatest interest are the nearby isolated main–sequence stars of spectral class F, G, K, and M with masses in the range 0.2 to 1.8 solar mass. Stars of secondary interest are the F, G, K & M stars in wide binary systems where existence of planetary systems appears possible. The main reason for the choice of these stars is their long life span in the main-sequence stage, because it is the long stable phase in the star’s life. For the same reason O, B, and A class stars are assigned a low probability for possessing planetary systems. Also majority of close binaries, multiple stars were rejected as possible candidates for life bearing planetary systems, because orbits of planets around them might not be stable enough to maintain a planet in a thermally habitable zone.

The NASA of USA, launched a massive search program, and asked Solar System Exploration Division established in 1988, to formulate ways and means for the discovery and study of other planetary systems. Based on their recommendations highly sensitive ground based radio interferometers and the two 10 meter optical Keek telescope at the Keek observatory on Mauna Kea, Hawaii (USA) are put into the task of surveying the space for Extra solar systems. The Hubble telescope is also being used to look around the other planetary systems. In the first phase of the search spanning over a period of ten years about 800 to 1000 stars within a distance range of nearly 100 ly from the earth are likely to be chosen as targets. The four general techniques that are employed for this purpose are astrometry, Doppler spectroscopy, Photometry and imaging. Jupiter like planets with a distance of 10 AU from their Sun can be detected by our present technology. Still smaller planets detection needs highly sensitive instruments and technology, which we may obtain in the 21^st century.

If there are about 400 billion stars in our galaxy and 85% are main-sequence stars, then there are about 340 billion main-sequence stars. Astronomers estimate that about 90% of main-sequence stars are of spectral class F, G, K, and M; this amounts to about 300 billion stars. One sixths of this i.e. 50 billion are probably not a member of binary systems. Thus 50 billion probable candidates are there in our galaxy, which may have planetary system.

Even though F, G, K, M main sequence stars may have planetary systems, life form could exist only within a region called Ecoshells. For example ecoshell for solar system lies between Venus and Mars orbit. This habitable zones or shells of life are essentially defined by where water would remain in a liquid state. The inner boundary is where water would boil; the outer boundary is where water would freeze.

 To define a habitable zone for life, why do we seek a temperature range in which water is in liquid form? Why not some other organic solvent, such as alcohol or ammonia? First, water is a simple molecule, consisting of hydrogen and oxygen atoms, of which hydrogen is the most abundant element in the universe, and the oxygen is among the most abundant elements after hydrogen and helium. Second liquid water can store a great deal of thermal energy before it vaporizes. Thus it acts as a buffer to day-night temperature changes that occur when a planet rotates. Water has a high surface tension, which can help concentrate solids at its boundaries. In a similar vein, carbon chemistry is expected to be more widespread basis for life than silicon or germanium chemistry (chemically Si and Ge behave chemically somewhat like carbon) this is so because carbon is for more abundant cosmically than either Si or Ge. Only a fairly massive planet lying in the ecosphere and retaining a thick atmosphere can be receptacle of a fair quantity of liquid water.

Of the 31 stellar systems within the radius of 15 ly from the Sun only 3 seem to meet the criteria needed for an ecoshell. These are the main-sequence stars Epsilon Eridani (K2), Epsilon Indi (K5), Tau Ceti (G8). Another star called Barnard’s star a red dwarf is also may have planetary systems, even though disagreement exists in this regard.

If we take the solar neighborhood as a representative sample, in which 3 stars out of 31 stellar systems within 15 ly of the sun have potentially habitable planets, the average distance between biologically suitable stars is about 17 ly. Within a radius of 1000 ly, then we should expect to find somewhat less than 1 million stars having suitable planets harboring some kind of life. Even if only 1 in 1000 of these planetary systems has an intelligent species, that still leaves 1000 sites of intelligent life within 1000 ly. If we conservatively extend this argument, estimating that only 1 million civilizations with a technology at least equal to ours are distributed throughout the galaxy. The average separation between them would require 600 years either way to send or receive a message.

From our discussion so for, we come to know that there is potentially a larger number of life supporting planetary systems in our galaxy. How to detect this planetary systems? Living beings could not come into existence in a planetary system out of blue. There should be a stage of chemical evolution up to the macromolecule stage followed by biological evolution. After life has evolved in a planet, it can convert the atmosphere to one with free oxygen through photosynthesis and then also can form an ozone layer to protect life on the planet from external hazards. Thus identification of the bands of O₂, O₃ and CH₄ or any of these in the spectra of a planet will be strong indication that the planet harbored carbon-based life. The proposed Lunar-IR-Array, may be built, within a decade on the moon and operated to identify some extra solar planets with an oxygen atmosphere. NASA’s Terrestrial Planet Finder (TPF), and Europa’s Infrared Space Interferometer (IRSI), which are scheduled for launching in the second decade of the twenty-first century can also detect such planetary systems.

If life exists elsewhere, Will the life form in another planet be like humans or what would they look like? These are questions that seem to be uppermost in everyone’s mind. Evolution is non-repeatable and so any extraterrestrial beings need not have humanoid characteristics. The nature of the environment could be crucial in determining the form of extraterrestrial life. It is quite possible that highly evolved species on other planets would differ in many ways from terrestrial life form, even if the key molecule DNA were there, because of the enormous number of combinations of nucleotides possible in the structure of DNA and the differences in environment. These evolved intelligent creatures should have a somewhat symmetrical body due to gravity. In addition they should have some kind of protected internal communication system, such as central nervous system, brain, and sensory organs. Regardless of whether such creatures are bipedal, multipedal, it is difficult to believe that their electromagnetic sensors would be equally distributed over their bodies. Also, their sensors must be depending on radiation from the parent star, it may be UV eyes, visible eyes or IR eyes. There should be a biological upper limit to the size; for too bulky a creature demands a large source of energy for locomotion and a massive skeletal support of some sort to resist gravity.

Will they have communication skills like us? Will they want communication with other in the universe? These are the questions come to our mind now. To have an answer for these questions, astronomers define an extraterrestrial civilization as a group of life forms technologically capable of and inclined by curiosity to communicate with other galactic civilizations. An equation developed by Frank Drake known as Drake equation is a general formula expressing the number of such galactic communities in terms of several functions:

   Number of communicative societies = astronomical factors x biological factors x sociological factors

i.e.,     N = R_sf_pn_p . f_l f_i . f_c L

Where R_s is the number of stars in our galaxy divided by the life span of the galaxy, f_p is the fraction of stars that live long enough to develop planetary systems, n_p is the number of planets in each planetary systems suitable for life, f_l is the fraction of planetary systems in which life actually appears, f_i is the fraction of evolving systems that evolve at least one intelligent species, f_c fraction of galactic societies technologically able and willing to take part in interstellar communications and L is the length of time the civilization continues in its communicative phase.

            An intelligent and reasonable guess yield a value N=L, i.e. the number of communicative civilizations in our galaxy now approximates to the average no of years spent in the communicative phase. When we think about the possibilities of our own destruction by nuclear holocaust, by biological disastrous from new mutants, by change in planet’s ecology and climatology, and by other calamities that could befall a civilized society, it is tempting to predict that the moment of civilized glory may indeed be brief in the span of an intelligent species. For a reasonable communication with an intelligent species else where, a species should survive in the communicative phase for more than 1000 years.

            Now we are in a juncture to say that intelligent beings like us in the galaxy will definitely try to communicate with other galactic societies nearby. Now the question is how they communicate? What medium they use for communication? In what way we can tell our presence to other ET’s?

            Around late 1950’s it become clear that communication over galactic distances were possible using radio waves. American physicists Giuseppi Cocconi and Philip Morrison pointed out in 1959, that any advanced intelligent society like us will definitely sent out radio signals, as it requires lesser energy when compared to other electromagnetic signals, and can travel with velocities of light.  Now it seems practical for us to listen for signals than to transmit them. Astronomers think that the frequency region between 1.4 GHz and 1.7 GHz is the most favorable frequency region for sending and receiving interstellar communications. Perhaps advanced extraterrestrial civilizations may have been transmitting messages for centuries and by now they have reached the solar system. But, the most advanced celestial communities could harness energy sources and communication technique for more sophisticated and powerful than we can realize, today. So, we may not be more aware of such electromagnetic message than New Guinea aborigines, who use drums for communications are aware of the international radio traffic constantly passing overhead.

            The first modern attempt to detect artificial signal from space was conducted by Frank Drake at national Radio astronomy Observatory, Green Bank, USA around the middle of 1960. Later Harvard-based BETA (Billion Channel Extraterrestrial Assay), University of California’s SERENDIP, a long-running program operated by Ohio State University and other such efforts are made to detect communication from ETs. No [positively identifiable signal of alien origin has yet been recorded by any of these searches. One signal detected by the Ohio State university team in 1977 remains a tantalizing and intriguing mystery. The signal, which was about 30 times stronger than the natural background at 1,420 MHz, moved across the sky at the same rate as the stars, was intermittent, then terminated as if it had been switched off. It was never seen again.

 NASA took up the task of searching for messages from galactic communities by inaugurating the program called High Resolution Microwave Survey in 1992 but it suddenly come to end in 1993. SETI institute in California started a similar program named Project Phoenix in 1993. For this purpose radio telescopes at the Arecibo Radio Observatory in Puerto Rico and the Goldstone tracking system in the Mojave Desert in California are used. In this program, signals from 1000 sun-like stars around a radius of 100 ly from sun is going to be survived and analyzed.

            We for our part also send out radio signals to the nearby galactic societies. In a great ambition a message has been sent in 1974 towards the globular cluster M13 in Hercules constellation from Arceibo observatory. The message was sent as a token of our anticipation that there may be an intelligent life in the M13. Even if a response is forth coming, we cannot expect it until approximately 52,000 AD. As an another way of sending message Pioneer-10 which is now sailing away from the solar system is carrying a message in a