A new path has been marked out in the quest for extra-terrestrial intelligence, says S.Ananthanarayanan.
The question of whether there is life in the universe has become more real with the growth in our means of accessing outer space. Spectroscopic analyses of light or other radiation from distant worlds, analyses of meteorite material, and more recently material brought back by space missions, all include the search for traces of biological activity. And then the search for signals of any kind from possible advanced civilizations in the cosmos.
The Search for Extra-Terrestrial Intelligene (SETI) Institute, a California based organisation that has been coordinating these efforts, has just set in motion a two-year programme of a concerted search for radio signals from a set of 20,000 Red Dwarfs, which are the oldest stars in the universe, as a project directed at an area of the greatest promise. Red Dwarfs are considered promising both because they are older and also because the ‘sun-like’ stars that we have been looking at so far are generally further away than many red dwarfs
While evidence of life forms detected in extra-terrestrial samples or in the spectra of light from deep space would be exciting, the only way we could become aware of intelligent life forms seems to be that we detect apparently well coded radio signals. The principles of what kind of signals to look for were first discussed by Giuseppe Cocconi and Philp Morrison in a classic paper in the journal, Nature, in 1959. They reasoned that if there were signals sent out by sentient beings in space, these would need to be low frequency waves which are the least dispersed and could last long distances. Within this range, the suitable frequencies per second would be above 1 megacycle, but below 30 megacycles, which is the frequency at which the gases in the atmosphere are strongly absorbent. Higher frequency radio signals, in any case, are not easy to detect. And within this permitted range, the band from 1 megacycle to 10 megacycles per second was considered the best.
But for all this, scanning for a signal at an unknown frequency within a band and making it out against the background emissions from the planet and its star would be challenging. Cocconi and Morrison hence suggested that an intelligent civilisation may select a specific frequency that pertained to hydrogen, the most prevalent element in the universe – the characteristic radio emission line of hydrogen, at 1,420 megacycles per second, or a wavelength of 21 cm, as a standard. Looking for this radiation, with allowance for spread, some 300 kilocycles per second, due to the motion of the emitter, was hence recommended as the way to go.
But even for this specific signal, there was considerable ‘noise’ or radiation from other hydrogen sources, which would made weak signal difficult to make out. The level of such disturbance was the most, about 40% stronger, along the plane of the Milky Way galaxy, in which the Solar System is located. It was hence recommended that the direction to look was towards the nearest stars which lay off the galactic plane.
The next question was of the kind of signals to look for. Cocconi and Morrison worked it out that the signal would be in pulses, maybe some one second apart, and as it would be decades, at least before a signal was detected, the train of pulses may continue for a period of years before it was repeated. And to be distinguished as being artificial, the pulses may encode marks of identification like “a sequence of small
While different SETI programmes have been in action since then, there has also been the quest, with new technology, for the ‘earth like’ planets of distant solar systems. We have looked for ‘earth-like’ planets because it is accepted that life, at least as we understand it, may not exist outside the narrow temperature belt in which water and a large number of carbon based organic substances are in the liquid state. Silicon based life is conceivable, but the temperature may be too high for many chemical processes or intra and inter-cellular communication to be possible for sustaining life. The quest has hence been of distant planets that are close enough, but not too close, to their parent stars.
There has also been a restriction of the kind of stars which may host earth-like planets. If the stars are too bright, things may be too warm for earth-like conditions, and then, if the star is too cool, the planet would need to be in a very near orbit to receive sufficient heat. This second case of the star being too cool , no doubt, does not rule out fair conditions and there are a great many of these cooler stars, the so called ‘red dwarfs’. But it has been considered, first, that there may not be many planets in the narrow zone very near the parent star and secondly, that at this near orbit, the planet would get ‘tidally locked’ and not rotate on its axis, like planets in general, but only once per revolution around the star. If this is the case, the star-facing side of the planet would get unbearably hot despite the star being cool, and the opposite side of the planet, which would face empty space, would be far below the freezing point of most substances. The quest has therefore been only of planets that orbit ‘sun-like’ stars and a great number have now been found.
Recent research has shown, however, that if near orbit planets of red dwarfs had oceans and an atmosphere, heat would move from the hot side to the cold and there would be areas that could support life. The discovery of exoplanets in last decades has also shown that near-orbit planets of red dwarfs are quite frequent. These discoveries point in the direction of red dwarfs being good prospects as the home of earth-like planets, and hence of intelligent life. Red dwarfs are also there in good numbers. “Significantly three-fourths of all stars are red dwarfs,” says Seth Shostak, of SETI Institute. As there are so many more red dwarfs, it would be possible to find a good number of them much nearer than sun-like stars, Shostak says.
Red dwarfs are smaller, low mass, and hence slower burning stars, which ‘simmer’, so to speak, while larger stars blaze. In smaller stars, the helium that is produced by nuclear fusion of hydrogen atoms at the core spreads to other parts and does not accumulate at the core. These stars hence continue burning their hydrogen content over a long period. Red dwarfs that formed soon after the beginning of the universe, over ten billion years ago, would hence still be there, which makes them the oldest, apart from the most numerous stars there are.
The SETI Institute at Mountain View, California sees these discoveries, both that planets in near orbit around red dwarfs can be earth-like and that such planets are there in good numbers, and also the fact that it is here that life has had the most time to arise, as good reasons to turn telescopes towards red dwarfs in the quest of extra-terrestrial radio signals. A list of 20,000 red dwarfs is being drawn up out of a list of 70,000 that has been compiled and also other data about nearby stars, including red dwarfs.
“We’ll scrutinize targeted systems over several frequency bands between 1 and 10 GHz,” says Institute scientist Gerry Harp. “Roughly half of those bands will be at so-called ‘magic frequencies’ -- places on the radio dial that are directly related to basic mathematical constants. It’s reasonable to speculate that extraterrestrials trying to attract attention might generate signals at such special frequencies.”
"The probability of success is difficult to estimate,
but if we never search the chance of success is zero.” -- Cocconi and Morrison
Dangers of ET encounters
The celebrated Stephen Hawking, while participating in the launch of initiatives to search for intelligent life in the cosmos, has cautioned that being found by such a civilisation may not be the best thing for the earth. The advanced aliens may have the capability to wipe out the human race and there is no reason for us to believe that they would show more compassion that what humans have shown, time and again, to creatures or peoples less capable than themselves, says Hawking.
Other studies have also compared contact with a technologically superior civilisation with similar contact of greatly different human civilisations. The contact of Europe with the Americas, after Christopher Columbus, resulted in the transfer of humans, ideas, animals, plants, microbes, disease, to the great advantage of the Europeans and the effective elimination of the original Americans
ET inititiatives thus limit themselves to detecting alien signals and not actively advertising the earth. But there are also contests, world-wide, seeking ideas for the best messages to include in broadcasts or ‘time capsules’ sent out into space. The only possible contact, as of now, may only be the decoding of a radio message, and actual visits by aliens or alien spacecraft are not seriously considered.