THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE

Posted by admin_akashic on Jun 27th, 2022 in Space

that life is more adaptable than it was formerly thought. If manned space exploration continues, it may only be one or two generations before (human) life exists on Mars.

The best conditions for the development of life are limited to planets located in a habitable zone around each star. Earth is the prime example. Applying the Goldilocks criterion to our own solar system, Venus is too hot and Mars is too cold. Therefore, life-sustaining planets would have to exist within a narrow thermal range. Life on Earth is precarious. A slight fluctuation in the Sun’s energy would wipe out most forms of life. A little hotter and the oceans would dry up; slightly cooler and the planet would be icebound. Fortunately the Sun has been stable for 5 billion years and it is likely to remain so for another 5 billion years. This is not the case with many other stars, as only the F, K, and G types are stable. Our Sun is a G-type, but the massive O-type stars, which burn fast and only exist for 10 million years as stable power sources, are not likely to be around long enough to support the evolution of life – certainly not in any advanced form. The size of the star is therefore extremely important. Generally the larger the star, the shorter is its life. The evolution of complex life-forms on Earth took 2 billion years. A star 50 per cent more massive than our Sun will only last 2 billion years before swelling up to become a red giant engulfing its planetary system.

Not only does life depend on the stability of the Sun’s energy output, it also depends on the fact that the Earth’s orbit is relatively circular, which guarantees stability of temperature. Thus a minimum condition for sustained life is that a planet remains in a thermally habitable zone. The larger the zone the better.

There is growing evidence regarding the existence of planets which orbit binary star systems. Astronomers have found evidence of a planet orbiting a binary star system in Sagittarius, which is 20,000 light years away. As between one-half and two-thirds of the stars in our neighbourhood are binary, this would suggest that planets are very widespread. However, planets associated with double star systems are generally unsatisfactory candidates for life, as their complex orbits are likely to take them out of a thermally stable zone. Irregular orbits would not have the strict constancy of solar heat, and this would rule out most stellar binary systems, which are the majority of stars. Nevertheless, there are some chances of stability, as some binary stars have close circular orbits. If close together, their planets could have a stable orbit as the stars’ gravitational effect would be approximate to each other. But if apart, then the planetary orbit would be such that they would at times be closer to one star than the other, not only experiencing varying rates of energy from the stars but adding to the risk of planetary collision. Although life could possibly survive in such a situation, extremes of temperature and risk of collision make it less likely than in a stable single star system. However, if we eliminate all double and multiple star systems from our calculations, this could still leave about 20 billion stars in the galaxy which are stable and could support life if they had planetary systems.

The size of the planet is also important. If too small the planet would not have enough mass to retain seas and consequently there would be no atmosphere; as

in the case of our Moon, where there is no liquid, and any gas emanating from the rocks will evaporate. If the planet is too large, like Jupiter, then the sheer quantity of atmosphere, its density, would prevent sunlight reaching the surface which would have an adverse affect on evolution. In a dense atmosphere it would take longer for the greater quantity of methane and ammonia to be replaced by oxygen, which would also inhibit evolution. However, a very large planet like Jupiter might provide its own internal heat in sufficient quantity to sustain the development of life on one of its satellites. It might be speculated that the newly discovered giant planets have satellites which could be almost as large and as rocky as the Earth. Whether or not one of the gas giants can capture an Earth-sized rocky planet is likely to remain a matter of conjecture for some time.

The planet’s rotation may be a crucial factor: if the speed of rotation is too slow it would heat up during the day and freeze at night, like Mercury, which turns on its axis once every two months. Thus in the day its temperature can be as high as 430ºC, and at night it is -170ºC, which is hardly compatible with the emergence of life. If the rotation is too fast it would create a strong atmosphere, even shedding matter into space. Even the inclination of the planet’s axis will determine how much of its surface is habitable.

Other factors may interrupt the development of life or extinguish it altogether. Dramatic solar flares, small variations in solar energy or supernovae occurring close to the planet so as to smother it with high energy radiation would inhibit the development of complex living systems. This has already happened more than forty times in the Earth’s existence and it is likely that the development of life on Earth has been interrupted many more times. Comets or large meteor impacts might wipe out most living beings, but even small impacts may cause dust clouds, poison water, and change the environment in ways that prohibit or interrupt the development of life.

Among the claims for ET life the most pervasive methodological fallacy is mistaking necessary conditions for life for the sufficient conditions. A suitable temperature and air are two necessary conditions but they are only two out of numerous other conditions, all of which are required for an environment to be habitable. Yet all too often, evidence of oxygen in an appropriate thermal range has been proposed as evidence of ET life. But such evidence is only conclusive if combined with other facts that together constitute the sufficient conditions of life. The materials necessary for life exist throughout the galaxy. Given a stable environment, the building blocks of life, such as the amino acids and other precursors of living matter could be formed out of methane, ammonia and cyanogen, which astronomers have detected in large clouds of complex molecules in space. The problem is whether or not all the other conditions which are sufficient for the development of life are present.

It is not known if there are any other planets that could sustain life, but even if they could we could not infer that they actually do sustain life. Percival Lowell’s (1909) belief that Mars was inhabited rested partly on empirical claims regarding a complex system of canals but mainly upon an assumption that life and the