reported two planets around the pulsar PSR 1257 + 12, which is about 1,600 light years away (Wolszczan and Frail, 1992). They were estimated to be about three times the size of the Earth. Pulsar PSR 1257 + 12 was observed to speed up and slow down over a period of 25.3 days, and Wolszczan suggested that this was the effect of an orbiting planet. He also claimed that two more distant planets were affecting the pulsar’s rotation every 66 and 98 days. Three years later sceptics put forward an alternative view, that the change in the signal from the pulsar was not due to the effect of a planetary mass, but rather was caused by a coronal hole in the Sun out of which comes a fast solar wind which periodically slows down radio pulses as they travel through space. This view has been contested and the matter has not been finally resolved (Seife, 1997: 12).
In the summer of 1991 scientists at Jodrell Bank, using the same pulsar timing method as the Arecibo group, claimed to have discovered a planet about ten times the size of the Earth orbiting a pulsar known as PSR 1829 – 10. However, in January 1992, Professor Andrew Lynn of the Department of Radio Astronomy, University of Manchester, acknowledged that they had been misled by an error in their calculations. Apparently they forgot to take account of the eccentric character of the Earth’s own orbit (reported in The Guardian, Katz, 1992).
The status of pulsar orbiting masses is unclear, and at the time they were first observed there were doubts among astronomers whether these alleged masses were actually similar to planets in our solar system. Ivan King, Professor of Astronomy at the University of California, Berkeley, argued that planets which orbit pulsars will differ greatly from planets like Earth: ‘They do not have anything to do with ordinary planets going around our Sun … the question of whether planets exist around ordinary stars is a completely unsettled one’ (quoted by Crosswell, 1991b: 11). In any case, there is no possibility of life on the ‘planets’ around PSR 1257 + 12, as it gives off deadly radiation. Moreover, the heat from it is baking the two inner planets to a temperature of many hundreds of degrees Celsius (Crosswell, 1992b). While pulsars are not inimical to life, it was nevertheless argued that if planets can be formed around pulsars they can also form around stars. Thus the detection of planets orbiting pulsars supports theories of accumulative processes in circumstellar disks. The observation of pulsars therefore provides the best confirmation of current theories of planetary formation which in turn favour the more generous predictions regarding ex-solar planets.
There are suggestions that pulsars would be promising targets for intelligent radio signals. According to Jean Heidmann (1997: 168–70), because pulsars are natural interstellar radio beacons, powerful and well distributed throughout interstellar space, and emit pulses of staggering regularity, and have a lifetime of a million years, they would make excellent cosmic lighthouses for possible interstellar civilizations. Heidmann suggests that we listen at the frequency of a selected pulsar multiplied by a universal mathematical constant which will obtain a frequency at the 1 to 10GHz SETI window. Heidmann has actually submitted
this proposal to an observational test and achieved an alert in the form of a strong signal from the region of the star, DM – 2398646. Unfortunately, when monitored three days later no repeat was detected.
Optimistic expectations regarding the numbers of ex-solar planets have encouraged SETI researchers. Steven J. Dick has concluded that the search for planets this century has consolidated a cosmological world-view that ‘assumed that planetary systems were common, that life has developed on many of those planets, and that intelligence may have evolved to the point where we can communicate by radio waves’ (Dick, 1996: 221).
What is life? There are few definitions which are free from both scientific and philosophical contestability. The concept of life is hard to define, and no satisfactory definition has been proposed that has sufficient universality to cover extraterrestrial life. However, a useful pragmatic definition might be ‘any object which feeds and reproduces’. These functions are so widely associated with life that their existence provides a conceptual foothold for considering something that feeds and reproduces as a candidate for life. One might find that this includes self-fuelling and replicating robots, and this might cause philosophical problems, but they could be regarded as examples of life of some sort, no more unusual or marvellous than some of the millions of living terrestrial objects. Of course the origin of robotic life – designed by some form of intelligence – can be apparent, whereas the origin of natural life is still unresolved. One approach to understanding the nature and origin of life is to consider the conditions favourable to its emergence and survival.
If there are many ex-solar planets, then what are the chances of life on them? The minimum condition for life of the terrestrial chemical type is a thermal range bounded by the liquidity of water, on the one hand, and the instability of proteins and other chemicals at high temperatures, on the other. There is, however, an important distinction between conditions which are favourable to the emergence of life and conditions to which life can tolerate or adapt. Many living beings can learn to cope with temperatures slightly below -20ºC and over 100ºC, but conditions suitable for the generation of life would be found somewhere in the middle of this range. Thus Mars, with extremes of low temperature, might not be capable of generating life, but under appropriate protective conditions it could sustain life. Bacteria and plants have been known to survive in simulated Martian conditions. Cacti have been grown in an assimilated Martian environment. Two other plants exposed to a simulated Martian environment for one month at the United Carbide Research Institute, New York, died due to temperature extremes but vigorous young shoots flourished from the dying mature plants (McGowan and Ordway, 1966: 83). It may be objected that these simulations are largely based on speculation regarding the nature of the Martian environment. Nevertheless, they do indicate
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