The Drake Equation: The ne Term ne: The number of planets within a planetary system capable of supporting life. When this factor was discussed at the Greenbank conference in 1961 it was concluded that the number lay between 1 and 5, so that every planetary system would have at least 1 suitable planet. Their estimate was based on the premise that other solar systems would be like ours. Sadly however, it is beginning to appear that this estimate was wrong. One interesting aspect of the planetary systems so far discovered is that, in general, they are unlike our own solar system, and this may well have implications on our estimate of the number of planets capable of supporting life. Two major differences are apparent, firstly a high percentage of those discovered have a Jupiter-sized planet very close to the central star. Secondly, those planets found at greater orbital distances appear to have orbits which are much more elliptical than those in our own solar system. Both factors appear to work against the presence of a terrestrial planet in a stable, near circular orbit capable of supporting life. Observational selection effects Nevertheless, it must be stressed that at this moment observational selection effects give us a quite distorted view of other solar systems. Due to the limitations of observational equipment and techniques, we can so far detect only rather large planets (the lightest found until now weighs about 50 Earth masses) and it is easier to detect those that are close to their central star. So it is perhaps not too surprising that it is these which figure highly in those planetary systems so far discovered. In time solar systems nearer to our own may well be found, but it would be wise not to assume that all solar systems would be capable of harbouring life. It could well be that earthlike planets in stable circular orbits, so providing a long term temperate climate, are in fact not very common. Our estimate of this factor will improve with time but it does appear that a value of 1 is too optimistic. Perhaps somewhere between one in five and one in fifty is likely to be nearer to the mark.