Let us now imagine that at least the potential for life is more or less universal. In doing so we shall conveniently neglect, but not entirely forget, the difficulties in understanding how it might have been assembled. I shall assume, perhaps incorrectly, that life is confined to planets with liquid water. So far as the search for planets beyond our Solar System is concerned, until a few years ago, matters were not very encouraging. Now, with the discovery of tens of what are referred to as extra-solar planets, everything is changing.33
Although remote planetary systems had long been postulated, the first clear hint of what would be found came with the discovery of discs of gas and dust surrounding stars. It is widely believed that planets nucleate from such discs. Of these structures perhaps the best known is the one associated with the Southern Hemisphere star 3 Pictoris, where an immense disc extends into space for a distance many times greater than the diameter of our Solar System. It is conjectural whether planets are forming in this particular disc,34 but in any event with the existing telescopes no direct image35 of any extra-solar planet is obtainable. How then is it possible to detect their presence? Nearly all the recent series of extraordinary discoveries depends on a method known as high resolution stellar spectroscopy. The principle of the method is based on the fact that although the star is much more massive than the planet, the latter also has a mass and so exerts a gravitational pull that slightly affects the star. Not surprisingly, given the enormously disparate masses of star and planet, the effect on the star is very small. Nevertheless, as the planet moves round in its orbit so the star is pulled in different directions. As a result, the wavelengths of the light from the star as seen by observers on Earth are very slightly shifted because of the relative motion between the source and the observer (the Doppler effect). The shift can be measured by interfer-ometry, and periodic shifts in the spectral lines indicate that a distant planet has been detected.
The measurements are very sensitive, and only massive planets capable of imposing measurable perturbations on their stars can be detected. In addition, the measurements need to be made over a protracted period, sometimes several years, before the evidence is sufficiently compelling. Even so, the sensitivity of the technique and the precision of the fits are both quite remarkable (Fig. 5.2). About 90 extra-solar planetary systems have so far been detected (Fig. 5.3). In most instances the available data indicate that there is one planet orbiting each star. This is not to say there are no other planets in each system. Quite possibly there are, but they are too small to exert a measurable influence on the star. There are, however, a number of observations where the data indicate a two-planet system (e.g. stars HD 12661, HD 37124, HD 38529), and in a few cases even a three-planet system.36 In Upsilon Andromedae, for example, the spec-troscopy of the star indicated a planet orbiting the star every 4.6 days, but there was a residuum of data; when analysed further this revealed the presence of two more planets (Fig. 5.4), respectively taking about 240 days and 3.5 years to orbit. So, too, 55 Cancri is now known to possess three planets: two close to their sun, and the third, substantially larger, in an orbit somewhat beyond the equivalent position of Jupiter.
The results to date are spectacular, but they are also deeply sobering.These remote planetary systems are patently very different from our Solar System. So far as the search for extraterrestrial life is concerned, this seems discouraging, but some qualification is needed. First, the limitations of the detection method mean that only very large planets can be discerned. These are typically somewhat larger than Jupiter, which itself is about 300 times more massive than the Earth. A number of the extra-solar planets are truly gigantic (and it is important to remember that the estimate is a minimum because it is the mass times the sine of the inclination of the orbital plane relative to the observer). The planets circling HD 162020 and HD 202206, for example, are each estimated to have a mass equivalent to about
figure 5.2 Detection of an extra-solar system, in this case a planet orbiting the 55 Cancri. The dots represent measurements of the Doppler velocities that arise as a result of minute shifts in the spectral absorption of the starlight as the gravitational tug of the orbiting planet distorts very slightly the shape of its star. The sinusoidal line is the predicted pattern; notice the goodness of fit. These data suggest the planet has a mass slightly less than Jupiter (0.84), orbits the star in about a fortnight (14.64 days), and is located ten times closer to the star than is the Earth (at 0.11 Astronomical Units). Note that subsequently evidence has emerged of 55 Cancri possessing two more planets; see Fig. 5.3. (Redrawn with permission from fig. 1 of R. P. Butler et al. (1997) Three new '51 Pegasi-type' planets. Astrophysical Journal, vol. 475, pp. L115-L118, copyright of the authors and the American Astronomical Society.)
14 Jupiters, while one of the planets in HD 168443 approaches 17 Jupiter masses. On such planets gravity would, by terrestrial standards, be immensely powerful,37 and outside any oceans (whose existence, as we shall see, may in any event be problematic) life would probably be only microscopic and present as thin films: a difficult place for a human to visit. A shower of rain would be like standing under flying gravel, and walking down steps would probably result in multiple fractures. In reality many of these enormous planets may be more similar to gas giants like Jupiter,38 effectively without a solid surface and where any sort of terrestrial ecology is probably
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