How likely is it that systems similar to the Solar System exist elsewhere in the Galaxy ? This, like many of the questions raised when discussing extraterrestrial life, is a highly controversial issue. It should be remembered that we have no direct evidence for the existence of any planetary system other than the Solar System. The issue thus rests on indirect arguments that are essentially theoretical.

The first point to note is that the Solar System is a dynamically stable system. The orbits of the planets have probably been much as they are now for most of the past life of the Solar System, and it is expected that this situation will persist for as long as the Sun is a stable main-sequence star. Such stability is not a characteristic of planetary systems in binary or multiple star systems in general, and so we should look at single stars in search of planetary systems. A large fraction of solar-type stars appear to be members of multiple stellar systems. However, the number of candidate-stars in our Galaxy for having solar systems probably exceeds 109 and may be as high as 1010.

A second point concerns the rate at which the Sun is spinning about its rotation axis. If a typical interstellar cloud of one or two HO Jar masses were to collapse to form a star, the resulting star would be spinning at least ten times faster than our Sun. It would therefore seem that the Sun’s rotation has been braked by some means during the formation process. One theory for the braking of the Sun’s rotation is that the rapidly spinning protoSun interacted with its surroundings in such a way as to cause the surrounding regions to rotate at the expense of the Sun’s own rotational energy. The interaction could be via magnetic fields such as are observed to pervade the Galaxy, or by generation of turbulence in the surroundings. Thus ‘slow’ rotation of a star might be construed as evidence in favour of a preplanetary system that absorbed the star’s rotational impetus. The observations on the rotation velocities of various kinds of stars indicate that G, K, and M stars have much lower rotational velocities than O, B, A, or F stars. The main-sequence G, K and M stars have rotation velocities around ten kilometres per second, while the main-sequence 0, B, A and F stars have rotation velocities of the order 100 kilometres per second or more. Thus, not only are the G, K, M stars intrinsically slow rotators they are slow compared with other stars. This could be sympto¬matic of the existence of planetary systems about these stars, though it has been suggested as an alternative view that the G, K, M stars are slow rotators because they are old and their stellar winds have had a long time to carry away a large fraction of the star’s rotational energy.

There are almost a hundred stars within twenty light years of the’ Sun. Most of these have masses in the range of 0.1 to 1 solar mass, but. only a few have luminosities similar to the Sun’s. Stars of spectral type GO to G5 have temperatures in the range 4000 K-V 6000 K, and luminosities in the range 0.05 to 1.5 solar luminosities they therefore make obvious candidates for a first search for evidence of planetary systems. There are 17 such stars within 20 l.y . of the Sun. all but five of which are components of multiple star systems. The stars which belong to multiple systems can be excluded on the grounds that they are less likely to possess stable planetary systems. This leaves ? Eri, ? Ind, ? Cet, ? Dra, ? Pav as the candidate stars for a first investigation. As remarked earlier, this list is constructed on the prejudice that solar-type stars are the ones most likely to possess planets with suitable environments. If we go to smaller and fainter stars, the size of the list increases significantly.

It is known that a number of stars have very small companions whose existence can only be inferred by observing the motion of the stars relative to more distant stars. The small companion exerts a force on the main star which causes the star to describe a small orbit about a point called the MASSCENTRE of the system. Thus the Sun describes a complicated orbit about the masscentre of the Solar System. This masscentre lies close to the surface of the Sun, and, as seen from afar, the Sun would appear to wobble about its path through space through a distance of about a solar radius. The planets Jupiter and Saturn are mainly responsible for this effect. Distant astronomers might detect Jupiter and Saturn by noticing that the Sun’s wobble has components with periods of about twelve and thirty years, though it should be remarked that the wobble is so slight that it would be almost undetectable by present terrestrial techniques from the nearest stars. There are seven stars for which there is evidence of a wobble in the motion across the sky. It is clear from the preceding comments that the wobble is caused by bodies considerably larger than Jupiter and Saturn combined, otherwise no peculiarities in the motion could have been detected.

The case where there is least doubt is that of Barnard’s star. BARNARD’S STAR, a 9.5 mag M5V star, is the nearest star in the northern equatorial hemisphere. The star has a large proper motion and photographic plates exist showing the star’s track across the sky during the past 60 years. The deviations from a smooth track amount to only a few hundredths of an arc second and show a roughly periodic structure. Analysis of these deviations show that the motion of Barnard’s star can be explained in terms of two planets orbiting the star in circular orbits with periods of 11.5and 20-25 years. The innermost of the planets has a mass very close to that of Jupiter ,while the outermost has a mass very close to that of Jupiter These are indeed planets! There is considerable confidence in the orbital characteristics of the inner planet, and the somewhat uncertain state of affairs regarding the outer planet should improve with continuing observations. One of the exciting aspects of this discovery is that evidence of a planetary system has been found in a very small sample of nearby stars where an effect could be detected. The planetary system of Barnard’s star might have been detected had Barnard’s star been three times further away, but of course there would be less certainty in the result. There are fewer than fifty stars in such volume.

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