fragments. Determinant factors will be the rate of spin and the angle of spin. If it is spinning slowly, it may end up as a single star surrounded by low density gas and dust which will eventually disperse. But if it is spinning quickly, when it breaks up it will form a double or multiple star system. About half the stars have formed such systems. Somewhere between these extremes the condensing fragment becomes a star surrounded by a nebula which may become a planetary system. It is therefore important that stars, like our Sun, have peculiarly small angular momentum. Usually, massive stars have a high angular momentum while lower mass stars have less angular momentum and the formation of a planetary system may well be the means by which the loss of angular momen-tum occurs. According to one estimate about ‘one in four and one in seven of all stars begin their lives with a nebula that has the potential to form planets’ (ibid.: 30). There are clear limits on the formation of stars with planets and it has been estimated that planets form about 0.1 per cent of all stars of solar mass in the galaxy (Ruzmaikina, 1988: 41).
The actual mechanism by which planets evolve can be described as follows: as the dust cloud contracts, the heat generated by it is trapped and huge amounts of energy are released so it becomes a sun. Around this sun orbits a disk of dust and ice particles which collide with each other and eventually form together into small bodies called planetesimals. After several collisions they grow into planets in a rather wasteful hit-and-miss process. But this process accelerates nearer the star where mobility is faster. Thus in the solar system the Earth was formed in one million years compared to the about 300 million years which it took for the outer planets, Uranus and Neptune, to be formed. Data from the Hubble telescope lends confirmation to the hypothesis that the solar system is surrounded by a disk of icy comets, called the Kuiper belt (after the planetary astronomer, Gerard P. Kuiper, who proposed the existence of such a belt in 1951) of which Pluto and its moon, Charon, may be part. This disk may be the debris which never developed into planets. These observations suggest that tens of thousands of planetesimals may be orbiting the Sun (Horgan, 1994: 3–14).
Opinions vary concerning the nature of the clouds of dust which have been observed around certain stars. Are they the building blocks of planets or are they, as some astronomers suggest, what remains after the formation of planets is completed? Disks have been observed around young stars which supports the suggestion that the disks are themselves part of the early process of planet formation. The Hubble telescope revealed a number of young stars surrounded by disks of dust which could be indicative of the early stages of planetary formation. Further supporting evidence of the nebula hypothesis has come from infrared observations of a young star, MWC 349, which some astronomers believe to be an example of the primordial nebula that produced the solar system. This particular star has about 30 times the mass of the solar system. Its luminosity appears to have decreased by 88 per cent during the forty years it has been observed. This is now attributed to the dissipation of disk material (Brahic,
1994). The most striking claims so far were made during the mid-1980s when astronomers observed a disk around the young star, Beta Pictoris, and similar reported observations were made regarding the T Tauri stars in the constellation of Taurus. It is believed that T Tauri stars have disks of dust around them which might be in the process of generating a planetary system. Evidence for this was derived from infrared radiation which is believed to come from the dust particles in the disks. Older stars do not appear to show signs of infrared radiation, so it is believed that the dust has settled into planetary masses. On the other hand, the dust might have been blown away by the stellar winds, i.e. streams of subatomic particles ejected from stars at high velocities. First-generation stars, consisting of hydrogen and helium, would be unlikely to be accompanied by a dust cloud.
A planetary system would normally be restricted to single stars, neither too large nor too small, with medium angular momentum. Although there still remains great uncertainty with regard to the formation of multiple star systems, their complicated gravitational systems indicate that they are unlikely places to find planetary systems – although they should not be ruled out altogether. About half the stars in the universe are in multiple systems. David Hughes (1992: 33) calculates that about one in twenty-four stars could have planetary systems and that the average number of planets around stars that could have them is 13.5. This is a possibility of 600,000 million planets in our galaxy which contains about one million stars.
It should be stressed that during the early 1990s such calculations and theories provided hints not evidence regarding ex-solar planets. These hints, however, had an accumulative effect. The Hubble telescope pictured several disks in the Orion Nebula. Supportive evidence also came from the IRAS (Infrared Astronomical Satellite) which found evidence for the proto-planetary nebulae around several nearby stars, such as Beta Pictoris. It would appear to be a very young disk and it is not known whether planets have evolved there yet, although spectroscope observations indicated that bodies of a few kilometres in diameter may be present in the disk (Vidal-Madjar, 1994: 415). Further hints were to come from observations of the rotation rates for stars. Our Sun, like other G-type stars, rotates slower than some of the large stars. One explanation is that it has been slowed down by the gravitational effects of the planets. Very large fast-rotating stars will have drawn their planets into them, like a skater who draws her arms inwards and spins faster. This feature increases the plausibility of the hypothesis that slowly rotating stars have planets. But critics might argue that all of this is based on an unfounded assumption of a uniform rate of rotation for all stars. In fact, there is evidence that some T Tauri stars with a similar mass to our Sun rotate as fast as the larger stars. Moreover, there are alternative explanations for the slow rotation rates of certain stars. Stars, like our Sun, for example, may decelerate their rate of spin through the gradual interaction of stellar winds with their magnetic fields, with the winds acting as brakes.
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