THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE

Posted by admin_akashic on Jun 27th, 2022 in Space

Theories concerning the formation of stars and planets are called cosmogonic theories. The most acceptable theories at present are versions of nebula theory, according to which the star and its planetary system are formed by the same cloud of contracting dust. Since everything in the solar system rotates in the same direction it is plausible to assume that the entire system originated at the same time out of matter that accumulated to form the individual planets. On these terms both the Sun and its planets originated out of the same cloud of dust. But there are still many theories and cosmogony is highly dependent upon speculation, although progress would be made with developments in astronomical techniques which could facilitate more accurate observations of planets.

While current theory is suggestive of a very large number of planetary systems, accident or catastrophe theories of planetary formation, which were prevalent during the eighteenth and nineteenth centuries, placed restrictions on the number of possible planetary systems. In general such theories were governed by the belief that the solar system was unique. One version of the catastrophe theory was that the original binary companion of the Sun either destructed or collided with a third body, leaving debris which later settled into planets. The fault with this explanation lies in the fact that the result of the self-destruction of a star is usually a supernova with the debris hurled out too far to form orbiting bodies, and thus what remains is a white dwarf or neutron star. Collision with a third body would scatter matter in all directions, and would be unlikely to produce planetary orbits. Among the main exponents of accident theory was Compte de Buffon, who speculated in De la formation des planètes, in 1745, that the solar system was composed of matter torn from the Sun following an encounter with a comet. This speculation did not survive mathematical computation which reveals that even if a comet were large enough to cause great masses to leave the Sun, the gravitational separating force would be too great to allow condensation into a planetary system. The most likely result of such a collision would be an envelope of solar gases.

During the nineteenth century several astronomers were attracted to various ‘tidal’ theories of planetary formation. Thomas Crowther Chamberlain and Forest Ray Moulton developed a tidal theory which became known as the ‘planetesimal hypothesis’, according to which an alien star bypassed the Sun, drawing off matter which then condensed into planetesimals. By steady accretion these small bodies grew in size and then became planets. Objections to the hypothesis were based on doubts concerning the ability of planetesimals to endure the many collisions that were expected. It was more plausible to assume that they would be dissipated. Other problems with the hypothesis were a failure to account for planetary orbits, their distances from the Sun, and the rapid rotation of large planets like Jupiter and Saturn.

In the early twentieth century Sir James Jeans’ and Harold Jeffreys’ near collision hypothesis – a version of the tidal theory – was favoured. Jeans and Jeffreys maintained that a star passed near the Sun and pulled out a tidal filament from which the planets were formed. Given that the chance of stars

colliding in our part of the galaxy is about one in a billion each year then the number of planetary systems was likely to be small. The main weakness of this hypothesis lay in explaining how the matter was to be drawn out of the stars as they passed each other. It is easy to imagine great tidal waves of matter drawn from each star as they attract each other, but the likelihood is that the matter would be drawn back to the original stars as they departed, not ejected freely into space. The thesis is now discredited because [1] the filament extracted from the Sun would not extend far enough; [2] the material drawn out would be too hot to condense into planets and would disperse instead; and [3] the very presence of deuterium and lithium on Earth suggests that planets were formed at lower temperatures than this model suggests.

Most current theories reject accidental explanations of planetary formation in favour of natural explanations which accordingly predict larger numbers of planetary systems. In some respects Kant and Laplace’s respective nebula hypotheses have come back into favour, as both held that planets and stars originated together. In his Allgemeine Naturgeschichte und Theorie des Himmels (1775; 1981) Kant depicted the Sun as a huge slowly rotating body of gas filling up the entire volume of what was to become the solar system. As it cooled it contracted and spun more rapidly, reaching a point where rings of gases would break away and condense into planets. This hypothesis was developed by Pierre-Simon Marquis de Laplace, with an additional assumption of an extensive nebula surrounding the Sun. As the nebula cooled and contracted it rotated faster, such that centrifugal forces outmatched gravity and the rings fell off, later to condense, forming planets. The major objection was that the centrifugal forces would not be powerful enough to overcome the gravitational forces, and if they were, then the matter would be hurled too far away for it to coalesce into planets.

According to the currently prevailing, but not undisputed, theory, planets are natural by-products of the formation of a star from a collapsing cloud of gas and dust. This explanation is also supported by some of the more plausible accounts of the formation of an atmosphere according to which atmospheric constituents are already present in the dust grains. There is also evidence that organic material may be present in the stellar clouds. Planets are products of a flattened disk of material left over after the collapse of a slowly rotating interstellar cloud of gas and dust, consisting of silicon, oxygen and carbon. This would suggest that planets do not orbit the older – first generation – stars which consist mainly of hydrogen and helium and are not accompanied by clouds of dust. It is likely that ex-solar planetary systems will be found near the younger stars. Stars and planets are thus formed simultaneously as the result of the protostellar cloud collapse where the angular momentum of the cloud is small enough, but not exceeding the momentum required for the formation of stars and planets. The spinning clouds of dust can be triggered when they pass into a dense region near the leading edges of the galaxy’s spiral arms, or by explosions of nearby supernovae (Hughes, 1992: 29). Eventually the cloud breaks up into smaller