Astronomy The origin of the solar system

Press, 30 August 1977, Page 30

Astronomy The origin of the solar system

The most acceptable theory of the origin of the Solar System considers that it condensed out of a cold cloud of dust and gas. The cloud collapsed rapidly under gravitation. Originally it was rotating, with the result that as the cloud collapsed, the residual rotation formed an extensive disc about a rapidly rotating protosun. The protosun was very much more massive than the Sun to-day. The fact that it was rotating very rapidly poses a problem, because some mechanism must be provided wehreby this rotation was slowed down, since the Sun now rotates much more slowly. The rotation must have somehow been transferred to the planets, which today have more than ninety-seven per cent of the angular momentum of the system, but only about 0.14 per cent of the total mass. Possibly the Sun was originally much less stable and more active, so that the solar wind flowing through the Solar System was much more intense than it is to-day. This would have retarded the solar rotation and also dissipated the solar nebula.

The energy generated as the nebula collapsed would have vaporised the dust in the system. There would have been recondensation as the vapours cooled, enabling the planets to be formed either by accretion or by quick condensations within the original cloud. Possibly the terrestrial planets, those with compositions akin to the Earth’s, were formed by the first process and the larger planets by the second. It is generally thought that the solar nebula was homogeneous, so that its chemical mixture was much the same throughout. Recent investigations of meteorites suggest that the solar nebula was not of a uniform chemical mixture. Meteorites are fragments of planetary bodies of asteroid size. The iron meteorites, in particular, show that they have been subjected to large stresses under intense pressure, such as would occur in the collisional break-up of an asteroid. They also reveal a crystalline structure that would be formed by slow cooling under moderate pressure, such as exists within asteroids. Additionally, their orbits are similar to those asteroids that have their perihelia within the Earth’s orbit. The age of meteorites is generally placed at a time older than the rocks of the Earth.

Compared with the Earth, meteorites show a large excess in the isotopes of oxygen and magnesium, in particular, as well as in nitrogen, carbon, neon and xenon. The composition of meteorites is thought to be similar to that of the original solar nebula. Therefore these excesses suggest that the

original nebula was not homogeneous, which implies that the Solar System might not have originated in a collapsing cloud of dust and gas. A new theory suggests that a supernova explosion was responsible for the enrichment of the nearby cloud with material from which the Solar System was formed. This idea suggests that he inner planets — Mercury, Venus. Earth and Mars — were formed by accretion of material frm the central region of the supernova, whilst the outer planets were enriched by material from the outer layers of the supernova. This neatly accounts for the entirely different composition of the inner and outer planets, but leaves many questions unanswered.

It is generally thought that stars, and presumably the planets which are likely to accompany star formation, originated in cool clouds. Such clouds may have started to compress under the extra pressure provided by a supernova explosion.

A star cluster in Canis Major consists- of stars having a common origin, and forms one of the youngest groups of stars known. Surrounding it is a circular nebula, where, recently, there has been found a neutral hydrogen shell. This shell is similar to expanding shells found around the remains of several old supernova. This shell is expanding at some 32 kilometres per second and has a radius of 30 parsecs. Its age is 600,000 years, which is also the estimated age of the stars in this cluster. It is suggested that this cluster originated in a supernova explosion, providing evidence that star formation is a natural result of such events. This is quoted as indirect proof that the Solar System may have been formed in a similar manner. It is all too easy to make such assumptions, but a great deal more research is necessary to place this theory on a firm basis.

This month the evening sky will be left without any bright planets as Mercury moves into the morn

F.M. Bateson

ing sky after reaching inferior conjunction on September 5. It will reach its greatest elongation west of the Sun on September 21. However, this elongation is not • favourable from our latitudes as the planet will be lost in the glare of the dawn sky. Venus, rising about 3 a.m., throughout the month will continue to dominate the dawn sky. It will pass close to Saturn on September 19, which may enable readers to locate Saturn in the dawn sky. Apart from this opportunity Saturn will be lost in the morning twilight. On September 5 Mars and Jupiter will be within half a degree of each other. There should be little trouble in identifying them, as Jupiter will be much the brighter of the two. There is also a distinct difference in colour, as Jupiter appears yellowish in contrast to the distinct red of Mars. Both these planets can be found in the constellation Gemini. If they are watched on several mornings an observer will note that Mars moves across the sky much more swiftly than Jupiter, so that Mars will pass over a much larger arc of the sky. However, they are not well placed on account of their positions well north of the celestial equator. It will be another two months before they are well situated in the evening.

Our spring equinox occurs on September 23, when the Moon is absent, celestial equator into the Southern Hemisphere. This also marks the season when the Zodiacal Light can be best seen in the evenings, because the ecliptic is then most nearly vertical to the horizon. Choose a dark, clear night wehn the Moon is absent. The Zodiacal Light will appear as a faint, conical beam of light somewhat broader at the base along the horizon and stretching up in a tapering cone along the Ecliptic. It is actually sunlight scattered by interplanetary dust and contributes about one third of the total light in the sky on a moonless night.