The Earth in space. The universe

The planet Earth is insignificant when compared to the universe at large. From the distance of the nearest stars it would be impossible, using techniques currently available to us. to see our planetary system. The planets comprise only 0.13 per cent of the mass of the solar system, and most of that planetary mass is due to the planet Jupiter.

This chapter reviews the properties of the Earth from an astronomical standpoint, relating its history to that of the Sun. the stars and the far universe. This is of importance in understanding the history of the planet, because the geological processes that have occurred in the past 4600 million years (Ma) were largely predetermined by the initial mass, composition and location in space of the Earth.

To compare the Earth with other objects in the universe it is convenient to refer to Figure 2.1 which gives the dimensions and masses of a range of astronomical objects.

2.1: Comparative values for the physical properties of astronomical objects. In some cases the values are not known with great precision

The most striking comparison is between the Earth and the universe: according to the astronomical evidence, the universe is about 10²⁸ times more massive than the Earth, and their radii differ by twenty powers of ten. This leads to an enormous difference in their relative densities : the Earth has the density normally associated with solid rock, whereas the average density of the universe amounts to around one hydrogen atom per cubic metre. Furthermore the age of the Earth is only about one-quarter the age of the universe.

The universe. Observations of the distant galaxies indicate that the universe as a whole is expanding, with the remotest objects receding at speeds approaching the velocity of light. This suggests that the density of the universe is decreasing, and that it is still expanding from an earlier, very dense phase. In 1963 radio astronomers detected weak extraterrestrial radiation uniformly distributed across the sky. This has been interpreted as a 'fossil relic' of the 'big bang', which marked the beginning of the universe as we know it.

The background radiation is a last remnant of the fireball from which the universe has expanded. Today this radiation is so dilute, so thinly spread around the vast radius of the universe, that the average temperature far away from any star is a mere 2.7 degrees above absolute zero. In the first few seconds of the primeval fireball the subsequent history of the universe at large was determined : its expansion rate, composition and ability to form galaxies, stars and planets.

Within the first few minutes the universe had settled down to the chemical composition it still possesses: nearly 75 per cent hydrogen and 25 per cent helium by mass. The degree of turbulence in this maelstrom has carried over into the rotational motions of the galaxies, stars and even the spin of the Earth on its axis. The elements heavier than hydrogen and helium did not exist in the early universe but were

created subsequently in exploding stars.

By about a million years after the big bang the universe had expanded sufficiently for the formation of clumps of matter which gave rise to galaxies and galaxy clusters (Figure 2.2). Ever since then these galaxies have been moving apart in a universal expansion.

2.2: The nearby galaxy cluster in the constellation Perseus. Many of its members seem to form a chain of galaxies. This cluster has spiral, elliptical and irregular galaxies, as well as a strong X-ray galaxy

Astronomers are not able to decide whether this expansion will continue forever, or whether the gravitational attractions between the galaxies might eventually reverse it. The answer is of philosophical rather than scientific importance perhaps, since a positive result favouring the second possibility might indicate that the universe runs through an infinite cycle of expansion followed by collapse, followed again by rebirth in a cosmic fireball. The present rate of expansion indicates that from thirteen to twenty billion veins have elapsed since the big bang.