The origin of the Jovian planets. Dynamics of the solar system

The outer planets, Jupiter, Saturn. Uranus and Neptune, are fundamentally different from the rocky masses of the inner solar system. They are gas-rich planets with lower densities than the terrestrial planets. The explanation may be that the inner planets had gaseous envelopes too, but that these have been ripped away by atmospheric tides raised by the Sun. Another possibility is that the ices of water, ammonia and methane condensed only in the cooler outer regions of

the solar nebula.

Snowflakes of these substances would readily stick together on contact, so that accretion may have been much swifter in the outer solar system. Eventually Jupiter and Saturn had grown sufficiently massive for their gravitational fields to capture even the lightest gases, which are scarce in the inner solar system today.

The outer planets have a profusion of natural satellites. Some may be captured asteroids. Others may have formed in the vicinity of their parent bodies in analogous manner to the accretion of the planets proper.

The minor planets, or asteroids, are probably planetsimals left stranded by chance during the great phase of accretion. Their combined mass is too small (less than 0.02 per cent of the Earth's mass) to make a proper planet. Comets may be icy planetesimals from the furthest outposts of the solar system.

2.12: The swirling clouds of Jupiter's upper atmosphere mark out several prominent circulation belts. Note the numerous small eddies trapped in these belts. The Great Red Spot is the largest of these whirlwinds. The two satellites are lo (left) and Europa. The scan lines are produced by the way the photograph was taken, rather like a television picture. This photograph was taken on 5 February 1979 by Voyager 1

Dynamics of the solar system. The solar system has now settled into a stable arrangement in which nine planets follow orbits that are widely spaced and nearly circular. If this were not the case it is obvious that collisions would occur.

The Earth's orbit has a fairly small eccentricity. Its mean distance is around 1 50 million km from the Sun. and the greatest and least distances are 153 million and 147 million km. The mean distances increase by roughly 75 per cent from one planet to the next, a remarkably regular arrangement first discovered in the eighteenth century.

The Earth's spin axis is inclined to its orbital plane by 23 1/2^o. Over long periods of time, the combined gravitational pull of the Sun and Moon can alter this slightly, and the angle is known to have changed by about half a degree in the past four thousand years. The direction in space to which the spin axis points also changes gradually, through precession, moving full circuit in about 26 000 years. The complex behaviour of the angle and direction of the rotational axis is important to studies of data from rock magnetism.

Other planetary systems. The possible existence of worlds other than our own is an ancient

astronomical problem, given a new twist as a theory for the origin of our own system is pieced together. In discussing this problem it is important to realize that our Sun is a solitary star, and such stars are in a minority.

Most stars are members of binary or multiple systems, and planets would not survive for long in such an environment. When we survey the single stars it is not possible to see any planets nor could we expect to do so with present techniques. In the future it may be possible to search for such planets directly DJ using large telescopes in space, or by detecting tiny but regular changes in the velocity of a star, which would indicate that unseen companions were in orbit around it. The theoretical approach suggests that planetary systems may be common around single solar-type stars. Nothing in the present theories assumes that our Sun has been subjected to any rare or unique process.

Hence plaints such as the Earth, Jupiter. Mercury, and perhaps even unlike any local examples, may be plentiful in the Galaxy and the universe at large. This conclusion has encouraged elaborate plans for searching for extraterrestrial intelligence, although none of these projects has received significant funding. For the present the Earth remains the only planet available for detailed scientific investigation.