giant planets

giant planets Our Solar System contains four giant planets which are so massive (Table 1) that their solid interiors lie deep within enormous liquid and gaseous envelopes that deny them Earth-like characteristics. Their atmospheres are composed mostly of hydrogen with some helium, but the remaining fraction of a percentage that forms the cloud layers prevents us from seeing deeper. The topmost cloud layer on Jupiter and Saturn is of ammonia, whereas the clouds are formed mostly of methane on Uranus and Neptune.

In Jupiter and Saturn gaseous matter persists to depths of tens of thousands of kilometres, until pressure is sufficient to compress hydrogen into a metallic state. Below this there is believed to be a water-rich layer (possibly a high-pressure form of ice) surrounding a central rocky core of several Earth-masses. Pressure within Uranus and Neptune is insufficient for metallic hydrogen to exist; instead their gaseous shells probably directly overlie the water–ice interiors, and their rocky centres are somewhat smaller.

We can study only the topmost part of the atmosphere of these planets directly. Atmospheric circulation is controlled more strongly by heat escaping from their interiors and by the rapid planetary spin than on the Earth, where the main driving force is solar heating. Major storm systems have been observed on all four giant planets, but only on Jupiter do they persist for more than about a year. The Great Red Spot (Fig. 1) is the most famous of these, and has persisted since the seventeenth century. All four giant planets have ring systems, made of dust and boulder-sized icy debris, but only Saturn's ring is wide and spectacular. Most ring-forming material probably originates from collisions of cometary material with satellites of these planets.

Table 1. The giant planets.

Name	Distance	Diameter	Mass relative	Density
from Sun	(km)	to Earth*	(tonnes m−3)
(millions of
kilometres)
*The mass of the Earth is 5.98 × 1024 kg.
Jupiter	778.3	142.8	317.8	1.33
Saturn	1427	120.0	569	0.69
Uranus	2870	51.2	14.5	1.29
Neptune	4497	48.6	102	1.64

Extrasolar planets have been detected around several nearby stars; those found so far they have all been giant planets in the Jupiter class. This is not surprising, because it is only the really large planets that we can expect to detect with current technology. It seems likely that smaller Earth-like planetary bodies must exist in abundance around other stars.

David A. Rothery

Bibliography

Rothery, D. A. (2000) Teach Yourself Planets. Hodder and Stoughton, London.
Beatty, J. K.,, Peterson, C. C.,, and and Chaikin, A. (1999) The new Solar System, (4th edn). Sky Publishing Corporation and Cambridge University Press, Cambridge.