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The Gas Giants

Two categories of planet orbit the Sun, the terrestrial planets and the gas planets. The terrestrial planets, Mercury, Venus, Earth and Mars are so called for their hard, rocky surfaces (even though two-thirds of the Earth’s surface is water, it is included in this group). The gas planets are distinguished by the huge amount of gas and proportionally very low amount of rocky and metallic material in their composition. Clearly, some grading process was operating at the creation of the Solar System so that the rocky planets formed close to the Sun, and the gas planets further out.

Like all stars, the Sun formed from a vast cloud of molecular hydrogen which collapsed under force of gravity, heating the core to 20 million degrees Kelvin, triggering nuclear fusion. In the immediate aftermath, the light gaseous material near the Sun was pushed to the outskirts of the Solar System by the force of the charged particles, or solar wind, generated in the fusion process at the Sun’s core. Heavier metallic and rocky materials were slower to move, so remained close to the Sun to be scooped up later in the formation of the terrestrial planets.

Meanwhile, in the outer reaches of the Solar System, the gas driven out by the solar wind began to coalesce into planets. Jupiter, the largest planet formed where the concentrations of gas was greatest in the primordial Solar System. Saturn collected most of the remaining gas, while the leftovers were collected by the much smaller Uranus and Neptune.

The gas giants, unlike terrestrial planets, do not have a clearly defined surface. What we see in photos is the surface of a tempestuous atmosphere that descends for thousands of kilometres towards the centres of the planets. Jupiter, due to its great size, is expected to have the greatest extremes towards its centre. Holding more hydrogen and helium than the other three, Jupiter is seen as a failed sun, which did not accumulate enough gas to ignite its core. However, the centre of Jupiter is still an exotic place. As one progresses towards the centre, pressures and temperatures increase until the hydrogen is so compressed it behaves like a liquid and then like a metal near the core. This metallic hydrogen, stirred by the planets fast rotation (ten hours) causes the powerful magnetic field around the planet. The same scenario is repeated to more modest extents in the other gas giants.

One maverick among the giants is Uranus. All the planets rotate on axes that are roughly perpendicular to their orbits. Uranus rolls like a barrel along its orbit, as its axis lies almost in line with the plane of its orbit around the Sun. When the probe Voyager II passed the planet in early 1986, its South Pole was pointed towards the Sun and astronomers were puzzled that the equator of the planet was still warmer than the South Pole. The reason remains a mystery.

Clear skies!
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