A gas giant is a large planet made mainly out of hydrogen and helium. Jupiter and Saturn are the gas giants of the solar system. The outermost parts of their atmospheres have many layers of visible clouds that are mostly made out of water and ammonia.
Jupiter has been covered in some detail elsewhere, so here we will concentrate on the other three giant planets.
Saturn is the sixth planet from the Sun and the second-largest in the solar system after Jupiter. It’s diameter is about nine times that of earth and it takes twenty nine and a half years to orbit the Sun. Binoculars will show an odd shaped disc. However even a modest telescope, such as a spotter scope, will reveal it’s rings.
Saturn was the ancient Roman god of agriculture. He was remembered in December during the most famous Roman festival of all – the Saturnalia. The Saturnalia was a time of feasting, free speech, gift-giving, role reversal, and revelry. Saturn the planet and Saturday are both named after him.
Saturn’s atmosphere usually appears bland and lacking in contrast. Ammonia crystals in it’s upper atmosphere give it a pale yellow hue. Wind speeds on Saturn can reach 1,800 km/h.
Saturn’s best known feature are it’s rings. These consist mainly of ice particles with a small amount of rocky debris and dust. Saturn also has sixty-two moons not including the hundreds of moonlets within it’s ring system. The largest moon is Titan. Bigger than the planet Mercury, Titan is the only moon in the Solar System to have a substantial atmosphere. One of Saturn’s most interesting moons is Enceladus. This moon is regarded as a potential habitat for life. In 2015 the Cassini probe flew through a plume on Enceladus and detected most of the ingredients needed to sustain some forms of primitive microbes.
The Ice Giants
An ice giant is a large planet made mainly out water, ammonia and methane, along with traces of other hydrocarbons.There are two ice giants in the solar system – Uranus and Neptune.
Uranus is the coldest planet in the solar system, with a minimum temperature of −224°C. It is the seventh planet from the Sun and it’s diameter is about four times the size of earth’s. It is the most distant planet that can be seen with the naked eye.
Uranus is the only planet whose name is derived from a figure in Greek mythology. Uranus or “Father Sky” was the husband of Gaia, “Mother Earth”. Together they were the ancestors of most of the other Greek gods.
Uranus takes 84 years to complete one orbit. It is also unique because it’s axis of rotation is tilted over at almost 900 compared to the other planets. This means that it’s seasonal changes are completely different to any other planet. Each pole gets 42 years of continuous sunlight, followed by 42 years of darkness. Around the equinoxes the rest of the planet has a normal day – night cycle, but around the solstices only a narrow strip round the equator experiences that normal rhythm.
Neptune is the eighth and farthest known planet from the Sun. It’s diameter is about 3.9 times that of earth and it takes a hundred and sixty five years to orbit the Sun. Although Neptune is not visible to the unaided eye it may be seen with binoculars.
Neptune was the god of freshwater and sea in Roman mythology. He was also the creator of horses as well as the owner of a powerful weapon, his Trident.
Neptune is the only planet in the Solar System that was found by mathematical prediction. During the first half of the nineteenth century Alexis Bouvard discovered unexpected changes in the orbit of Uranus that led him to realise that it was being affected by an unknown planet. From 1843, John Adams, and later from 1845, Urbain Le Verrier began work to calculate the position of the new planet. Johann Galle was the first person to see it through a telescope on 23 September 1846, within a degree of the position predicted by Le Verrier.
Image credits: NASA