Starwatch: The ice giants

Alan Pickup on our system's giant outer planets – including the two farthest, now known as the ice giants, Uranus and Neptune

neptune and uranus
Illustration: NASA, E Karkoschka, ESA, H B Hammel and STScI Photograph: Graphic

When Pluto was exiled to the class of dwarf planets in 2006, it left a solar system of eight planets that could be divided neatly into two sets of four. Closer to the Sun we have the smaller rocky worlds of Mercury, Venus, Earth and Mars while beyond the asteroid belt are the gas giants Jupiter, Saturn, Uranus and Neptune.

The largest is Jupiter which is 11 times wider and 318 times more massive than the Earth. Now resplendent in the E before dawn, it stands near the waning earthlit Moon on Saturday, the 20th. Indeed, Jupiter is more than twice as massive as all the other planets combined. Its gaseous bulk, some 90% being hydrogen and helium, is seasoned with enough other material that it might still contain a dense core much larger than the Earth and consisting of rock perhaps churning with hot metallic hydrogen.

Second in size is the ringed beauty Saturn. Currently to the right of Mars and low in the SW in our early evening, it is as wide as nine Earths, 95 times more massive and probably has an internal structure that mirrors that of Jupiter.

The other gas giants, Uranus and Neptune, are near-twins and sufficiently distinct that they are now dubbed the ice giants. Holding less raw hydrogen and helium, perhaps 10% and mainly in their outer layers, most of their mass is made up of ices of water, ammonia and methane which may overlay an Earth-sized core of iron, nickel and silicates.

Our illustration shows false-colour images acquired by Hubble and its Nicmos camera a few years ago. Combining optical and selected near-infrared wavelengths, they also show some of their moons and the Uranian rings. The darker hues represent lower atmospheric levels where methane dominates while the brighter colours show cirrus-like clouds, also perhaps of methane. To the eye, though, they present mainly featureless discs in ordinary light. Uranus is described as a mild cyan while the more pronounced azure of Neptune is the obvious factor in naming it following discovery in 1846.

Both ice giants are about four times wider than the Earth but Uranus, with a diameter of 51,118km, is 1,600km wider than Neptune despite it being the least massive, by 14.5 Earth-masses to Neptune's 17 – clearly, Neptune is more compact. They have a similar rotation, too – about 17 hours for Uranus versus 16 hours for Neptune.

Uranus' unique attribute, though, is that its axial tilt is 98° so that it orbits the Sun on its side every 84 years, presenting first one pole towards the Sun and then the other. At present, the Sun is overhead at a Uranian latitude of 26°N, and creeping 4° further N by this time next year.

Neptune appears as a magnitude 7.8 binocular-brightness object in Aquarius as it crosses the meridian at midnight BST tonight, while Uranus verges on naked-eye visibility at magnitude 5.7 in Pisces as it transits at 02:30. Telescopically, Uranus is 3.7 arcsec wide and Neptune only 2.3 arcsec. A web search should bring up useable finding charts.

• This article was amended on 16 September in order to correct the diameter of Uranus.


Alan Pickup

The GuardianTramp

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