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The Outer Gas Giants
These distant worlds include the Solar System's largest
Distance from Sun: 778 million km
The largest planet in the Solar System, Jupiter has more mass than all of the other planets put together and is second only to the Sun in terms of gravitational power. In 1994 it enticed comet Shoemaker-Levy 9 to fragment and crash into its swirling clouds — other likely comet crashes were recorded in 2009, 2010 and 2016. Jupiter is mostly gas, its composition of hydrogen and helium similar to that of the Sun.
With a good pair of binoculars the first things you'll notice are its four most famous moons: Io, Europa, Ganymede and Callisto, spied by Galileo Galilei in 1610. With a telescope you'll see a slightly squashed sphere. This is due to its fast spinning 'day' of just under 10 hours, which causes the equator to bulge outwards and the poles to flatten. Jupiter's cloudy atmosphere is revealed as dark bands separated by white zones. The longer you look, the more features appear, so keep an eye out for spots, wisps and kinks. The most famous feature is the Great Red Spot; twice the width of Earth, this is a gigantic storm with winds reaching up to 644km/h.
Distance from Sun: 1.43 billion km
Saturn is known for its spectacular rings, made from millions of chunks of water-ice spread out into a thin disc only a few tens of meters thick but stretching 100,000km from the planet's surface. The rings form bands, some broad, some narrow. Scores of moons orbit within the rings, some carving out wide gaps. As with Jupiter, a handful of them are visible to observers.
Saturn's brightness varies due to the way the rings are tilted and how much sunlight they reflect. The planet is not so bright when the rings are edge-on to us, but its brightness increases over 7.5 years as the rings open up to observers on Earth. Then it fades again over the same period. If you're wondering why this takes 7.5 years, it's a quarter of the time that Saturn takes to go around the Sun.
The best way of understanding Saturn's tilting effect is to go out and look at the planet — it really is one of the telescopic marvels of the Solar System. It doesn't matter if you have a small scope — the sight of this tiny, ringed world hanging in a large, inky black field of view is magical. The view of larger scopes will start to show detail in the rings and on the planet itself.
Distance from Sun: 2.87 billion km
The first planet to be discovered with a telescope, found by William Herschel in 1781. Its blue-green hue comes from the abundance of methane ices in its hydrogen and helium atmosphere, which also contains water and ammonia ices. Like Venus, Uranus spins from east to west, but its axis of rotation is tilted almost 90° from the plane of its orbit, suggesting that it might have been knocked over by a collision. Five rings were discovered in 1977 — in 1986 the Voyager spacecraft identified a further six, and two more were found by the Hubble Space Telescope in 2005, bringing the total to 13.
Visually, Uranus doesn't have much going for it, whether you use your eyes, a pair of binoculars or a telescope. By simply turning your head upwards, you can just about see this gaseous world as a very faint star at the limits of visibility (around mag. +5.6). You won't see much from anywhere with light pollution, however — the sky has to be very black. The view improves a little through a telescope, showing a greenish speck.
Distance from Sun: 4.5 billion km
Neptune's composition is similar to that of Uranus, being mainly hydrogen and helium with methane ices, water ices and ammonia ices mixed in. But unlike featureless Uranus, Neptune is wracked by stormy weather, with giant tempests boiling among the clouds. Its winds are the fastest in the Solar System, reaching an incredible 600m/s (that's 2,200 km/h). Neptune has six known rings. They appear to have bright clumps within them, which may be short-lived collections of debris.
At around mag. +8.0 you need at least binoculars to see Neptune. When looked at through a telescope it looks like a 'star' with a hint of blue, but it is not as spectacular as its larger, closer compatriots. If you have a very large scope you can also catch a glimpse of Neptune's largest moon, Triton, which is mag. +13.5.
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