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  Jupiter, Saturn, Uranus and Neptune are gas giants because they are mostly made of gas, which causes them to have more mass.
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  Christiaan Huygens discovers saturns rings, which is a common feature of gas giants
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  Uranus is the third largest gas giant, with a diameter of 51,200 KM
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  Pluto is not a gas giant, but is found around other gas giants.
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  Scientists suspect gas giant rings are mostly made up of dust and ice.
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  Voyager Space Craft discovers thin ring around Jupiter
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  Voyager's pictures reveal Neptune's rings.
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  Extrasolar gas giant found. Size and mass approximate to Jupiter
