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Gas Giants
Giant gas planets are sometimes called Jovian planets and exceed 10 Earth masses. Scientists know they are composed mostly of gas by observing the planets' apparent differential rotation. Astronomers noticed the equator rotated slower than the poles and deduced the planets are not made of solid matter. Gas giants are similar in composition to the sun, but aren't big enough to initiate a nuclear fusion reaction at the planets' hot molten core.
Ice Giants
The outer planets of the solar system are known as ice giants. According to the standard accretion theory, the ice giants Uranus and Neptune formed around a core of accreted solar nebular gas. The ice giants accumulated heavier elements such as oxygen, carbon, nitrogen and sulfur. The elements sank to the planets' centers and formed molten rocky cores heated by radioactivity and intense pressure. Hydrogen and the lighter elements rose to form an atmosphere over icy layers of water, ammonia and methane.
Jupiter
Jupiter is the solar system's most massive planet. Its outer layers are gaseous molecular hydrogen while its small core of heavy elements is surrounded by a zone of pressurized metallic hydrogen. Spectroscopic telescopes and the Galileo probe determined that Jupiter is 79 percent hydrogen, 20 percent helium and 1 percent heavy elements. Extreme turbulence on Jupiter causes hurricane-like storms. The Great Red Spot is the largest of these storms. Jupiter has many moons; the four biggest are called the Galilean Moons.
Neptune
Neptune is the planet farthest from the sun. It gets a distinctive baby-blue color from the presence of methane gas in the upper atmosphere. Neptune may be windier than Jupiter, with surface winds estimated at 1,300 mph. A giant rotating storm system called the Great Dark Spot travels through Neptune's southern hemisphere. Neptune has 13 moons, the largest is named Triton. Voyager 2 confirmed the presence of faint rings and arcs around Neptune.