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Outer planets of the solar system.

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Outer planets.
Outer planets: From top to bottom: Neptune, Uranus, Saturn, and Jupiter (sizes not to scale).

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. The Outer planets Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of "ices", such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, "Uranian planets," or "ice giants." All four of the gas giants exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth's orbit (thus consisting of the outer planets plus Mars).

Outer planet Jupiter.

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely Hydrogen and helium is not very different from that of the Sun, and the planet has been described as a "failed star". Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. The four largest of its 63 Satellites, Ganymede, Callisto, Io, and Europa (the Galilean satellites) share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, has a diameter larger than Mercury.

Outer planet Saturn.

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 56 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than Mercury; it is also the only satellite in the solar system with a substantial atmosphere, similar in composition to that of the atmosphere of the early Earth.

Outer planet Uranus.

Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold compared with the other gas giants, and radiates very little heat into space. Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda.

Outer planet Neptune.

Neptune (30 AU), though slightly smaller than Uranus, is denser and slightly more massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense "arcs" of material separated by gaps. Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen, and is the only large satellite to revolve around its host planet in a prograde (clockwise) motion. Neptune possesses a number of Trojan asteroids.

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