The planets that lie in the exterior of the solar system are extremely different from the terrestrial ones closer to the Sun. These large worlds are composed of gas and are classified as gas giants.
The four exterior planets, Jupiter, Saturn, Uranus and Neptune are increasingly being studied more and a few un-manned probes have been sent to get a closer look. Still, much remains a mystery about these big impressive gaseous worlds.
Planet Jupiter
This gas giant was named after the King of the Gods and it is the largest planet by far in the Solar System. Its size is 88,700 miles in diameter. Jupiter lies about 484 million miles from the Sun and takes almost 12 years to complete an orbit around it. Its day, however, is only 10 hours due to Jupiter's fast rotation.
The surface of Jupiter is made of gaseous material that gets denser as it gets deeper. The gas giant is about 90% hydrogen and 10% helium by atom numbers but it is 75/25% by mass, there are also traces of methane, water, ammonia and rock. The composition of Jupiter is similar to the Solar Nebula from which the Solar System was formed. Jupiter has faint rings, but they are hardly visible and nothing like Saturn's.
What is observed on gas planets when viewed are the tops of their clouds in their atmosphere. These are what form the famous 'red spots' on Jupiter, which are huge storms that last for hundreds of years. Although Jupiter is a gas planet its core is believed to be solid, made of rocky material and about as big as 18 Earth masses. The core can get as hot as 20,000 F.
Planet Saturn
Saturn was named after the god of agriculture. It is the sixth planet from the Sun and the second largest in the Solar System, measuring about 75,000 in diameter. Galileo was the first to observe Saturn in 1610, but was baffled by its odd appearance, because of the planet's prominent rings. It takes Saturn 29.5 years to orbit the Sun, but a Saturnian day is just 10 hours because it rotates fairly fast. It lies about 840 million miles from the Sun.
Saturn's surface composition is quite similar to Jupiter's, consisting of 75% hydrogen and 25% helium along with some traces of methane, ammonia and rock. Saturn's clouds are not as prominent as Jupiter's being very wide at the equator.
Saturn has a very low density so it would actually float on water. Its interior is also much like Jupiter's consisting of a rocky core layered by liquid metallic hydrogen and molecular hydrogen. There are some traces of ice as well. The temperature down at the core is very hot, reaching 12000 K.
Planet Uranus
The seventh planet from the Sun and third largest is the deity of the Heavens, Uranus. The planet was discovered in 1781 by Herschel. Uranus is about 1.7 billion miles from the Sun and It takes the planet 84 years to go around it once, while its day is about 17 hours.
The surface of Uranus is about 83% hydrogen, 15% helium and 2% methane. The planet has bands of clouds, but they are not very visible. The blue color on the surface is due to the methane in the upper atmosphere absorbing red light and reflecting the blue. Uranus does not have a rocky core but it is composed largely of rocky material and some ices along with hydrogen.
Uranus' rotation is very odd and its axis is almost perpendicular to the ecliptic plane. It's hard to say which is its north pole. This gas giant has very faint rings, but unlike other gas planets, Uranus' rings are constantly moving and disappearing.
Planet Neptune
Neptune is named after the god of the Sea. It is the fourth largest planet and the eighth from the Sun. Neptune was discovered in 1846. Neptune orbits the Sun from about 3 billion miles away and it takes 165 years to complete one orbit. A day in Neptune is 16 hours long.
Most of the information known about Neptune has been learned through the Voyager 2 spacecraft, which visited the planet in 1989. There are some ground and Hubble Telescope observations that have added more information as well. The composition of Neptune is mostly ices and rock with about 15% hydrogen and some helium. The atmosphere is made up of hydrogen, helium and some methane.
Like the other gas planets, Uranus has very fast winds and large storms on its surface. The winds on Neptune are actually the fastest in the Solar System reaching 1243 miles an hour. This gas giant also had a 'spot' feature called the Great Dark Spot, but recent Hubble observations show that it is now gone. There are faint rings around Neptune that are not very visible.
The majestic gas giants are a wonder, but much more remains to be discovered. With today's advances in research there will be much more information available and many new mysteries on these planets will be learned.
Sources:
Nine Planets - "A Multimedia Tour of the Solar System", William Arnett, February 9th, 2009. Nine Planets Org.
Solstation - Sol Company, 2009.
NASA's High Energy Astrophysics Science Archive Research Center
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