The first four planets closest to the Sun are called the inner planets. They are small and dense terrestrial planets, with solid surfaces. They are made up of mostly rock and metal with a distinct internal structure and a similar size. Three also have an atmosphere. The study of the four planets gives information about geology outside the Earth. Most asteroids are also often counted with the inner planets
The Oort cloud is separate from the trans-Neptune region, and much farther out. It contains the long-period comets.
The "plane of the ecliptic" is defined by the Earth's orbit around the Sun. All of the planets orbit the Sun roughly around this same orbital plane. The farther away from this plane a planet orbits, the more "inclined" is its orbit to the ecliptic. If you could look at the solar system "edge on" then all the planets would be orbiting more or less in the plane of the ecliptic.
Saturn
Saturn is the sixth planet from the Sun in the Solar System. It is the second largest planet in the Solar System, after Jupiter. Saturn is one of the four gas giant planets, along with Jupiter, Uranus, and Neptune.
Inside Saturn is probably a core of iron, nickel, silicon and oxygen compounds, surrounded by a deep layer of metallic hydrogen, then a layer of liquid hydrogen and liquid helium and finally, an outer gaseous layer.
Saturn has 67 known moons orbiting the planet. 38 are officially named and 29 are waiting to be named. The largest moon is Titan, which is larger in volume than the planet Mercury. Titan is the second-largest moon in the Solar System. The largest moon is Jupiter's moon, Ganymede. There is also a very large system of rings around Saturn. These rings are made of ice with smaller amounts of rocks and dust. Some people believe that the rings were caused from a moon impact or other event. Saturn is about 1,433,000,000 km (869,000,000 mi) on average from the Sun. Saturn takes 29.6 Earth years to revolve around the Sun.
Saturn was named after the Roman god Saturnus (called Kronos in Greek mythology). Saturn's symbol is ♄ which is the symbol of Saturnus' sickle.
Saturn is an oblate spheroid, meaning that it is flattened at the poles, and it swells out around its equator. The planet's equatorial diameter is , while its polar diameter (the distance from the north pole to the south pole) is ; a 9% difference. Saturn has a flattened shape due to its very fast rotation, once every 10.8 hours.
Saturn is the only planet in the Solar System that is less dense than water. Even though the planet's core is very dense, it has a gaseous atmosphere, so the average specific density of the planet is 0.69 g/cm. This means if Saturn could be placed in a large pool of water, it would float.
The outer part of Saturn's atmosphere is made up of about 96% hydrogen, 3% helium, 0.4% methane and 0.01% ammonia. There are also very small amounts of acetylene, ethane and phosphine.
Saturn's clouds show a banded pattern, like the cloud bands seen on Jupiter. Saturn's clouds are much fainter and the bands are wider at the equator. Saturn's lowest cloud layer is made up of water ice, and is about thick. The temperature here is quite low, at 250 K (-10°F, -23°C). However scientists do not agree about this. The layer above, about thick, is made up of ammonium hydrosulfide ice, and above that is a layer of ammonia ice clouds thick. The highest layer is made up of hydrogen and helium gases, which extends between and above the water cloud tops. Auroras are also known to form in Saturn in the mesosphere. The temperature at Saturn's cloud tops is extremely low, at 98 K (-283 °F, -175 °C). The temperatures in the inner layers are much higher than the outside layers because of the heat produced by Saturn's interior.
Saturn's winds are some of the fastest in the Solar System, reaching 1,800 km/h (1,118 mph), ten times faster than winds on Earth.
Saturn's atmosphere is also known to form oval shaped clouds, similar to the clearer spots seen in Jupiter. These oval spots are cyclonic storms, the same as cyclones seen on Earth. In 1990, the Hubble Space Telescope found a very large white cloud near Saturn's equator. Storms like the one in 1990 were known as "Great White Spots". These unique storms only exist for a short time and only happen about every 30 Earth years, at the time of the summer solstice in the Northern Hemisphere.
Great White Spots were also found in 1876, 1903, 1933, and 1960. If this cycle continues, another storm will form in about 2020.
The Voyager 1 spacecraft found a hexagonal cloud pattern near Saturn's north pole at about 78°N. The Cassini−Huygens probe later confirmed it in 2006. Unlike the north pole, the south pole does not show any hexagonal cloud feature. The probe also discovered a hurricane-like storm locked to the south pole that clearly showed an eyewall. Until this discovery, eyewalls had only been seen on Earth.
Saturn's interior is similar to Jupiter's interior. It has a small rocky core about the size of the Earth at its center. It is very hot; its temperature reaches 15,000 K (). Saturn is so hot that it gives out more heat energy into space than it receives from the Sun. Above it is a thicker layer of metallic hydrogen, about deep. Above that layer is a region of liquid hydrogen and helium. The core is heavy, with about 9 to 22 times more mass than the Earth's core.
Saturn has a natural magnetic field that is weaker than Jupiter's. Like the Earth's, Saturn's field is a magnetic dipole. Saturn's field is unique in that it is perfectly symmetrical, unlike any other known planet. This means the field is exactly in line with the planet's axis. Saturn generates radio waves, but they are too weak to be detected from Earth. The moon Titan orbits in the outer part of Saturn's magnetic field and gives out plasma to the field from the ionised particles in Titan's atmosphere.
Saturn's average distance from the Sun is over 1,400,000,000 km (869,000,000 mi), about nine times the distance from the Earth to the Sun. It takes 10,759 days, or about 29.8 years, for Saturn to orbit around the Sun. This is known as Saturn's "orbital period".
Voyager 1 measured Saturn's rotation as being 10 hours 14 minutes at the equator, 10 hours 40 minutes closer to the poles, and 10 hours 39 minutes 24 seconds for the planet's interior. This is known as its "rotational period".
Cassini measured the rotation of Saturn as being 10 hours 45 minutes 45 seconds ± 36 seconds. That is about six minutes, or one percent, longer than the radio rotational period measured by the Voyager 1 and Voyager 2 spacecraft, which flew by Saturn in 1980 and 1981.
Saturn's rotational period is calculated by the rotation speed of radio waves released by the planet. The Cassini−Huygens spacecraft discovered that the radio waves slowed down, suggesting that the rotational period increased. Since the scientists do not think Saturn's rotation is actually slowing down, the explanation may lie in the magnetic field that causes the radio waves.
Saturn is best known for its planetary rings which are easy to see with a telescope. There are seven named rings; A, B, C, D, E, F, and G rings. They were named in the order they were discovered, which is different to their order from the planet. From the planet the rings are: D, C, B, A, F, G and E.
Scientists believe that the rings are material left after a moon broke apart. A new idea says that it was a very large moon, most of which crashed into the planet. This left a large amount of ice to form the rings, and also some of the moons, like Enceladus, which are thought to be made of ice.