Solar System/Outer Planets

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This article is about the outer planets of our solar system. For Solar System event in general, see Solar System.

The inner planets of our solar system are those between the Sun and the asteroid belt: Mercury, Venus, Earth, and Mars. The outer planets are those beyond the asteroid belt: Jupiter, Saturn, Uranus, and Neptune.


Jupiter is the fifth planet from the sun at a distance of 778.3 million km. It is also the largest planet with a radius of 71,492 km. Jupiter holds most of the non-solar mass in the solar system. It gives off more energy than it receives from the sun, so it is believed that Jupiter is a failed star. That means that Jupiter could have formed into a star, but was not under the right conditions. Contrary to popular belief, Jupiter has three small rings around it. These rings are made out of tiny particles. Its day is the shortest of the solar system's planets at about 10 hours, and its year is equal to 12 earth years. It has an atmosphere composed of hydrogen and helium. The outer layer is the thin visible cloud bands that we see this is also the zone that contains the circular storm known as the Great Red Spot, which has been churning for centuries. This is followed by a thick layer of liquid hydrogen. Beneath that is a nearly same size level of liquid hydrogen that, because of the pressure, behaves like a metal. Finally, beneath that is an iron-silicate core.


Saturn is the sixth planet from the sun, and it is 1.427 billion km away from the sun. It is also the second largest with a 60,330 km radius. Its day is only 10 hours and 40 minutes, and its year is about 30 Earth years. It is the least dense planet, and if placed in a large enough body of water, it would float. It has the largest and most spectacular ring system in the solar system. They have a diameter of 275,000 km, but they are only a few hundred meters thick. The rings are made up of particles that vary in size, from dust like particles, to the moons Janus and Epimetheus. Sheperd moons such as Atlas, Prometheus, and Pandora keep the rings in line. Saturn, like Jupiter, is made up of only Hydrogen and Helium, and gives off more energy than it receives. It is not as large as Jupiter, so it is not believed to have ever had the potential to be a star.


The pronunciation preferred by astronomers is /ˈjʊərənəs/ (yoor-uh-nuhs).
Uranus is the third largest planet (25,560 km radius), and the seventh planet from the sun (2.87 billion km away). It was the first planet discovered after prehistoric times, because it is so far away from Earth. It was discovered by William Herschel. Uranus is known for having its axis of rotation almost parallel to its plane of orbit. This gives it seasons that seem rather strange on Earth - 42 years of almost complete darkness followed by 42 years of consistent sunlight. Its 9-ring ring system is also parallel to its plane of orbit. These rings are different from those of Jupiter and Saturn, because they are more like hoops than rings of particles, and they have large gaps between them. Its day is about 18 hours long, and its year is 84 Earth years long. Its outer atmosphere is composed of hydrogen, helium, and methane, which gives it its blue green color. Beneath the outer layer is a layer of high pressure solid water, methane, and ammonia. Then, beneath that layer is a ball of rocky material that is very similar to Earth, but its surface is distorted by the dense inner ocean of water and methane.


The 8th planet from the sun, Neptune is 4.479 billion km away from the sun. It is the fourth largest planet with a radius of 24,765 km. It was discovered in 1846 after calculations in Uranus's orbit revealed that its motions were disturbed by a more distant planet. Its day is about 19 hours, and its year is 165 Earth years. The outer third of Neptune is made of hydrogen, helium, water, and methane, which, as on Uranus gives it a blue tint. The inner two thirds are made of molten rock, liquid water, liquid ammonia, and methane. Neptune's most apparent feature is a storm similar to the Great Red Spot, the Great Dark Spot. Here, winds can reach more than 1600 kph, making it one of the windiest places in the solar system.