Outer Planets

home    history    our sun    inner planets    outer planets    moons    kuiper belt    links


Outer Planets

The next four planets out from the sun are Jupiter, Saturn, Uranus, and Neptune. They are collectively referred to as the Jovian planets (means Jupiter like). They are significantly different from the terrestrial planets. Besides being farther from the sun, the Jovian planets are much larger, are more gaseous, have lower densities, and all have some sort of rings. Their large sizes are responsible for increased gravity which can hold onto lighter elements like gases.

A belt of asteroids (fragments of rock and iron) between Mars and Jupiter separate the four inner planets from the five outer planets. Pluto, which fits in with neither the terrestrial or the Jovian planets, is actually considered a Kuiper Belt object which we will see in more depth on the Kuiper Belt page.


courtesy of JPL/Caltech/NASA

Jupiter is the largest planet in the solar system. It's diameter is about 11 times that of Earth. You could fit over 1,000 Earths inside! It is a giant ball of gas with no solid surface. It does however have a liquid center due to the high pressures forcing it into a liquid and a small solid core. Jupiter has 63 natural satellites (or moons). The largest four are Io, Europa, Ganymede, and Callisto. Viewed through a large telescope, Jupiter is stunningly colorful and looks like a disk covered with bands of blue, brown, pink, red, orange, and yellow. Its most distinguishing feature is the Great Red Spot, an intense, giant windstorm larger in size than Earth, which has continued for centuries without any signs of dying down.

Saturn is the second-largest planet and has majestic rings surrounding it. Saturn's seven rings are flat and lie inside one another. They are made of billions of ice covered particles so they are not solid bodies they are orbiting debris. Ice reflects light, that is why they are so bright and we can see them through a regular telescope from Earth. Saturn's density is lighter than water (0.7) which means if you had a body of water big enough to hold it, it would float! It's composition is 73% Hydrogen and 26% Helium and 1% other which is similar to both Jupiter and the sun.


(courtesy of NASA)


Uranus is a greenish-blue planet, twice as far from the Sun as its neighbor Saturn. It's axis of rotation is sideways and it takes 84 years to revolve around the sun! It has approximately 20 years of daytime and 20 years of nighttime because one pole faces the sun for a couple of decades and then the other for a couple. Uranus wasn't discovered until 1781 by William Herschel who named it Georgium Sidus (the Georgian star) after the English king, George III. Later its name was changed to Uranus, after an ancient Greek sky god, since all the other planets had been named after Roman and Greek gods.


courtesy NASA's Hubble Space Telescope

Neptune, the 8th planet from the sun, is named for an ancient Roman sea god, and is a stormy blue planet about 30 times farther from the Sun than Earth. Close-up photographs from Voyager revealed the "Great Dark Spot", a large (19,000 mile wide) high-pressure cyclone rotating counter-clockwise. It has recently disappeared however. Neptune was discovered when astronomers realized that something was exerting a gravitational pull on Uranus, and that it was possible that an unknown planet might be responsible. Through mathematical calculations, astronomers determined there was indeed an undiscovered planet out in space a year before it was actually seen for the first time through a telescope (in 1846).

The Voyager 2 spacecraft flew by Neptune in 1989. It showed that the planet is, basically, a twin of Uranus based on its size, density, and chemical composition. Like Uranus, Neptune is distinctly colored light blue, from the CH4 in its atmosphere. Neptune's period of revolution is 165 years.

Pluto was the coldest, smallest, and outermost planet in our solar system until a few years ago, when more objects were discovered. Pluto and its moon, Charon, were called double planets because Charon is so large it seems less of a moon than another planet. During each revolution around the sun, Pluto passes inside Neptune's orbit for 20 years, making Neptune the outermost planet for that time. Pluto was predicted to exist in 1905 and discovered in 1930. It is the only planet that has not yet been studied closely by a space probe. We will get close up views in July 2015 when the New Horizons spacecraft flies past Pluto and sends back detailed images of Pluto and it's 3 moons.

Pluto & Charon

courtesy NASA's Hubble Space Telescope

The debate continues regarding Pluto's status as a planet. We now realize that it is among many other objects in the Kuiper Belt and it doesn't fit the new "planet" definition. It is now called a "dwarf planet".

The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, defines a "dwarf planet" as a celestial body that, within the Solar System,

The term "dwarf planet" was adopted in 2006 as part of a three-way classification of bodies orbiting the Sun. Objects that are large enough to have cleared the neighbourhood of their orbit are defined as "planets", while those which are too small to be in hydrostatic equilibrium are defined as "small solar system bodies".

top of pagearo

XHTML 1.0!