Astronomy 101: Exploring the Outer Solar System

Lesson 10: Completing Our Visit Close To Home

outer solar system
The gas and ice giants of the solar system, shown with their interiors in cutaway. NASA

Our final lesson in this part of Astronomy 101 will concentrate primarily on the outer solar system, including two gas giants; Jupiter, Saturn and the two ice giant planets Uranus, and Neptune. There's also Pluto, which is a dwarf planet, as well as other distant small worlds that remain unexplored. 

Jupiter, the fifth planet from the Sun, is also the largest in our solar system. Its average distance is approximately 588 million kilometers, which is about five times the distance from Earth to the Sun. Jupiter It has no surface, though it may have a core composed of comet-like rock-forming minerals. Gravity at the top of the clouds in Jupiter's atmosphere is about 2.5 times Earth's gravity

Jupiter takes about 11.9 Earth years to make one trip around the Sun, and it's day is about 10 hours long. It is the fourth brightest object in Earth's sky, after the Sun, the Moon, and Venus. It can be seen easily with the naked eye. Binoculars or a telescope may show details, like the Great Red Spot or its four largest moons. 

The second-largest planet in our solar system is Saturn. It lies 1.2 billion kilometers from Earth and takes 29 years to orbit the Sun. It is also primarily a giant world of condensed gas, with a small rocky core. Saturn is perhaps best known for its rings, which are made of hundreds of thousands of ringlets of small particles.

Viewed from earth, Saturn appears as a yellowish object and can be easily viewed by the naked eye. With a telescope, the A and B rings are easily visible, and under very good conditions the D and E rings can be seen. Very strong telescopes can distinguish more rings, as well as the nine satellites of Saturn.

Uranus is the seventh most distant planet from the Sun, with an average distance of 2.5 billion kilometers. It is often referred to as a gas giant, but its icy composition makes it more of an "ice giant". Uranus has a rocky core, completely covered with watery slush and mixed with rocky particles. It has an atmosphere of hydrogen, helium, and methane with ices mixed in. Despite its size, Uranus's gravity is only about 1.17 times that of Earth. A Uranus day is about 17.25 Earth hours, while its year is 84 Earth years long

Uranus was the first planet to be discovered using a telescope. Under ideal conditions, it can barely be seen with the unaided eye, but should be clearly visible with binoculars or a telescope. Uranus has rings, 11 that are known. It also has 15 moons discovered to date. Ten of these were discovered when Voyager 2 passed the planet in 1986.

The last of the giant planets in our solar system is Neptune, fourth largest, and also considered more of an ice giant. Its composition is similar to Uranus, with a rocky core and huge ocean of water. With a mass 17 times that of Earth, it's volume is 72 times Earth's volume. Its atmosphere is composed primarily of hydrogen, helium, and minute amounts of methane. A day on Neptune lasts about 16 Earth hours, while its long journey around the sun makes its year nearly 165 Earth years.

Neptune is occasionally barely visible to the naked eye, and is so faint, that even with binoculars looks like a pale star. With a powerful telescope, it looks like a green disk. It has four known rings and 8 known moons. Voyager 2 also passed by Neptune in 1989, nearly ten years after it was launched. Most of what we know was learned during this pass.

The Kuiper Belt and Oort Cloud

Next, we come to the Kuiper Belt (pronounced "KIGH-per Belt"). It's a disk-shaped deep-freeze containing icy debris. It lies beyond the orbit of Neptune. 

Kuiper Belt Objects (KBOs) populate the region and are sometimes called Edgeworth Kuiper Belt objects, and sometimes are also referred to as transneptunian objects (TNOs.)

Probably the most famous KBO is Pluto the dwarf planet. It takes 248 years to orbit the Sun and lies some 5.9 billion kilometers away. Pluto can only be seen through large telescopes. Even the Hubble Space Telescope can only make out the largest features on Pluto. It's the only planet not yet visited by a spacecraft.

The New Horizons mission swept past Pluto on July 15, 2015 and returned the first-ever closeup looks at Pluto, and is now on its way to explore MU 69, another KBO. 

Far beyond the Kuiper Belt lies the Oört Cloud, a collection of icy particles that stretches out about 25 percent of the way to the next star system. The Oört Cloud (named for its discoverer, astronomer Jan Oört) supplies most of the comets in the solar system; they orbit out there until something knocks them into a headlong rush toward the Sun. 

The end of the solar system brings us to the end of Astronomy 101. We hope you enjoyed this "taste" of astronomy and encourage you to explore more at!

Updated and edited by Carolyn Collins Petersen.