Solar System

What is the Solar System?

Our Solar System

Our Solar System is the Sun and everything that travels around it, which includes the nine known planets, their moons, asteroids and comets. The Sun is the biggest object in our solar system. It contains more than 99% of the solar system's mass. Astronomers think the Solar System is more than 4 billion years old.

All nine planets can be seen with a small telescope. Large observatories continue to provide much useful information. But the possibility of getting up close with interplanetary spacecraft technology has revolutionized planetary science.

We will look at Jupiter, Saturn, Neptune, Uranus and Pluto on this page. Information for the remainder of the nine planets can be found by clicking the following links: Earth, Mars, Venus and Mercury.



Jupiter is the fifth planet from the Sun and by far the largest out of the nine planets. Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright "wandering star".

Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope.

Jupiter is a gas planet and therefore have no solid surfaces. Their gaseous material simply gets denser with depth. What we see when look at these Jupiter is the tops of clouds high in their atmospheres.

Jupiter is about 90% hydrogen and 10% helium with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.


An image of Saturn taken by Voyager 1

Saturn is the sixth planet from the Sun and the second largest. Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust.

Saturn was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present. Saturn's interior is hot (12000 Kelvin at the core) and Saturn radiates more energy into space than it receives from the Sun.


An image of Neptune taken by Voyager 2

Neptune is the eighth planet from the Sun in our Solar System and fourth largest.

Neptune and Uranus are very much alike. They are both large gas planets that look like big blue balls in the sky. Neptune's blue color is caused by the methane (CH4) in its atmosphere; this molecule absorbs red light. The planet has a hazy atmosphere and strong winds.

Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus's orbit was disturbed by an unknown object which turned out to be another gas giant, Neptune).

Neptune has large, dark circles on its surface which astronomers believe to be storms. Neptune has two thick and two thin rings which surround it. Neptune also has at least eight moons. Four of these moons orbit the planet within the rings. One of Neptune's moons, Triton, orbits the planet in a direction opposite that of the seven other moons.

Due to Pluto's unusual elliptical shaped orbit, Neptune is actually the farthest planet from the Sun for a 20 year period out of every 248 Earth years.



Uranus is the seventh planet from the Sun in our Solar System. It is one of the smaller gas giants, but it is still large enough to hold 64 planets the size of Earth.

Since Uranus is a gas planet, it doesn't have a solid surface like Earth. The top layer of gas is far from quiet. By studying the pictures sent back by the Voyager spacecraft, scientists were able to see that there are winds blowing at over 645 km (400 miles) an hour.

Uranus has at least fifteen moons. There are probably more, but this won't be known for sure until another spacecraft is sent to investigate. The largest, and furthest away, of Uranus' moons are Miranda, Ariel, Umbriel, Titania and Oberon. The moons are named after characters in the stories of William Shakespeare and Alexander Pope. The remaining ten moons are much smaller and a lot closer to the planet. They were discovered by the Voyager 2 spacecraft as it sped past the planet in 1986. Some of the smaller moons act as 'shepherds', helping to keep some order in some of the rings. The others may be comets or asteroids that have been 'captured' by the planet.



Pluto is the ninth and furthest planet from the Sun. It is also the smallest planet and indeed is smaller than several moons. Pluto itself has a large moon named Charon; two small moons were discovered in 2005.

Pluto is the only planet that has not been visited by a spacecraft. A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.

Pluto's thin atmosphere is most likely nitrogen and carbon monoxide, in balance with solid nitrogen and carbon monoxide ices on the surface. As Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere freezes. When it returns to a closer proximity to the sun, the temperature of Pluto's solid surface increases, causing the nitrogen ice to turn into gas - creating an anti-greenhouse effect.

It takes about two hours for the Suns ray to reach Pluto. Pluto is so far away, that it takes 200 years for it to make a single orbit around the Sun.