Exploring the Solar System

Updated: May 16

The solar system


The solar system is the name given to our immediate neighbourhood in space. It is made up of a star (the Sun), eight planets, more than 100 moles and an assortment of comets, asteroids and other space rocks and dust. All of these are held captive by the Sun's gravity.

Inner planets

The asteroid belt (made up of millions of rocky bodies) circles the Sun. Mercury, Venus, Earth, and Mars are on the inner side of the belt

Outer planets

The planets outside of the asteroid belt are Jupiter, Saturn, Uranus, and Neptune. Pluto used to be the most distant planet, but it failed the new planet test. Pluto now considered as dwarf planet.

What's in a name?

Most of the planets were named after Roman gods

  • Mercury is the winged messenger (because it appears to move swiftly)

  • Venus is named after the goddess of love the king of the gods (because it is the brightest and looks a beautiful planet)

  • Mars is the god of war (because of its red, blood like colour)

  • Jupiter is named after the king of the gods (it is the largest planet)

  • Saturn is the father of Jupiter and the god agriculture

  • Uranus is the Greek god of the sky and Neptune is the Roman god of the sea (named for its colour)

What is a planet?

To qualify as a planet, an object has to meet a number of conditions

  • It must be in orbit around a star, just as Earth orbits the Sun.

  • It must be large enough for its gravity to make it round.

  • It must have cleared its orbit of other objects (which Pluto hasn't done).

  • It must not be a satellite (as, for example, the moon is a satellite of Earth)

Jupiter:- Jupiter has sixteen moons. The largest four (Ganymede, Callisto, lo, and Europa) can be seen in from Earth with binoculars.

Saturn:- Every fifteen year Saturn appears sideways to us and the rings seem to disappear.

Mercury:- The smallest planet in the solar system.

The Sun:-- Our Sun is a star, but it is closer to us than any other star. Like all stars, it is a massive ball of burning gas, fed by constant explosions. Without it, our planet would be lifeless. Sun is spinning and it spin on its axis like other planets of the solar system. The planets circle or orbit the sun spinning as they move. The size of the Earth is smaller as compared to the Sun. The Sun is white, but false color images such as this allow astromers to identify different features on its surface. The Sun’s heat about eight minutes to reach in Earth.

Long lived

The Sun was born just under five billion years ago. Although it burns four million tonnes (tons) of fuel each second, it is so big that it will continue to burn for another five billion years.

A hot spot?

White areas show places where the Sun's surface temperature is higher than elsewhere. Cooler, dark areas called sunspots, sometimes appear on the surface. These hotspots are called faculae.

Solar wind

The Sun sends out a stream of invisible particles called the solar wind. When these pass Earth's North and South Poles, they can create stunning colours. Shimmering colours can light up the skies towards the Earth's polar regions.

Solar flares

Blasts of hot gas sometimes flare up from the Sun's surface in huge arcs or loops. They reach thousands of kilometres (miles) into space.

Investigating the Sun

Various space probes have been designed to study the Sun,

  • Ulysses was launched in 1990 to look at the Sun's polar regions

  • SOHO was launched in 1995 to observe the Sun and solar activity

  • TRACE was launched in 1998 to study the Sun's atmosphere.

Eclipse of the Sun

It's a sunny day, but a shadow falls over the land. This is darker than a cloud covering the Sun, the light dims completely and for a few moments day turns to night. This is a solar eclipse.

You are lucky if you see a total eclipse. You could wait hundreds of years to see two in the same plane. The sun has been covered by the moon. The streaming light is the Sun's corona people at the centre of the moon’s shadow experience a total solar eclipse.

What is a solar eclipse?

Solar eclipses occur when the moon passes between the Sun and the Earth. By doing this, the moon stops some of the Sun's light from reaching Earth. The resulting shadow means that temporarily day turn on light in certain places.

The stages of a solar eclipse

Time lapse photography shows how the moon covers the Sun in stages. In a total eclipse, the Sun will be completely covered this is called totality for up to eight minutes. The Sun's outer atmosphere, the corona, shows clearly at this time.

The moon's shadow sweeps across the Sun at about 1,700 km (1,100 miles) per hour. It takes about one hour for the moon to block the Sun's light, once its shadow begins to move across the Sun.

Ringed wonder

In the instant before the Sun disappears behind the moon, sunlight sometimes streams between mountains on the moon's surface, producing a stunning effect known as a diamond ring. The diamond ring effect lasts for just a few seconds.

Plotting eclipses

Solar eclipses occur once every 15 months or so and maps are used to plot the path of future eclipses. The shadow from a total eclipse follows a narrow path and often falls on an ocean so it won't be seen (unless you're on a boat!). About one hundred years top speed take if we could drive to the Sun.


The closest planet to the Sun and far smaller than Earth, Mercury has blistering hot days but freezing nights. The nights get cold because Mercury has no atmosphere to trap the Sun's heat. It takes 88 days for Mercury to orbit the Sun. So it has the shortest year of all the planets in the solar system.

Ted hot

As Mercury faces the Sun, temperatures reach a sizzling 425°C (800 F), hot enough to melt lead. Mercury is the second hottest planet, after Venus.

Surface mapping

In 1974 and 1975, the space probe Mariner 10 flew within 327 km (203 miles) of Mercury's surface. It took hundreds of photographs, covering just under half the planet.

An easy target

Mariner 10 provided close-ups of Mercury that showed a heavily scarred surface. Rather like our moon, this planet has been battered by comets and meteors. This is partly because there's no protective atmosphere in which meteors can burn up.

A small planet

Mercury is the smallest planet in the solar system. Pluto, which is smaller, has now been reclassified as a warf planet.

Long journey

Launched in 2004, space probe Messenger set out on a journey to Mercury. Its aim to reach the planet in 2008, fly by three times, then enter orbit around it in 2011. This close to the Sun, Messenger's heat shield will reach 370°C (700F).

Do you know?

One of Mercury's craters (the Caloris Basin) is so large that the British Isles could fit comfortably into it.

Many of Mercury's craters are named after famous painters, authors, and musicians So you'd find Mozart Beethoven Michelangelo, and Bach there.

  • In Roman mythology Mercury was messenger to the Gods.

  • A patchwork picture of planet Mercury created using images taken by Mariner 10.

  • A temperature map of Mercury red shows where the most heat is found.

  • Cross-section of Mercury showing its molten iron core.

Morning Star

You wouldn't want to visit Venus. You'd be crushed in an instant and remains fried to a crisp. This barren your planet is covered in acid clouds and it has an incredibly dense atmosphere.

Venus is the brightest planet. It can be seen in the early morning or early-evening sky, depending on where it is in its orbit around the Sun. That's why it's known as the morning or evening star.

Venus has very long days since it takes 243 times longer to rotate on its axis than Earth does. Venus planet is the nearest in size to Earth. Its diameter is just 650 km (400 miles) smaller than Earth.

A mass of clouds

The cloud layer is too thick to let much sunlight penetrate, but it does reflect a lot of light. In fact, after the moon, Venus is the brightest object in our night sky.

So what is it like?

This false colour picture of Venus was made from data collected by probes, including the Magellan probe, sent to Venus between 1989 and 1992. The blue areas represent huge plains of solid lava. The white, green, and brown areas are higher land, such as hills, mountains, volcanoes, and valleys.

Surface of Venus

This view of the surface of Venus exaggerates the height of Maat Mons, the highest volcano on Venus, to show its slopes in more detail. Maat Mons is named after Ma'at, the Egyptian goddess of truth and justice.

The surface temperature is about 482°C (900°F). Venus spins in the opposite direction to Earth, meaning the Sun rises in the west ond sets in the east

Third rock from the Sun

Our home planet, Earth is the only one in the solar system capable of supporting life as we know it. It's the right temperature because it's neither too close to the Sun, nor too far from it.

  • Earth is the third planet from the Sun. It takes Earth 365.25 days to orbit the Sun.

  • Earth is constantly spinning. It takes 24 hours to turn completely on its axis.

  • Earth is surrounded by a thin halo, our precious atmosphere.

  • Earth is the only planet in our solar system with an atmosphere that animals and plants (as we know them) can breathe.

  • Moon doesn't produce so much light. It reflects the sun's light.

Take a deep breath

All planets have an atmosphere made up of gases. Earth's is mostly nitrogen and oxygen, with traces of carbon dioxide and other gases mixed in.

A warm blanket

Earth's atmosphere and oceans play a crucial role in keeping its temperature stable. They absorb the Sun's heat and move it around the planet. This helps keep the temperature suitable for life.

A moving crust

Earth's surface layer or crust is a shell of solid rock. This crust is broken up into plates, which shift around constantly on a middle layer of molten or liquid, rock, Earth's core is solid. Molten lava erupts from a volcano in Hawaii, which is located where there is a weak place in the Earth's crust.

The moon

Earth has one natural satellite, the moon. It is the brightest object in the night sky (though it doesn't produce its own light), It is a bleak place, with no water, no plants, no air, and no life. Only one side of the moon is ever seen from the Earth. The moon may have been formed after an immense collision that sent debris into orbit around the Earth.

Spinning around and around

It takes 27 days for the Moon to travel around, or orbit, Earth. As it travels, it spins. Slowly. It actually spins just once during each orbit of the Earth.

How did it form?

Nobody really knows, but it was possibly after Earth was hit by another planet some 4.5 billion years ago. The moon's gravitational pull gives Earth's oceans a bulge. Sea levels change in particular areas as Earth spins.

Tide control

The twice-daily rise and fall of Earth's oceans is mainly caused by the moon's gravitational pull, which makes the ocean bulge a few metres in one direction. This bulge moves very slowly, but appears to sweep round the Earth as it turns.

  • Away from the bulge it's low tide

  • As a place passes through the bulge, its high tide.

Moon missions

Many unmanned space probes have been sent to investigate the moon, including these below.

  • Luna 3, a Russian probe, took the first pictures of the far side of the moon

  • Lunar Prospector, discovered ice near the Moon's poles in 1999.

  • Luna 9, a US probe made the first soft landing on the Moon in 1966.

  • A battered past

  • The moon has been badly battered by meteors in its long history, leaving its surface full of craters.

The red planet

Our nearest neighbour, Mars, was named by the Romans after their god of war, because its red colour reminded them of blood.

Mars is the fourth planet from the Sun. It has an atmosphere, seasons, huge mountains, and icy poles. A Martian day lasts a little over 24 hours (24 Earth hours 39 minutes and some seconds). It is called a sol.

An alternate Earth?

Mars is half the size of Earth and conditions are very different from those on our planet. Although Mars has a thin atmosphere and seasons, nothing grows there. Its red, desert-like surface is littered with dust and rocks. Mars's sky is not blue. If you stood on Mars you would see a pink sky.

I spy two moons

Mars has two lumpy moons that were discovered in 1877. They are so tiny that astronomers think they were asteroids pulled into orbit around Mars by its gravity. They are Deimos and Phobos.

  • Deimos (which means terror" in Greek) is 16 km by 12 km (10 miles by 7 miles).

  • Phobos (which means "fear" in Greek) measures 28 km by 20 km (17 miles by 12 miles).

Is there water?

Surface temperature on Mars are too low for liquid water to exist and Mars has no rivers, seas, or oceans. However, there was water once. We know this because of the existence of dried-up water channels.

A face on Mars

In 1976 Viking Orbiter 1 sent a series of shots that showed a "face" on the planet's surface. Many saw this as an enormous sculpture built by intelligent life. It's actually a mountain.

That's definitely Martian!

  • There are a number of notable features on the surface of Mars.

  • The planet experiences strong winds that create immense dust storms.

  • Mars has a huge polar ice cap. If melted, the water would cover the planet.

  • The Olympus Mons volcano is the largest in the solar system.

  • The Valles Marineris canyon would stretch across the USA.


Saturn is the second largest planet in our solar system. It is huge. You could line up nine Earths in a row across Saturn, but as it is largely composed of gas, you couldn't land a craft on its surface.

  • Saturn named after the Roman God of agriculture

  • Saturn is the sixth planet from the Sun in the Solar System.

  • Saturn spins around once every 10 hours 39 minutes, making it bulge at the middle as it spins.

A ringed beauty

Saturn isn't the only planet with rings, but it is the only planet whose rings are visible to us because of their large area. The rings are made up of ice, dust, and rock.

The seven main rings are made up of about 10,000 ringlets. They extend more than 300,000 km (180,000 miles). Ice in Saturn's rings reflects light. That's why we see them so well. Saturn's rings are less than 1km (6 miles) thick.

Blown away

Even if you could land on Saturn, you'd be blown away pretty quickly by the incredibly strong winds. Winds around the planet's equator can reach 1,800 kilometres (1,100 miles) an hour. High winds an Earth would be very light winds on Saturn.

A mission to Saturn

In 1997, the Cassini spacecraft blasted off for Saturn with a spaceprobe named Huygens on board. Cassini went into orbit around Saturn in June 2004 and Huygens was dropped onto its largest moon, Titan, in January 2005.

Titan is very difficult to study because of its thick orange clouds. Huygens is now the furthest human-made object ever to land on a celestial body. I'm still here!

Huygens transmitted data for about five hours, but only about two hours was picked up before Cassini lost the signal after moving over Titan's horizon and therefore out of range. It remained active for far longer than anyone had hoped.

Vital statistics

  • Cassini sent back lots of information because of some amazing features:

  • The craft contains more than 12 km (7.5 miles) of wire.

  • It is the size of a bus coach, but is only a little heavier in weight than an elephant.

  • More than half its weight is made up by its fuel.

  • It carries a camera that could spot a coin from almost 4 km (2.5 miles).

Do you know?

Jupiter, Saturn and Uranus all have rings, even though we can't see them.

Distant Twins

Uranus and Neptune are the seventh and eighth planets from the Sun, and are often referred to as twins because of their similar size and make-up.


Uranus was discovered in 1781 by William Herschel. He named it Georgium Sidus, or George's Star, in honour of King George III of Great Britain. This name was not popular, so was later renamed Uranus.

  • Unlike the other planets, Uranus orbits the Sun on its side.

  • In Greek mythology Uranus is the god of heaven.

Unusual seasons

It takes 84 years for Uranus to orbit the Sun. The poles each experience 42 years of “winter", then 42 years of “summer". For 21 of those years, they are each in continual darkness or light.


Uranus looks a lot plainer than this false colour suggests, but this image gives astronomers a lot of information about the planet. Uranus is encircled by at least 11 narrow rings made of rocks and dust. This close up picture of uranus shows rings.


A blue planet, Neptune is named after the ancient Roman god of the Sea, Neptune takes 165 years to orbit the Sun. So since its discovery in 1846, it hasn't completed a full orbit. It has done on 2011.

These two pictures show the rings of Neptune. The planet has at least four faint rings made up of dust particles. Neptune is a very cold planet. This isn't surprising as it is 30 times further away from the Sun than the Earth. Imagine, from Neptune the Sun probably looks as small as the stars look to us. A day in Neptune lasts 16 hours 7 minutes

Neptune's Moon

Neptune has 13 known moons. Its largest moon, Triton, is bitterly cold and has a heavily pitted surface as well as active volcano-like eruptions.

Big storm

Neptune is incredibly stormy. In 1989 Voyager 2 discovered a storm the size of Earth on Neptune's surface. The storm lasted several years.

Pictures from space

Voyager 2 has flown past both Uranus (in 1986) and Neptune (in 1989). It discovered ten of Uranus's moons and six of Neptune's. Most information had to be gathered in just a few hours as it sped on its way.

Neptune, taken by Voyager 2 in 1989, showing two dork storms (the Dark Spots) and the fast-moving clouds Scooter


Pluto, a ball of ice and rock, was discovered in 1930, and became our solar system's ninth planet. However, in 2006 it was reclassified as a dwarf planet.

Sun light take five and half hours to reach Pluto. Pluto doesn't lie in the same plane as the eight planets of the solar system. Pluto's surface is thought to be frozen nitrogen, which gives off gas if its temperature is raised. Scientists learned a lot about Pluto when New Horizons reaches the planet.

Pluto has three moons. The largest is called Charon. It moves around Pluto every six days. Two smaller moons, Nix and Hydra, were discovered in 2005.

Why Isn't It a planet?

Pluto was reclassified as it is just one of many objects in what is known as the Kuiper Belt. Other named dwarf planets in our solar system are Ceres and Eris.

The Kuiper Belt is a band of comet-like objects that orbit the Sun beyond Neptune.

A frozen world

At times, the surface of Pluto has an atmosphere. It appears when Pluto is closer to the Sun and its ice is warmed to release gas, but it freezes and disappears when Pluto moves farther away from the Sun.

So how big is Pluto?

Pluto is estimated to be about 2,300 km (1,400 miles) in diameter. Eris is some 3,000 km (1,864 miles) in diameter. Even some of the solar system's moons are larger than Pluto, including our moon!

Pluto's orbit

Pluto orbits the Sun slightly differently to the main planets. Its orbit also takes it nearer the Sun than Neptune for part of the 248 years of its orbit.

Do you know?

  • Eris was only discovered in 2003.

  • Ceres is the largest asteroid in the asteroid belt.

A mission to Pluto

In 2006, the US space agency NASA launched a spacecraft called New Horizons that is headed for Pluto. It takes ten years to make the 5 billion km (3 billion mile) journey.

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