EARTH

EARTH
From space our home planet, Earth, appears to be mainly blue in colour. This is because of the colour of the oceans which cover over two-thirds of its surface. The land areas, or continents, cover less than a third. The layer of air above the surface is thin, but makes life on Earth possible.

What causes day and night?

Almost every place on Earth has a time when it is light(day), followed by a time when it is dark(night). Day and night come about because Earth spins round in space, and different parts of its surface face the Sun. It is daytime when a place is on the side of Earth facing the Sun. It becomes night when the place is on the side of Earth facing away from the Sun.

What makes Earth different?

A number of things make Earth different from other planets. It is covered with great oceans of water, and its atmosphere contains lots of oxygen. The atmosphere also acts like a blanket, holding in enough of the Sun’s heat to keep Earth at comfortable temperature. The water, the oxygen and the temperature make Earth a suitable place for living things - at least one-and-a-half million different kinds of plants and animals.

How has Earth changed?

Earth formed about 4,600 million years ago when bits of matter in space come together. At first Earth was a great molten ball. It gradually cooled and the atmosphere and oceans eventually formed. In time, it changed into the world we know today, made up of layers of rock with a metal core. Our world is still changing. Currents in the rocks beneath the crust widening the oceans, and driving the continents further apart.

What causes the seasons?

The changes in weather that we call the seasons happen because of the way Earth’s axis is tilted in space. Because of this tilt, a place leans more towards the Sun and is warmer at some times of the year than at others. It is this that causes the changing seasons. The place tilted towards the Sun has summer, while the place leaning away has winter.

EARTH DATA

Diameter at equator : 12,756 km

Average distance from the sun : 149.6 million km

Turns on axis : 23 hours 56 minutes

Year length : 365.25 days

Surface temperature : -89°C to 58°C

Satellites : 1(the Moon)

NEPTUNE AND PLUTO

NEPTUNE AND PLUTO

Neptune and Pluto were the last planets to be discovered. They lie thousands of millions kilometres away from Earth, at the edge of the Solar System. Neptune is a gas giant, very like Uranus. Pluto is a tiny ice ball, smaller than our own Moon.

Why is Neptune blue?

Neptune is a lovely blue colour, rather like Earth. This colour comes about because the atmosphere contains a gas called methane. Methane absorbs the red colours in sunlight, and makes the light coming from Neptune’s atmosphere appear blue. Dark spots that sometimes appear in Neptune’s atmosphere are violent storms.

Does Neptune have moons?

Through a telescope, we can see two moons circling around Neptune – Triton and Nereid. When Voyager 2 visited the planet, it found six more. One, Proteus, was slightly bigger than Nereid, but the others were tiny. Triton is by far the biggest moon, measuring some 2,700 kilometres across. Unusually, it circles the planet in the opposite direction from most moons.

What is Neptune like?

Neptune has a similar make-up to its twin planet, Uranus. It has an atmosphere made up mainly of hydrogen, together with some helium. Beneath this there is a huge, deep, hot ocean of water and liquid gases, including methane. In the centre, there is a core of rock, which may be about the same size of Earth.

When did Voyager 2 visit Neptune?

Neptune was the last planet Voyager2 visited on its 12 – years journey. Launched in 1977, Voyager 2 passed about 5,000 kilometres above Neptune cloud tops on August 24, 1989 – close than to any other planet. By then, it was more than 4,000 million kilometres from Earth, and its radio signals too more than four hours to get back.

Who found Pluto?

United States astronomer Percival Lowell built his own observatory, and led a search for a ninth planet. An astronomer who worked there, Clyde Tombaugh, finally discovered it in 1930.

What do we know about Pluto?

We do not know much about Pluto because it is so far away. At the furthest, it travels more than 7,000 million kilometres from the Sun. Even in powerful telescope, it looks only like a faint star. So far, no space probes have visited the planet. All we know is that Pluto is a deep – frozen ball of rock and ice. It probably has a covering of ‘snow’, made up of frozen methane gas.

What is special about Charon?

Pluto’s only a moon, Charon, is a unique in the Solar System, as it is half as big across as Pluto itself. No other moon is as big compared with its planet. Also, circles Pluto in the same time it takes Pluto to spin round once. This makes Charon appear fixed in the Pluto’s sky.

What would Charon look like from Pluto?

Because it appears fixed in Pluto’s sky, Charon can only be seen from one side of the planet. From that side, Charon would appear huge, much bigger than the Moon does on Earth. This is because Charon circles very close to Pluto, only about 20,000 kilometres away. Our Moon circles 20 times further from us.

NEPTUNE DATA

Diameter at equator : 49,500 million km

Average distance from Sun : 4,500 million km

Minimum distance from earth : 4,300 million km

Turns on axis : 17 hours 6 minutes

Circles Sun : 165 Earth-years

Temperature at cloud tops : -210°C

Satellites : 8


PLUTO DATA

Diameter at equator : 2,250 km

Average distance from Sun : 5,900 km

Minimum distance from earth : 4,300 km

Turns on axis : 6 Earth-days 9 hours

Circles Sun : 248 Earth- years

Surface temperature : -230°C

Satellites : 1

MERCURY

MERCURY

Mercury is the planet closest to the Sun. It is also the fastest-moving planet, whizzing round the sun in just 88 days. Being close to the Sun, Mercury gets extremely hot. Its surface is covered in thousands of craters, making it look rather like a moon.

How big is Mercury?

Mercury is the smallest of the rocky, Earth-like planets. With a diameter of only 4,880 kilometres, it is less than half the size of the Earth. The planet Pluto, a deep-frozen ball of rock and ice, is even smaller.

What is strange about Mercury’s orbit?

Most planets have a nearly circular orbit, or path, around the Sun. Mercury, however, has an oval orbit. At times it travels as far as 70 million kilometres away from the Sun. At others, it gets as close as 46 millions kilometres.

Why does Mercury get so hot?

As it travels around the Sun, Mercury spins so slowly on its axis that a point on its surface stays in the Sun for nearly six Earth-month at a time. With the Sun so close and shining for so long, surface temperatures on Mercury soar to 430°C- hot enough to melt metals such as tin and lead.

What has shaped Mercury’s surface?

Billions of years ago, all the planets were bombarded by huge meteorites. On Earth, most craters made up by the meteorites have been worn away by the action of the weather. Mercury has no weather because it has almost atmosphere. So all the craters that formed ages ago remain, and the whole planet is covered with them. A huge one, called the Calories Basin, was made by a giant meteorite that send shock waves throughout the planet.

What is Mercury made of?

Like the Earth and the other rocky planets, Mercury is made up of different layers. Underneath a rocky crust there is a rocky mantle, and at the centre, a huge metal core. The shrinking of the core has caused great ridges, up to 3 kilometres high, to appear on the surface.

Have any space probes visited Mercury?

Only one space probe has flown to study Mercury. Named Mariner 10, it flew to the planet in 1974, after visiting Venus. Its picture revealed for the first time that Mercury looked rather like some parts of the Moon. Mariner 10 flew past Mercury twice more. On the last occasion, in March 1975, it skimmed only about 300 kilometres above the surface.

MERCURY DATA

Diameter at equator: 4880km

Mass : 0.06 times Earth’s mass

Average distance from the Sun : 58 millions km

Minimum distance from the Earth : 91 millions km

Length of day : 59 Earth-days

Length of year : 88 Earth-days

Temperature : -185°C to 430°C

Satellites : 0

VENUS

VENUS

Venus is the planet whose orbit comes closest to Earth. We often see it shining in the western sky after sunset, which is why it is known as the Evening Star. Venus is a near twin of Earth in size, but it is waterless world with a scorching climate.

Why does Venus change shape?

From Earth, Venus seems to change its shape and size as time goes by. This is because it orbits closer to the Sun than Earth. When it is on the far side of the Sun, we see it as a small circle. As it moves closer to Earth, it gets bigger, but it as a part-circle. Finally, it is just a thin crescent.

What is the surface of Venus like?

Space probes have shown that great plains cover much of Venus’ surface. There are two big highland regions, which we can think of as continents. One is found in the north, and is called Ishtar Terra. The other lies near the equator, and is called Aphrodite Terra.

Why is Venus so cloudy?

We cannot see Venus’ surface from Earth because of thick clouds in its atmosphere. These clouds are not like the clouds we find on Earth, which are made up of tiny water droplets. Venus’ clouds are made up of tiny droplets of sulphuric acid, one of the strongest acids we know. The sulphur has found its way into the atmosphere from the many volcanoes that have erupted on Venus over the years.

How can we see through Venus’ clouds?

Space probes can see through Venus’ clouds and show us what the planet’s surface is like. But they do not ‘see’ in ordinary light. They ‘see’ with radar beams, because radar beams can go straight through the clouds. The most successful radar probe, name Magellan, mapped the whole planet between 1990 and 1992.

What is Venus made of?

Venus is nearly the same size as Earth, and is probably similar in make-up. It has a hard rocky crust, but no great oceans as Earth has. Venus is far too hot for water to remain in liquid form. Beneath the crust is a mantle of heavier rock, and at the centre is a metal core, which may b partly liquid.

Why is Venus so hot?

The average temperature in Venus is more than twice as hot as an oven set on ‘high’. This is because its atmosphere contains mainly carbon dioxide – a heavy gas that trap heat. Over the years it has caused the atmosphere to trap more and more heat, as a greenhouse does. The cloud layers trap the heat too, making the temperature reach a scorching 480°C.

VENUS DATA

Diameter at equator : 12,100 km

Average distance from Sun : 108 million km

Minimum distance from Earth : 42 million km

Turns on axis : 243 Earth-days

Circles Sun : 225 Earth-days

Surface temperature : 480°C

Satellites : 0

THE PLANETS

THE PLANETS

The nine planets are the most important members of the Sun’s family. In order of distance from the Sun , they are Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune and Pluto. The first four are small rocky bodies. The next four are giants , made up mainly of gas . Pluto is a tiny ball of rock and ice.

Which is the biggest planet?

Jupiter is by far the largest of the planets. It has more mass than all other planets put together. It measures nearly 143,000 kilometres across, which is 11 times bigger than Earth. Even though it is so big, it takes less than 10 hours for it to spin round once. This means that its surface round at a speed of 45,000 kilometres an hour. This is 30 times faster than Earth spins.

Which planets have rings?

Once it was thought that Saturn was the only planet that had rings around it because they were the only one that can be seen through a telescope. But close up photographs taken by the Voyager space probes have shown us that the other three gas giants- Jupiter, Uranus and Neptune-have rings too. The rings around the these other planets are much thinner , narrower , and darker than Saturn’s.

Why is Uranus sometimes called ‘new’?

Astronomers have been studying the planets for thousand of years. They have watched the way they move, or ‘wander’ across the night sky, unlike the stars. But the ancient astronomers could only see five planets in the night sky. It was not until 1781 that someone built a telescope powerful enough to spot another planet, which came to be called Uranus. Uranus was the first of three ‘new’ planets to be discovered. Neptune was discovered in 1846, and Pluto in 1930.

What is special about Saturn?

Two things are outstanding about Saturn. One is obvious when you look at the planet through telescope. The planet is surrounded by a set of bright, shining rings. Many people think that these make Saturn the most beautiful object in the Solar System. The other special thing about Saturn is that it is the lightest(least dense) of all the planets. It is lighter even than water. This means that if you could place it in huge bowl of water, it would float.

Which planet is furthest from the sun?
As far as we know, the most distant planet from the Sun is Pluto, the last ‘new’ planet to be discovered after Neptune. But Pluto is not always the furthest away. For 20 years between 1979 and 1999, Neptune was further still because during this time Pluto was travelling inside Neptune’s orbit. Neptune will become the furthest planet again in a little over 200 years’ time. Pluto travels more than 7000 million kilometres away from the Sun. It takes nearly 248 Earth-years to circle the Sun once.

URANUS

URANUS

Uranus is the third biggest planet, and is four times bigger across than Earth. It is so far from the Earth that it is barely visible with the naked eye. Because of this it was not discovered until 1700s, with the help of a telescope

Who discovered Uranus?

In March 1781, an English astronomer named William Herschel was looking at the sky through a telescope. He spied what he thought must be a new comet. But it was actually a new planet. Until then, astronomers knew of only six planets. The new planet, which was late called Uranus , turned out to be twice as far away from the Sun as Saturn.

Why is Uranus sometimes called the topsy-turvy planet?

All planets spin as they orbit the Sun. We say they spin round their axis (an imaginary line that goes through their north and south poles). In most planets the axis is nearly upright as the planet spins. But Uranus spins on an axis at right-angle to normal, so it is as if Uranus is lying on its side. This means that, at times in its orbits, Uranus’ poles point straight at the Sun. As a result, they become hotter than the rest of the planet, instead of always being colder, as on Earth.

How many rings does Uranus have?

Astronomers use to think that Saturn was the only planet that had rings circling it. But, in 1977, they discovered that Uranus had rings too. There are about 11 main rings, made up of bits of rock up to a metre across, which whizz round the planet at high speed. The particles in some of the rings are kept in place by tiny ‘shepherd’ moons.


Which probe has visited Uranus?

We can find out very little about Uranus through telescopes because it is so far away. Most of what we know comes from Voyager 2 space probe, which visited Uranus in 1986. Voyager 2 had earlier visited Jupiter(1979) and Saturn(1981). It has now gone far beyond the planets, and will soon leave the Solar System and begin a journey to the stars.

What is Uranus made of?

Uranus has a thick atmosphere of hydrogen, helium and methane ice, and a mantle of water, ammonia and methane ice. At the centre there is an iron silicate core.
What is Uranus’ moons like?
We can only see the five largest of Uranus’ moons from Earth – Miranda, Ariel, Umbriel, Titania and Oberon. Ten smaller moons were discovered by Voyager 2. The large moons are great balls of rock and ice, pitted with craters, and with long cracks in their surface. Titania is the biggest moon. It is about 1,600 kilometres across.

What is special about Miranda?

Miranda is the smallest moon that can be seen from Earth, with a diameter of only about 500 kilometres. Close-up photographs show it to be the most interesting moon of all. Its surface is a patchwork of different kinds of landscape – craters, grooves, cliffs and valleys. Astronomers think that, ages ago, Miranda shattered into pieces when it collided with another body. Then the pieces came together to create the landscape we see today.

URANUS DATA

Diameter at equators : 51,000 km

Average distance from Sun : 2,870 million km

Minimum distance from Earth : 2,600 million km

Turns on axis : 17 hours 14 minutes

Circles Sun : 84 Earth-years

Temperature at cloud tops : -200°C

Satellites : 15

SATURN

SATURN

Saturn is the biggest planet, after Jupiter. Like Jupiter, it is a giant ball of gas. Saturn is a favourite planet among astronomers because of its shining rings. The rings appear to change shape year by year as the planet makes its way round the Sun.

What are Saturn ‘s rings made of?

Saturn is surrounded by many rings, but only three can be seen from Earth. The other rings were discovered by space probes. The rings look like solid sheets, but they are not. They are made up of millions upon millions of bits of ice, whizzing round the planet at high speed. The bits vary in size from specks of dust to large chunks. In places, the rings are less than 50 metres thick.

Why is Saturn so cloudy?

Saturn is a very cloudy planet. The clouds form into bands parallel to the equator because the planet is spinning round so fast. These bands are not as easy to see as they are on Jupiter because of the haze that tops the atmosphere. There seem to be three main cloud layers of Saturn, located at different level, with clear areas in between. The upper layers of cloud are made up of ammonia and ammonium compounds. At the lowest level, the clouds seem to be made up of water and ice particles, like the clouds wa have on Earth.

What’s Saturn like inside?

Saturn is a gas giant, which means that it is composed mainly of gas and iquid gas. Its cloudy atmosphere is made up almost entirely of hydrogen and helium. Below that lies a vast, deep ocean of liquid hydrogen. Deeper down is a layer of hydrogen in the form of a liquid metal. At the centre of the planet, there is a small core of rock.

What are Saturn’s moons like?

Saturn has at least 18 moons – more than other planet. Only five have a diameter greater than 1,000 kilometres – Tethys, Dione, Rhea, Titan and Lapetus. The smallest, Pan, is only about 20 kilometres across. Biggest by far is Titan. With a diameter of 5,140 kilometres, it is the second largest moon in the whole Solar System, and the only one that has a thick atmosphere.

What is Titan’s surface like?

Titan’s thick atmosphere is made up mainly of nitrogen gas. It is orange in colour and full of hazy clouds that stop us seeing what its surface is like. Astronomers reckon that it may be covered with great lakes or seas of liquid methane, and there may be land areas covered with methane ice and snow. In 2004, the Cassini space probe will drop a landing probe(Huygens) on to the surface, which should tell us what conditions are like there.

SATURN DATA

Diameter of equator : 120,000 km

Diameter of visible rings : 270,000 km

Average distance from Sun : 1,427 million km

Minimum distance from Earth : 1,200 million km

Turns of axis : 10 hours 40 minutes

Circles Sun : 29.5 Earth-day

Temperature at cloud tops : -170°C

Satellites : 18 known