Download - children s illustrated encyclopedia Stars and Planets...galaxies. Matter inside the galaxies went on clumping together until stars were created (see page 7). Our own Sun was born in

cchhiillddrreenn��ss iilllluussttrraatteedd eennccyyccllooppeeddiiaa

SSttaarrss aannddPPllaanneettss

Orpheus

18 MERCURY

19 VENUS

20 EARTH

21 MOON

22 MARS

23 JUPITER

24 SATURN

25 URANUS

26 NEPTUNEVoyager 2

27 PLUTO

28 MOONSSeven largest moons

30 COMETS

31 ASTEROIDS

32 INDEX

C O N T E N T S

3

CONTENTS

U N I V E R S E

4 UNIVERSEComposition of the Universe • Big Bang

6 GALAXIESMilky Way Galaxy

7 STARSLife cycle of a star

8 BLACK HOLESOrigins of black holes • Einstein’s General Theory • Quasars

10 CONSTELLATIONS

S O L A R S Y S T E M

12 SOLAR SYSTEMMembers of the Solar System • Early astronomers

14 THE SUNInternal layers • The surface of the Sun Death of the Sun

16 THE PLANETSRelative sizes and distances • Exploring the planets • The Planets form

First published in 2009 by Orpheus Books Ltd., 6 Church Green, Witney, Oxfordshire OX28 4AW England

www.orpheusbooks.com

Copyright © 2009 Orpheus Books Ltd

Created and produced by Orpheus Books Ltd

Text Nicholas Harris

Consultant David Hawksett, researcher in planetary science, University of Lancaster

Illustrators Susanna Addario, Elisabetta Ferrero, Giuliano Fornari, Andrea Ricciardi di Gaudesi,

Gary Hincks, Shane Marsh, Lee Montgomery, Steve Noon, Sebastian Quigley, Alessandro Rabatti, Eric Robson, Claudia Saraceni, Roger Stewart, Thomas Trojer, Mark Wilkinson, Martin Woodward, David Wright

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical,

photocopying, recording or otherwise, without the prior written permission of the copyright owner.

ISBN 978 1 905473 56 4

A CIP record for this book is available

Printed and bound in Singapore

C O N T E N T S

2

Photograph on page 9: The Illustrated London News Picture Library

About a billion years after the Big Bang,the clouds of gas started to form intogalaxies. Matter inside the galaxies went onclumping together until stars were created(see page 7). Our own Sun was born in thisway about 5 billion years ago. Its family ofplanets, including our Earth, was formedfrom the debris spinning round the infantSun (see page 17).With billions and billionsof stars and planets forming in the sameway across the Universe, it seems almostcertain that life will have also evolvedelsewhere. Will we on Earth one day makecontact with these alien life-forms?The expansion of the Universe is slowing

down. Some astronomers think that gravitymay eventually bring the expansion to ahalt, then collapse all matter once more to asingle point in a “Big Crunch”. Othersbelieve that there is not enough material inthe Universe to do this and that theUniverse will carry on expanding forever.

U N I V E R S E

5

UNIVERSE

EVERYTHING that we can think of —and everything else that exists—all

belong to the Universe. From grains of sandto tall buildings, from particles of dust togiant stars and planets, from microscopicbacteria to people—all are part of theUniverse. It even includes empty space.The Universe is unimaginably vast:

billions upon billions of kilometres wide.Distances in the Universe are so great thatwe have to use a special measure to recordthem. This is a light year, or the distancethat light, which moves at a speed of about300,000 kilometres per second, travels inone year: about 9,460,528,405,000kilometres. The nearest star to Earth (afterthe Sun), Proxima Centauri, is 4.2 lightyears away. The most distant objects weknow in the Universe are more than 13billion light years away from Earth.

It is possible that the Universewill carry on expanding forever. In

this sequence, the Universe is createdin an immense explosion called the BigBang. It expands rapidly, with all thegalaxies moving away from one another asthe Universe inflates like a balloon.

The Universe iscomposed ofmany galaxysuperclusters,themselvesmade up ofclusters ofgalaxies. One ofthese contains theMilky Way Galaxy, aspiral-shaped mass ofabout 200 billion stars, oneof which is our own Sun, parentto a family of nine planets.

The third planetfrom the Sun is

Earth, orbited bythe Moon. Earth is

the only world in theUniverse where life is

known to exist, but wemay discover others one day.

Many scientists think that all matterin the Universe will eventually collide:the “Big Crunch”. Vast amounts of invisible“dark matter” in the Universe may exertsufficient gravity to halt its expansion andcause the galaxies to compress together.

U N I V E R S E

4

Nearly all the matter in the Universe iscontained in galaxies, enormous masses ofstars, gas and dust (see page 6).There may beabout about 100 billion galaxies, eachcontaining hundreds of billions of stars.Galaxies are grouped into giant “clouds” ofgalaxies, called superclusters. These arespread round the Universe like a net, madeup of strings and knots. In between thereare gigantic empty spaces.The superclusters are, themselves, made

up of smaller clusters of galaxies. One ofthese, a cluster of 30 galaxies or so, is calledthe Local Group. It contains the Milky WayGalaxy, the vast spiral of stars to which ourown local star, the Sun, belongs.Astronomers have discovered that all

galaxies are rushing away from one another.This means that, a long time ago, they wereonce all close together. So the Universe hada definite beginning—and may have an end.

Universe Living environment

Earth andMoon

(AB)UK TextJOB NO:E7-05129 TITLE:SCIENCE

150# DTP:48 PAGE:32 (AB)UK TextJOB NO:E7-05129 TITLE:SCIENCE

150# DTP:48 PAGE:33

Galaxy

SolarSystem

B I G B A N GMany astronomers believe that the Universebegan life in a single momentous event.This was an incredibly hot, dense explosioncalled the Big Bang, which took placeabout 15 billion years ago. During thisexplosion, all matter, energy, space—andtime itself—were created.In the first few millionths of a second, the

particles that make up atoms, the buildingblocks of all matter, were formed. It tookabout 100,000 years for the first atoms,those of the gases hydrogen and helium, tocome together. By this time, the searingheat of the Big Bang had cooled, space hadexpanded and the gases began to spreadout. Gradually, however, gravity drew thegases together, leaving vast regions of emptyspace in between.

A star begins its life as a dense mass of gas and dust called aprotostar (1). The core becomes so hot that nuclear reactionsstart deep inside it. Gas and dust are blown away (2),although some remain in a disc surrounding the new star.Planets may form here (3). The star is now a main sequencestar (4). When the fuel it uses to produce energy runs out, thecore collapses and the star swells into a red giant (5). Amassive star will become a supergiant that will blast apart ina mighty explosion called a supernova (6). It ends its days asa neutron star or a black hole (7). A red giant will puff awayinto space, leaving behind a white dwarf.

U N I V E R S E

7

GALAXIES

GALAXIES are gigantic collections ofstars. The galaxy in which the Sun is

situated, the Milky Way Galaxy, is a vastspiral of about 200 billion stars measuringabout 100,000 light years across. There arebillions more galaxies in the Universe, mostof which are elliptical (oval) in shape. Thereare also others that have irregular shapes.The Milky Way has a bulge at its centre,

the nucleus, where older red stars areconcentrated. Four giant arms radiate outfrom the nucleus. These contain youngerblue stars as well as areas of gas and dust—the raw material for the creation of newstars. The whole spiral spins at a speed ofabout 250 kilometres per second.

This illustration (below) is a view of the Milky Way Galaxyseen from the side. It looks like a pair of fried eggs stucktogether back-to-back. The “yolks” form the central bulge,

or nucleus, while the “whites” form the spiral-shaped,flattened disc surrounding it. The Sun lies about 25,000light years (halfway out) from the central point.

The Horsehead Nebula is really a giganticcloud of dust and gas that has taken on afamiliar shape. It is one of many clouds inour Galaxy where stars start to form.

U N I V E R S E

6

STARS

STARS are giant spinning balls of hotgases. Like massive nuclear powerstations, they produce vast amounts ofenergy in the form of heat and light, whichthey radiate across space as they shine.They may look like tiny points of light in

the night sky, but many stars are incrediblybig. Betelgeuse, in the constellation ofOrion, is 800 times the size of the Sun, ourlocal star. Stars vary enormously accordingto the amount of light they emit. Some ofthe most powerful give off more than100,000 the light of the Sun, while othersare 100,000 times weaker.Stars are born when clouds of dust and

gas in space, known as nebulae, compresstogether under the force of gravity tobecome dense “blobs”, called protostars. It isnot certain why this happens. Maybe thepressure of an exploding star nearby at theend of its life triggers the process. After a star has formed it becomes a

stable “main sequence” star. The Sun is atypical star of average brightness. Moremassive stars, like Rigel (also in Orion),glow blue-white, while at the other end ofthe scale, a white dwarf, the collapsed coreof an old star, is no bigger than the Earth.

Sun

Rigel

Betelgeuse



150# DTP:48 PAGE:35

The Sun compared insize to a white dwarf(right).

A red giant’s outerlayers eventuallyflake off into space,leaving a planetarynebula (right). Asupergiant goes outin a supernova. TheCrab Nebula is theremains of one(below).

The Milky Way Galaxyclosely resembles theAndromeda Galaxy, whichlies 2.25 million lightyears away. The Sun issituated on one of thespiral arms about halfwayout from the nucleus.Here are mostly yellowand orange young-to-middle aged stars.

1

4

7

3

5

2 6

If it weighs more than the three Suns, itsqueezes further. An escape velocity of thespeed of light would be needed to travelaway from it. Any light rays would be pulledback in, so the object is invisible: a blackhole.

E I N S T E I N ’ S G E N E R A L T H E O R YThe great German physicist Albert Einstein(1879-1955) found another way to explainhow space, light and matter would behaveclose to a black hole. In his General Theoryof Relativity of 1915, Einstein proposed thatthe gravitational pull of an object wouldresult in the “curving” of space, in the sameway that a person can curve a trampoline. Amassive object creates a large “dent” in spaceinto which light and matter would fall. Thedenser the object, the greater the dent. Sothe Sun would make only a shallow dent,whereas a neutron star would create a verydeep dent. A black hole, the densest object ofall, creates a dent so deep that nothing canescape from it.

U N I V E R S E

9

BLACK HOLES

BLACK HOLES are the strangest objectsin the Universe. No-one has ever seen

one, but most astronomers are convincedthat they exist. They are tiny regions ofspace surrounded by a force of gravity sostrong that nothing, not even light, canescape from them.All bodies in space exert a force of

gravity, the force which attracts other thingstowards them. The greater an object, thestronger its gravitational pull, and the harderit is to escape from it. A rocket launchedfrom Earth must go faster than 40,000kilometres per hour (its “escape velocity”)to escape Earth’s gravitational pull. The Sunis many thousands of times more massivethan Earth, so a rocket would have to travelmuch faster: more than 2 million kilometresper hour. If there was an object muchbigger or denser than the Sun, an escapevelocity equal to that of the speed of lightmay be needed to escape from it.Where might an object of such high

density be found? Stars more than 10 timesas heavy as the Sun burn up their fuel in amuch shorter time—a few million years,compared to the Sun’s 10 billion years.Theyswell into massive supergiants beforeblasting apart in supernovas (see page 7). Asupernova’s core compresses in seconds to atiny, super-dense body called a neutron star.

Black holes are invisible, but it is possibleto detect them by studying their effects.Astronomers observing a star calledCygnus X-1 saw that it was giving offenormous amounts of energy (a suresign of violent activity in theUniverse). They discovered thatthis huge, hot blue star wasbeing dragged around in acircle by an unseen objectwith a huge gravitationalpull. That unseen object,astronomers now believe, isa black hole, which istearing gas from the star.The gas forms a whirlingdisc before plummetinginto the black hole. As itfalls, it travels faster andfaster until it movesalmost at the speed oflight itself. Close to thehole, the gas becomesso hot it emits massiveamounts of energy.

Billions of light years away, a huge, disc ofgas and dust swirls around a giant blackhole at the core of a quasar. The incredibleenergy blasts two jets of particles—thecomponent parts of atoms—out into space.

Imagine a star in space as ball on a rubbersheet. A massive object like a star will“bend” space and anything close to it willfall in towards it. If the ball were so heavythat the sheet stretched into a long, deeptube, the result would be a black hole.

U N I V E R S E

8

To Albert Einstein, gravity was a propertyof space and not a force between objects.

Q U A S A R SIncredibly powerful, massive black holesmay, astronomers think, be found lurking atthe centres of galaxies. There could even beone at the centre of our own Milky WayGalaxy. Astronomers have detected a ring offast-moving, hot gas swirling around thecentre. The ring of gas is probably in thegrip of a powerful gravitational pull—mostlikely, astronomers suspect, to be the workof a black hole.The activity at the centre of our Galaxy

is as nothing compared to that of quasars.These objects look like stars, but they lie atincredible distances from us: the farthestquasars are 13 billion light years away. To bevisible at that distance means they must begiving off immense amounts of energy.Quasars are the centres of extremely violentgalaxies containing supermassive blackholes, weighing up to 100 billion Suns. Thebrilliant light comes from the disc of hotgas and dust spiralling into the black hole.


150# DTP:48 PAGE:36JOB NO:E7-05129 TITLE:SCIENCE

150# DTP:48 PAGE:37UK Text (AB)

This chartshows the stars

you can see if youlive in the northern

hemisphere. Turn the bookaround so that the present month is atthe bottom. Then face south at 10 pm.On a clear night you should be able tolocate many of the stars on the chart.

This chartshows the stars

you can see if you livein the southern hemisphere.

Turn the book around so that thepresent month is at the bottom. Then face

north at 10 pm. On a clear night you should beable to locate many of the stars on the chart.

U N I V E R S E

11

A line runningfrom two stars inthe constellationUrsa Major(Great Bear)points to thePole Star, almostexactly duenorth. Years ago,seafarers usedthis observationfor navigation.

U N I V E R S E

10

CONSTELLATIONS

CONSTELLATIONS are areas of thesky, divided up for the purpose of

identifying stars, galaxies and other objectsin the heavens. Years ago, before telescopeswere invented, early astronomers groupedthe stars together into patterns, imaginingtheir shapes to look like gods, heroes andsacred beasts from popular legends. The 88constellations that exist today include 48known to the ancient Greeks, whoinherited some from the Babylonians.

Orion, a hunter inGreek myths (left), isan easy constellationto spot. Three starsin a diagonal lineform his belt, whileothers make up hisdagger and shield.The belt stars pointdown towards Sirius,the brightest star inthe night sky. InGreek myths,Centaurus (right) washalf man, half horse.

ANDROMEDATRIANGULUM

ARIES

AQUILA

AURIGA

BOÖTES

CAMELO-PARDALIS

CANCERCANESVENATICI

CANISMINOR

CASSIOPEIA

CEPHEUS

COMABERENICES

CORONABOREALIS

CYGNUS

DELPHINUS

DRACO

EQUULEUS

GEMINI

HERCULES

LACERTA

LEO

LEO MINOR

LYNX

LYRA

PISCES

Galaxy

AQUARIUS

PEGASUS

OPHIUCHUS

ORION

TAURUS

CETUS

MONOCEROS

HYDRA

SEXTANSVIRGO

SERPENSCAUDA

SERPENSCAPUT

SCUTUM

URSA MAJOR

URSAMINOR

VULPECULA

Arcturus

Vega Capella

Procyon

Altair

Betelgeuse

Merak

Dubhe

Aldebaran

Pleiades

Polaris

ANTLIA

APUS

AQUARIUS

AQUILA

ARA

CANIS MAJOR

CAPRICORNUS

CARINA

CENTAURUS

CETUS

CIRCINUS

COLUMBA

CORONAAUSTRALIS

CORVUS

CRATER

CRUX

DORADO

ERIDANUS

FORNAX

GRUS

HOROLOGIUM

HYDRA

HYDRUS

INDUS

LEPUS

LIBRA

LUPUS

MENSA

MICROSCOPIUM

MONOCEROS

MUSCA

OCTANS

OPHIUCHUS

ORION

PAVO

PHOENIX

PICTOR

PISCES

PISCISAUSTRINUS

PUPPIS

PYXIS

RETICULUM

SAGITTARIUS

SCORPIUS

SCULPTOR

SCUTUM

SERPENSCAUDA

SEXTANS

TRIANGULUMAUSTRALE

TUCANA

VELA

VIRGO

VOLANS

Sirius

Canopus

Large MagellanicCloud

Alpha Centauri

Rigel

Achernar

Betelgeuse

Procyon



150# DTP:48 PAGE:39


13

Nicolaus Copernicus (1473-1543)


12

SOLAR SYSTEM

THE SOLAR SYSTEM consists of theSun and an array of objects that orbit

it. These objects include the eight knownplanets, their moons, a number of dwarfplanets, asteroids, comets, meteoroids andhuge amounts of gas and dust. The Sun’sgreat size relative to the other objects in theSolar System gives it the gravitational pullto keep those objects, including Earth,permanently in orbit around it. The planets orbit the Sun in the same

direction (anticlockwise in this illustration)and in elliptical (oval-shaped) paths. All theplanets, and most of their moons, travel onapproximately the same plane, with theexception of Mercury, which has a slightlytilted orbit. Pluto, now re-classified as adwarf planet, has an orbit somewhat moreelliptical and tilted than the true planets.Constantly streaming away from the Sun

in all directions is the solar wind, made upof electrically-charged particles (parts ofatoms). Travelling at more than 400kilometres per second, it produces electriccurrents inside a giant magnetic “bubble”called the heliosphere. The heliosphereprotects the Solar System from cosmic raysarriving from space. Its edge, some 18billion kilometres from the Sun, marks thetrue boundary of the Solar System.

they moved in epicycles. It was left to theGerman astronomer Johannes Kepler(1571-1630), who showed that the planetsmoved in elliptical, rather than perfectlycircular, orbits. The shapes of their orbitsalso explained the “wandering” that soperplexed earlier observers, thus disprovingthe idea that the planets moved in epicycles.The Italian astronomer Galileo (1564-

1642) was the first to use a telescope. Fromhis observations of the moons of Jupiter inorbit around that planet, and the changingshape of Venus as it orbited the Sun, heconcluded that Copernicus had beencorrect: the planets do orbit the Sun.

circular paths around Earth, the centre ofthe Universe. Ptolemy, who lived in the 2ndcentury AD, observed that, while the starsmoved across the night sky along regularpaths, the planets appeared to “wander”from theirs. He proposed that they eachmoved in their own small circles, calledepicycles, as they orbited Earth.The Polish priest and astronomer,

Nicolaus Copernicus, challenged Ptolemy’sview of the Solar System, declaring that theSun lay at the centre of a system of orbitingplanets. Only the Moon orbited the Earth.Copernicus wrongly believed that theplanets’ orbits were perfect circles and that

E A R LY A S T R O N O M E R SThousands of years ago, in the time of theancient civilizations of Egypt and China,people thought that the Sun and Moonwere gods, the Earth was flat and the skywas a great dome suspended above it. In later years, astronomers from ancient

Greece proved that the Earth was round.Many believed that the stars were fixed to agreat sphere that rotated around the Eartheach day. One Greek astronomer,Aristarchus, proposed that the planets,including Earth, orbited the Sun, a star, butmost astronomers of this time thought thatthe Sun, Moon and planets all travelled in

Sun

Jupiter

Saturn

Uranus

Neptune

Pluto

Mercury

Venus

Mars

Earth

A s t e r o i d s

Orbit of Halley’s Comet



150# DTP:48 PAGE:41

On this diagram of the SolarSystem, the orbits of theplanets are represented bycoloured rings. (The size ofthe planets and their relativedistances are not to scale.) Anumber of unmanned space-craft, called space probes,have been launched to explorethe Solar System. They includePioneer 10 (the first probe),Voyagers 1 and 2, and Galileo. Galileo space probe

Pioneer 10 space probe

Voyager 2 space probe

Voyager 1 space probe


15

About 7 billion years fromnow, the hydrogen that theSun uses as fuel to createenergy will start to run out.Eventually the Sun willballoon into a red giant,engulfing Mercury andperhaps Venus, too. This iswhat Earth’s landscape maylook like when this happens.Its oceans and atmospherehave gone and its rockysurface is melting intemperatures of 1500˚C.Venus is seen as a black dotagainst the Sun. It may soonbe swallowed up by thecolossal star.

By coincidence, the Moon and Sun appearto be the same size in the sky. So whenthe Moon passes between the Earth andthe Sun (seen in the sequence 1-3), it mayblock out our view of the Sun, a solareclipse. During a total eclipse, an eventonly rarely witnessed, the Moon covers theSun’s surface entirely and the coronashines out from behind a black disc. For ashort while, dusk falls. In a partial eclipse,part of the Sun still remains visible.


14

THE SUN

THE SUN is an ordinary star. To us onEarth it is of crucial importance since

no life could exist without it, but it issimply one of billions of stars in the MilkyWay Galaxy (see page 6), itself one of billionsof galaxies in the Universe. For a star, theSun is below average size—someastronomers classify it as a “yellow dwarf ”.Yet it is massive when compared to theplanets. The Sun contains more than 99 percent of all the matter in the Solar System.Its diameter of 1,400,000 kilometres ismore than 100 times that of Earth.The Sun is a spinning ball of intensely

hot gas made up almost entirely ofhydrogen (three-quarters of its mass) and

Invisible lines of magnetic force that twistaround the Sun’s globe are the cause ofmany extraordinary features. Huge arches offire, called prominences, can be held upabove the Sun by magnetism. Flares, sudden,massive explosions of energy, burst forthwhen the magnetic field shifts. Wheremagnetic field lines erupt through thephotosphere, there are dark, cooler areas(about 4300˚C) known as sunspots. Beyond the chromosphere lies the

corona, the Sun’s hot, shimmering outeratmosphere. This is visible from Earth onlyduring a total solar eclipse.

T H E S U R FA C E O F T H E S U NThe Sun’s outer shell, the photosphere, isonly about 500 kilometres thick and, at5500˚C, much “cooler” than at the core. Itis in a state of constant motion, like waterin a boiling kettle. Hundreds of thousandsof flaming gas jets, called spicules, leap up to10,000 kilometres into the Sun’s atmos-phere, known as the chromosphere.

D E AT H O F T H E S U NWhen the Sun’s fuel of hydrogen starts torun out, it will grow into a much biggerand brighter star, called a red giant. It willeventually shed its outer layers into space.All that will remain of the Sun itself will be,at first, a small, extremely dense star (awhite dwarf), before it eventually cools andwastes away (a black dwarf).

helium. It produces massive amounts ofenergy by “burning” about four milliontonnes of hydrogen every second.

I N T E R N A L L AY E R SAt the centre of the Sun is the core, aregion of incredible pressure (200 billiontimes that on the Earth’s surface) andintense heat—about 15 million˚C. This isthe Sun’s nuclear furnace, where the energythat keeps it shining is released. Hydrogenatoms fuse together to form helium. Energyfrom this reaction flows out from the corethrough the radiative zone to the convectivezone. Here, in a continuous cycle, hot gasbubbles up to the surface before sinkingdown to be reheated again.

Core

Earth

(to scale)

Jupiter

Photosphere

3

Radiative zone

Convective zone

Sunspots

Flare

Prominence

2

1

321

Moon

Earth

Sun



150# DTP:48 PAGE:43

In thissequence, theSun grows to thesize of a red giant.


17

2 The collapsedcloud becomesa swirlingdisc ofmatterwith abulge atits centre.

3 Smallfragmentsof rockclump

together inlarge blocks

calledplanetesimals.

4 The core of thedisc becomes aball of hot gas—a star. Thesolar windstrips thefour innerplanets oftheiratmospheres.

5 The SolarSystem today:

the Sun, planets,moons and small

fragments of rock, gas,dust and ice left over from theSolar System’s origins.


16

THE PLANETS

APLANET is a large object in orbitaround a star. It can be made of rock,

metal, liquid, gas or a combination of these.Planets do not produce light, but theyreflect the light of their parent star.In our own Solar System, there are eight

planets, including Earth, orbiting the Sun,our parent star. Observations of other starsmade by astronomers using powerfultelescopes indicate that they, too, haveplanets. There could therefore be billions ofother planets in the Universe.The Earth is the largest of the four inner,

or “terrestrial”, planets: Mercury, Venus,Earth and Mars. They are dwarfed by thefour “gas giants”, Jupiter, Saturn, Uranusand Neptune, so called because they havecomparatively small rocky cores surroundedby thick layers of liquid and gas. Pluto—once a considered a planet, too—is knownas a “dwarf planet”, along with a number ofother small objects in the Solar System.

The solar wind stripped away anyremaining dust and gas, including theatmospheres around the four inner planets.The giant planets lay beyond the solarwind’s fiercest blast, so they were able tohold on to their thick blankets of gas.Jupiter’s gravitational pull caused nearby

planetesimals to destroy one another ratherthan build up into another planet, leaving abelt of rock fragments, known as asteroids,still orbiting the Sun, as they do today.

The diagram at the foot of this pageshows the relative distances of the planetsfrom the Sun. Pacing out their positionswould give an even better idea of the hugedistances between them. If the Sun were afootball, Mercury would be a pinhead 10paces away from it. Earth (the size of apeppercorn) is a further 16 paces on fromMercury, with the Moon a thumb’s lengthaway from Earth. Another 209 paces wouldbring you to Jupiter (a large marble), whilethe dwarf planet Pluto lies 884 paces further.To reach the nearest star, Proxima Centauri,you must walk another 6700 kilometres!

E X P LO R I N G T H E P L A N E T S Because the giant planets lie so far fromEarth, it would take too long for people totravel to them. So space probes have beenlaunched to “fly by” every planet and sendback pictures. Voyager 2 (see page 26) madethe greatest journey. Space probe Cassinivisited Saturn in 2004-05.

1 A shock wave,possibly from anearby supernova,causes a cloud ofgas and dust tocollapse under itsown gravity.

This diagram (below) shows the relative distances of theplanets from the Sun. 1 AU (Astronomical Unit) is the averagedistance between Earth and the Sun.M

ercury 0.39 AU

Venus 0.72 AU

Earth 1 AU

Mars 1.52 AU

Asteroids

Jupiter 5.2 AU

Saturn 9.5 AU

Uranus

19.2 AU

Neptune

30.1 AU

Pluto 29.6 AU

(when nearest the

Sun)

Pluto

49.3 AU

JOB NO:E7-05129 TITLE:SCIENCE150# DTP:48 PAGE:44

JOB NO:E7-05129 TITLE:SCIENCE150# DTP:48 PAGE:45

2

4

3

1

5

6

7

8

UK Text (AB) UK Text (AB)

T H E P L A N E T S F O R MThe Solar System began life as a cloud ofgas and dust drifting across the Milky WayGalaxy. It is thought that a supernova mayhave sent shock waves racing across space,striking the cloud and somehow causing itto collapse under its own gravity.Within 100,000 years, the collapsed cloud

became a swirling disc, called a solar nebula.Under pressure from gas and dust spirallinginwards, the centre became hotter anddenser and began to bulge. It would soonevolve into the infant Sun.Away from this central furnace, particles

of dust began to clump together likesnowflakes, first into small fragments ofrock, then becoming large boulders. Overmillions of years, some grew into blocksseveral kilometres across, calledplanetesimals. These eventually started tocollide with one another, building up likesnowballs to become the four rocky innerplanets, Mercury, Venus, Earth and Mars, andthe cores of the four gas giants, Jupiter,Saturn, Uranus and Neptune.

1 Mercury2 Venus3 Earth4 Mars5 Jupiter6 Saturn7 Uranus8 Neptune

VENUS

ABOUT THE SAME SIZE as Earth,Venus is shrouded in thick, unbroken

clouds made of droplets of deadly sulphuricacid. Because its cloud cover reflects thelight of the Sun from its surface, Venus is avery bright object in the night sky. Some 25 kilometres thick, the clouds

prevent most sunlight from reaching thesurface. But another kind of radiation fromthe Sun, called infrared, does get thoughand Venus’s dense atmosphere stops it fromescaping. The result is a constant surface

Beneath the clouds, Venus’sbarren surface features tensof thousands of volcanoes(some possibly still active)surrounded by vast lavaplains. Lava flows have cutchannels in the ground thatlook as if they may have beencarved by rivers. Odd, dome-shaped volcanoes, or“pancakes”, as they havebeen described, have formedwhere lava has oozed to thesurface, then cooled as itspread out in all directions.

The interior ofVenus is similar tothat of Earth,although its metalliccore is much largerthan Earth’s.

Venus is covered bythick clouds.

They raceround in theplanet injust fourdays.

temperature hotter than the melting pointof lead and the hottest in the Solar System.If any space explorer landed on Venus, he orshe would be simultaneously incinerated,suffocated by the unbreathable carbondioxide air, dissolved by acid and crushed byair pressure about 90 times that on Earth.Venus spins slowly on its axis, actually

taking longer to complete one rotation thanto orbit the Sun. Relative to all the othertrue planets, it spins backwards.


19

MERCURY

MERCURY, the closest planet to theSun, is the second smallest planet in

the Solar System. Because it is so near theSun, it can be seen from Earth only withdifficulty—low in the dawn or twilight skyclose to the Sun. Mercury’s surface looks quite similar to

that of our Moon. Bare and rocky, it iscovered with craters, the result of continualbombardment by meteorites during the firstbillion years of its existence. Originallymolten, Mercury’s surface shrank as it

The landscape of Mercury isdominated by thousands ofcraters. The huge Sun burnswith a fierce heat—turning tosevere cold when this face ofthe planet is turned awayfrom it. Large boulders fallingfrom space have producedcraters in Mercury’s surfacemeasuring many kilometresacross, some with smallercraters inside. Because thereis hardly any atmosphere,Mercury’s skies remain blackeven during the day.

Mercury, thedensest planetapart from Earth,has a large metalcore made of ironand nickel,surrounded by a thinrocky shell.

Mercury’s surfaceis made up ofthousands ofcraters, aswell asmountainsand lavaplains.

cooled after the bombardment eased,resulting in “wrinkles”—long mountainchains. With no winds or water to erode therocks, Mercury’s landscape has remained thesame ever since. Mercury’s orbit has an unusual shape. The

other planets have nearly circular orbits butMercury’s is elliptical—more like an oval. Atits closest, Mercury is 46 million kilometresfrom the Sun, 70 million kilometres away atits most distant.Mercury has great extremes of

temperature. Where it faces the Sun, it canexceed 400˚C, but during the long nights(lasting about 59 Earth days) and with noatmosphere to keep the heat in,temperatures can plummet to -170˚C.

When a meteorite strikes the surface ofMercury, it punches a saucer-shapedcrater in the ground. Debris is blasted outin all directions, creating long streaks.


18

F A C T F I L EDiameter: 4880 kmDay: 58.6 daysYear: 88 days

Average distance from the Sun: 58 million kmSurface temperature: -180 to +430°C

Atmosphere: traces of heliumMoons: none

Crust

Mantle

Core

Mantle

CoreCrust



150# DTP:48 PAGE:47

This is what Venus would look like if itwere not permanently obscured by clouds.The dark areas are lava plains.

F A C T F I L EDiameter: 12,105 kmDay: 243 daysYear: 225 days

Average distance from the Sun: 108 million kmSurface temperature: 490°C

Atmosphere: carbon dioxide, traces of nitrogenNumber of moons: none

MOON

THE MOON is neither a star nor aplanet. It is a ball of rock that travels

around Earth, taking about 27 days tocomplete the circle. It is the brightest objectin the night sky, although the light it“shines” is reflected from the Sun.The Moon may have formed when a

large object or planetesimal (see page 17)collided with the newly-formed Earth morethan four billion years ago. The impact“splashed” into space vast amounts of debristhat later came together to form the Moon.

With neither air nor liquidwater, it is impossible forplants or animals to live onthe Moon. The barren lunarlandscape is pitted withcraters, blasted out bymeteorites crashing to itssurface. Scattered debris hasleft streaks radiating fromsome craters. The Moon alsohas wide, smooth lava plains.Early astronomers thoughtthese were seas. They arestill called by the Latin namefor sea, mare.

The Moon’sinternal structure issimilar to Earth’s,its crust is thickerand not divided intotectonic plates.

A completely barrenworld, the Moon’s

surfaceconsists ofcrateredhighlandsand wideplains.


21

EARTH

OUR OWN PLANET, Earth, is thelargest of the four inner planets.Third

in order of from the Sun, 71% of its surfaceis taken up by oceans. Water is also presentas droplets or ice particles that make up theclouds, as vapour in the atmosphere and asice in polar areas or on high mountains. Liquid water is essential for the existence

of life on Earth, the only body in the SolarSystem where life is known to be present.Earth’s distance from the Sun—neither tooclose nor too far—produces exactly the

In contrast to the barrenlandscapes of the otherplanets, much of Earth’s iscovered by vegetation,including forest, scrub andgrassland. Different climatesdetermine the types of plantsand animals that live indifferent places. Large areasshow the important influenceof humans: for example,farmland, roads and cities.Land areas are continuallysculpted by the weather andmoving water or ice.

At Earth’s core is ahot ball of solidiron surrounded byliquid iron. Abovethat is a thick, rockymantle topped by athin, rocky crust.

right temperature range. The atmospheretraps enough of the Sun’s energy to avoidtemperature extremes. It also screens theharmful rays of the Sun and acts as a shieldagainst bombardment by meteoroids.Earth’s magnetic field is generated by

electrical currents produced by the swirlingmotion of the liquid inner core.Themagnetic field protects Earth from the solarwind (see page 12).Earth’s outer shell, made up of the rocky

crust and partly-molten upper mantle, isdivided into about 15 separate pieces, calledtectonic plates. Volcanoes and earthquakesoccur where plate edges meet.

When Earth lies directly between the Sunand the Moon it casts its shadow on theMoon. This is called a lunar eclipse.


20

F A C T F I L EDiameter: 12,756 km

Day: 23 hours 56 minutesYear 365.26 days

Average distance from the Sun: 149.7 million kmSurface temperature: -70 to +55˚C

Atmosphere: nitrogen, oxygen, water vapourNumber of moons: 1

P H A S E S O F T H E M O O NThe shape of the Moon appears to

change from one night to the next. Thishappens because, as it travels round Earth(above), it spins only once, so the same faceremains pointed towards us at all times. It isour view of the sunlit part that changes.When the face pointed towards us is turnedaway from the Sun, we cannot see theMoon at all: a New Moon (1). When it isturned towards the Sun, we see a completedisc: a Full Moon (5). In between, it passesthrough crescent (2), quarter (3) andgibbous (4) phases, and back again (6-8).

F A C T F I L EDiameter: 3476 km

Average distance from Earth: 384,600 kmDay: 27.3 days

Surface temperature: -155 to 105˚CAtmosphere: none

Inner core

Outercore

Mantle

Crust

1

234

5

67

8

1 2 3 4

5 6 7 8

Beneath swirlingwhite clouds, thegreens andbrowns of theland andblue of theocean canbe seen.

Lowermantle

Upper

man

t le

core

Out e

r

Innercore

Crust



150# DTP:48 PAGE:49

JUPITER

JUPITER is the largest planet in the SolarSystem. Large enough to contain more

than 1300 Earths inside it, Jupiter is moremassive than all the other planets combined.Along with Saturn, Uranus and Neptune,Jupiter is known as a “gas giant”, because itis mostly made of gas with no solid surfaceat all.The colourful patterns of red, brown,

yellow and white on Jupiter’s surface areproduced by the chemicals sulphur andphosphorus in the swirling atmosphere.Jupiter’s extremely quick rotation isprobably responsible both for separating theclouds into different colour “zones” (thelighter bands) and “belts” (the darker bands),and for the continual storms. The GreatRed Spot, its most famous feature, is such astorm. The quick rotation also causes Jupiterto bulge at its equator, so that it measures7500 kilometres less from pole to pole.

Surrounding Jupiter’s rocky core is a layer ofhydrogen so dense that it has metallic properties.

Above that is liquid hydrogen and a thick atmosphere.

The beautiful, ever-changing patterns onJupiter’s globeare violentwinds.

Jupiter has a system of rings consisting ofdark grains of dust. The four largest of itsmoons are bigger than Pluto.

Large enough to containtwo Earths, the Great RedSpot is actually a giantstorm that has beenraging for at least 300years. Its topmost cloudsrotate in an anticlockwisedirection, taking aboutsix days to make acomplete turn. Thissequence shows smallerstorms (the white ovals)and turbulent air currentsflowing past the Spot.

Jupiter’s largest moonsshown in order of

distance from theplanet (top is

nearest).


23

MARS

ALTHOUGH Mars is much smallerthan Earth, the two planets have a

number of similarities. The Martian day isonly a little longer than ours and its angleof tilt means that Mars has four seasons, justas we do on Earth. Daytime temperatures atthe equator in midsummer can sometimesreach 25˚C. Thin clouds of water vapour orearly morning surface frosts can alsosometimes be seen. Like Earth, Mars hasvolcanoes, mountains, dried-up river beds,canyons, deserts and polar icecaps.

A number of valleys andchannels have been carvedinto the Martian plains. Fromthe evidence of sediments—muds and silts deposited bywater—it seems likely thatthere were once rivers, lakesand even seas on Mars. Theonly water left on the surfacetoday is frozen in the polaricecaps. The rest may havebeen lost to space due toMars’ weak gravity, or hiddenfrom view as a deep-frozenlayer beneath the surface.

Mars has quite alow density and avery weakmagnetic field. Thissuggests that it hasonly a relatively smallball of iron at its core.

The “Red Planet”has two tinymoons (below,not to scale).They wereonceasteroids.

Mars’ four massive volcanoes stand on theTharsis bulge. Olympus Mons, which rises20 km, is seen here compared to MaunaKea, Hawaii (green summit, at far right)the tallest mountain on Earth measuredfrom base to summit (10,205 m).

Deimos

Phobos


22

F A C T F I L EDiameter: 6797 kmDay: 24.6 hoursYear: 687 days

Average distance from the Sun: 228 million kmSurface temperature: -120 to +25°CAtmosphere: carbon dioxide, nitrogen

Moons: 2

F A C T F I L EDiameter: 143,884 kmDay: 9.8 hoursYear: 11.8 years

Average distance from the Sun: 778 million kmSurface temperature: -150°CAtmosphere: hydrogen, helium

Number of moons: 63

For these reasons, Mars is thought to bethe only other planet where life may oncehave existed. However, analysis of theMartian soil by space probes Viking 1 and 2,which touched down on the planet in1976, and Pathfinder in 1997, failed to findany sign of past or present life. Mars is a barren planet. Its reddish colour

comes from iron oxide dust (similar to rust).From time to time, large dark regionsappear on the surface. These are areas ofbare rock, exposed when storms remove thedusty covering.The Martian landscapefeatures some dramatic landforms. The SolarSystem’s highest mountains and its deepestcanyon, Valles Marineris, are found on Mars.Mantle

Core

Metis

Adrastea

AmaltheaThebe

Io

Europa

Ganymede

Callisto

LedaHimalia

Lysithea

Elara

AnankeCarme

Sinope

Pasiphae

Atmosphere



150# DTP:48 PAGE:51

Crust

Mantle

Core

Liquid

h ydr

ogen

Metallichydrogen

Core

URANUS

URANUS was discovered in 1781 byWilliam Herschel, an amateur German

astronomer living in England. Morerecently, astronomers found that Uranus istilted 98˚ from the vertical, meaning that itorbits the Sun almost on its side. So formuch of the 84-year-long journey, bothpoles face long periods of continuousdaylight, followed by continuous night.

Uranus’ relatively small, rockycore is surrounded by a slushyocean of water with someammonia. Its thick atmosphereis composed mainly of hydrogen.

Uranus has a family of 11faint rings, none morethan 10 km wide, eachmade up of pitch-blackblocks, measuring only afew metres deep. Theycircle Uranus’ equator.

Uranus’ moons, in orderof distance from theplanet (top is nearest).Twelve more moons haverecently been discovered.

Uranus’ moon Miranda may once havebeen smooth (1) before it was blastedapart by a meteoroid (2). The fragmentswere reassembled by gravity (3), butMiranda now became a lumpy ball ofmixed-up pieces (4).


25

SATURN

ALL FOUR gas giants have rings, butSaturn’s, visible from Earth through

even a small telescope, are broad, bright andmagnificent. As detailed photographs takenby Voyager 2 show, the rings are made up ofbillions of blocks of ice and rock, ranging insize from boulders as large as houses downto tiny fragments the size of snowflakes (topright). They are only a few tens of metresthick. Some astronomers think that therings are the fragmented remains of a moonthat was smashed apart by a passing comet.Three rings can be made out from Earth.

The outer ring ( A ring) is separated fromthe other two lying inside it (B and C) by agap called the Cassini Division.Voyager 2spotted fainter rings beyond A ring. It alsorevealed that each ring was, itself, dividedinto thousands of ringlets.Saturn has a large family of moons, many

of which are small, irregularly shaped bodieswith some even sharing the same orbits.

Saturn’s internal structure(above) is similar to Jupiter’s.

Saturn’s spinning axis is tiltedso our view of its rings altersas it orbits the Sun (below).At stages 1 and 7 they areinvisible. At 4 and 10 theyare at their widest angles.

24

F A C T F I L EDiameter: 120,514 km

Day: 10.2 hoursYear: 29.5 years

Average distance from Sun: 1427 million kmSurface temperature: -180˚CAtmosphere: hydrogen, helium

Number of moons: 60

F A C T F I L EDiameter: 51,118 kmDay: 17.2 hoursYear: 84 years

Average distance from the Sun: 2869 million kmSurface temperature: -210˚C

Atmosphere: hydrogen, helium, methaneNumber of moons: 27 Cordelia

Ophelia

Bianca

CressidaDesdemona

JulietPortia

RosalindBelinda

Puck

Miranda

Ariel

Umbriel

Titania

Oberon

Pan

Atlas

Prometheus

Pandora

Epimetheus

Janus

Mimas

Enceladus

Tethys

Telesto

Calypso

Dione

Helene

Rhea

Titan

Hyperion

Iapetus

Phoebe

1 2 3 4

5

6

8910

11

12

7

Atmosp h

ere

Water andammonia

Core

12

3

4Atmosph

ere

Liquidhydrogen

Metallichydrogen

Core



150# DTP:48 PAGE:53

Swirling clouds andstorms can sometimesbe seen as ripples onSaturn’s globe. Saturnrotates very quickly,producing a distinct bulgeat its equator. It is the leastdense of the planets: if a largeenough bathtub could be found,Saturn would float in the water!

Eighteen of Saturn’s moons, inorder of distance from theplanet (top is nearest). Theyare drawn to scale, relative toone another.


PLUTO

PLUTO was first identified in 1930 by theAmerican astronomer Clyde Tombaugh.

He compared photographs of part of the skytaken six days apart and noticed that apinprick of light had moved slightly againstthe background of stars. In recent years,many objects similar to Pluto have beendiscovered in the outer Solar System, notablyEris. In 2006, Pluto was placed in the newcategory of “dwarf planets”, along with Erisand the asteroid Ceres. Pluto has threemoons, Charon, Hydra and Nix.

Pluto’s surface is probably an“icescape” of frozen nitrogen,carbon monoxide andmethane. There may becraters made by collisionswith rock and ice fragments.Seen from Pluto, the Sunlooks no more than a bright,distant star. It still providesjust enough heat to evaporatesome of the surface frost andcreate an extremely thinatmosphere. Charon, Pluto’slargest moon, featuresprominently in the sky.

Pluto is denser thanthe icy moons ofUranus and Neptune,suggesting that it hasa relatively large,rocky core.

It is likely thatPluto looks quitesimilar toNeptune’smoon,Triton.

Pluto has a very elongated orbit, rangingbetween 7400 and 4400 million kilometresfrom the Sun, bringing it inside the orbit ofNeptune for part of the journey. Pluto’slargest moon, Charon, is just over half itssize and lies only 19,640 kilometres awayfrom it. Both spin in a direction opposite tothat of all the true planets except Venus.

Thousands of icy objects may exist in theouter reaches of the Solar System. Theymay form either a belt (the Kuiper Belt) ora cloud (the Oort Cloud). This could be thebirthplace of comets (see page 30).


27

NEPTUNE

NEPTUNE was discovered by Germanastronomer Johann Galle in 1846. Its

largest moon, Triton, was recorded a fewdays later. Besides that, very little wasknown about Neptune until the spaceprobe Voyager 2 visited it in 1989.A bright blue globe, Neptune almost

completely lacks surface features. At thetime it was photographed by Voyager, astorm system, called the Great Dark Spot(which later disappeared), could be seenracing in a direction opposite to the planet’srotation. Winds on Neptune blow at morethan 2000 kilometres per hour.Like the other gas giants, Neptune has a

system of rings. There are four extremelyfaint rings, composed of dark, icy fragments.

A layer of warm water, with someammonia and methane, surrounds

Neptune’s rocky core.

V OYA G E R 2The greatest journey by a space probe so farundertaken was made by Voyager 2.Between 1979 and 1989, it flew close byJupiter, Saturn, Uranus and Neptune,transmitting superbly clear pictures of theplanets and their moons.Voyager has sincesped away from the Solar System, althoughit continues to send back signals—20 billiontimes weaker than those of a watch battery!

Voyager is playing itspart in the search for lifein other solar systems.Should aliens ever comeacross the space probethey will find an audio-visual disc on board. Ifthey play it, they willhear, among other things,the sounds of whales, ababy crying and greetingsin 55 languages.

(Bottom) Eight of Neptune’smoons are shown in orderof distance from the planet(top is nearest). They aredrawn to scale, relative toone another.

Neptune’s blueness arisesfrom the small amounts ofmethane found in its

atmosphere. Thewhite streaks

are fast-movingclouds.


26

Naiad

Thalassa

Despina

Galatea

Larissa

Proteus

Triton

Nereid

F A C T F I L EDiameter: 50,538 kmDay: 16.1 hoursYear: 164.8 years

Average distance from the Sun: 4496 million kmSurface temperature: -220°C

Atmosphere: hydrogen, helium, methaneMoons: 13

F A C T F I L EDiameter: 2300 kmDay: 6.4 daysYear: 248 years

Average distance from the Sun: 5914 million kmSurface temperature: -220°C

Atmosphere: probably nitrogen and methaneNumber of moons: 3

Kuiper Belt

Oort Cloud

Solar System

Mantle

CoreCrust



150# DTP:48 PAGE:55

Core

Water, ammoniaand methaneA

tmosph

ere

Callisto, Jupiter’s second largest moon, isheavily cratered. Measuring 600 kilometresacross, its most prominent crater, calledValhalla, is surrounded by a series of ripples.Io, the third of Jupiter’s Galileans, with itscrust a vivid mixture of yellows, oranges,reds and blacks, looks a little like a pizza. Infact it is peppered with active volcanoes andlakes of molten rock.

The blue-green globe ofUranus, about 129,000 kmaway, dominates the skies ofone of its moons, Miranda.The landscape of this moon, ajumble of cliffs, canyons andcraters, looks as if it hasbeen patched together frommany different landscapes(see page 25). There is adistinctly V-shaped zone of“grooves”. One great cliff is20 km high, well over twicethe height of Earth’s highestmountain, Mount Everest.

The surface of Triton,Neptune’s major moon, ismade up of a deep layer ofmainly granite-hard nitrogenice. Sometimes the ice meltsto a slush before quicklyrefreezing, producing ridgesand cracks. Nitrogen in itsgaseous form collectsbeneath the solid ice crust.The pressure builds up andeventually the nitrogen gaserupts through weak points inthe crust. Gas and dust arespurted up to 8 km high.

Our own Moon is the fifth largest moonin the Solar System, although it would take81 Moons to make up a world the size ofEarth. The Moon’s lava plains indicate pastvolcanic activity, but there are no activevolcanoes there today. Next in order of size comes Europa, the

fourth Galilean and an object of greatinterest amongst astronomers. Looking likea cracked egg, its surface consists of icesheets that are continually melting and re-solidifying. It is by no means impossiblethat, beneath those ice sheets, there is awarm ocean of liquid water. Could it bethat life has also evolved on Europa and thatthere are life-forms swimming in its oceans?Future space probe missions may find out.Triton is Neptune’s largest moon. Its

surface is the coldest place known in theSolar System. At -235˚C, the temperature islow enough to freeze nitrogen. Triton wasphotographed in stunning detail by Voyager2, the last of its close encounters, in 1989.


29

MOONS

MOONS, also known as satellites, arerelatively small worlds that orbit the

planets of the Solar System. Earth has onemoon, known simply as the Moon (see page21), but other planets have many more—Saturn, for example, has at least 60 moons.

Moons are very varied in size and form.Many have unusual landscape features thatintrigue astronomers.Moons are created in different ways.

Some are the result of fragments of rock orice being pulled together by gravity to forma globe. Others are asteroids that have been“captured” by a planet’s gravitational force.

The surface of Jupiter’s moonIo may look like this. Thereare volcanoes everywhere.When they erupt, plumes ofsulphur dioxide gas burstthrough from Io’s rockyinterior, jetting some 300 kminto space. Astronomers thinkit is likely that Io is beingaffected by the enormousgravitational pull of Jupiter(and nearby moon Europa),which tugs at the moon,creating sufficient heat insideit to cause the eruptions.

Ganymede

Titan

Callisto

Io

Moon

Europa

Triton

All seven of the moons illustrated here(below) are larger than the dwarf planetPluto, while the largest moons, Ganymedeand Titan, are even bigger than Mercury, thesmallest true planet. Jupiter’s four largestmoons are all in the top seven. They arecalled the “Galileans” after the Italianscientist Galileo Galilei who first discoveredthem with one of the first telescopes in1610. Ganymede has an icy surface withcratered plains and areas showing strange“grooved” patterns.

Titan, Saturn’s largest moon, is the onlymoon to have a thick atmosphere, mademainly of nitrogen. On its surface are lakesof methane, sandy plains and mountains.


28



150# DTP:48 PAGE:57

F A M O U S C O M E T SThe English astronomer Edmund Halley(1656-1742) was the first to realise thatcomets were orbiting objects. He oncemade a famous prediction: a comet that heobserved in 1682 would return to the skiesin 1758. Halley believed that cometsrecorded in 1531 and 1607 were simplyearlier sightings of the one he saw in 1682.Halley did not live to see his predictioncome true. Halley’s Comet, as it has beenknown ever since, was duly sighted onChristmas Day 1758 and has reappearedevery 75 to 76 years. When Halley’s Cometappeared in March 1986, the space probeGiotto flew within 600 kilometres of it,sending back pictures and sampling thegases and dust particles given off by it.Comet sightings are always big events.

Comet Hale-Bopp in 1997 and McNaughtin 2007 were the most spectacular of recentyears. Comets can also be destructive if theypass too close to a planet. In July 1994,drawn in by gravity, fragments of CometShoemaker-Levy smashed into Jupiter,creating fireballs on impact.

On 30th June 1908 there was a hugeexplosion in the Tunguska region ofSiberia, Russia. Trees in an area about100 km across were felled by the blast,but no crater was found. The Tunguskafireball may have been a comet explodingat an altitude of about 6 km.

ASTEROIDS

ASTEROIDS are small, mostly rocky,irregular-shaped bodies. They are found

orbiting the Sun in a band filling the 550-million-kilometre gap between Mars andJupiter. The largest, Ceres, measures justunder 1000 kilometres across, but only ahandful have diameters greater than 100kilometres. About 4000 have been recorded,but there are many thousands more toosmall to be identified.Astronomers believe that, during the

formation of the Solar System (see page 17),Jupiter’s strong gravitational pull causednearby planetesimals to smash into oneanother rather than build up into anotherplanet. This left the belt of fragments we callthe asteroids.

The asteroids have continued to collidewith one another since their formation,producing smaller fragments calledmeteoroids. These have occasionallycrashed on to Earth’s surface (when they areknown as meteorites). It is feared that oneday a large meteorite may devastate Earth,causing climatic change sufficient to wipeout many life-forms.

A close-up view of the irregularshaped objects that make up theasteroid belt between Mars andJupiter. From study of asteroidfragments that have fallen toEarth, scientists have datedthe age of the Solar Systemto 4.6 million years ago.

Most asteroids are rocky, indicating theycome from the outer layers of a formerminor planet. But some are metallic—theycome from the core of such a planet.


31

COMETS

COMETS are potato-shaped lumps ofdust measuring only a few kilometres

across, but accompanied by (when near theSun) tails of gas or dust that stretch forhundreds of millions of kilometres acrossspace.The lump of dust is fused together byfrozen gases and water ice. Like all otherobjects in the Solar System, comets orbitthe Sun, although their orbits are often veryelliptical (elongated ovals), looping intowards the Sun from distant reaches of theSolar System. When a comet approaches theSun, part of its ices melt and the gas and

This is a view(below) of a comet’s nucleus,

a lump of dust frozen together. When itcomes near to the Sun, the ices melt, the outer

crust of the nucleus cracks open and jets of dustand gas gush out to form a cloud called a coma.

A comet has two tails: a straight gas tail anda broader, curved dust tail. They grow asthe comet approaches the Sun. Thecomet passes round the Sun. Itstails, still pointing away fromthe Sun, shorten anddisappear as the cometretreats to the outerSolar System.

dust escape, forming a surrounding cloud,or coma. As it rounds the Sun, the coma isswept back into two tails, a straight gas tailand a broader, curved dust tail, alwayspointing away from the Sun.

Sometimes, small pieces of debris breakoff from comets. Great showers of thesefragments, called meteors, sometimes comequite close to Earth. Millions of tinyparticles burn up in Earth’s atmosphere.Commonly known as shooting stars, theyappear to us as split-second streaks of lightin the night sky.


30

Gas tail

Dust tail


150# DTP:48 PAGE:58 (AB)UK Text

AAchernar 11Aldebaran 10Alpha Centauri 11Altair 10Arcturus 10Ariel 25Aristarchus 12asteroids 12-13, 16-17,

22, 31astronomers, early 12-

13atmosphere 15, 17, 18,

19, 20, 23, 25, 26atoms 8

BBetelgeuse 7, 10-11Big Crunch 5Big Bang 4-5black holes 7, 8-9

CCallisto 23, 28-29Canopus 11carbon dioxide 19carbon monoxide 27 Cassini Division 24Centaurus 11Ceres 31Charon 27chromosphere 15Comet, Hale-Bopp 31 Halley’s 13, 31Shoemaker-Levy 31

comets 12, 13, 24, 27, 30-31

constellations 10-11Copernicus, Nicolaus

12-13corona 15cosmic rays 12craters 18, 21, 27Cygnus X-1 9

DDeimos 22density 8, 18, 22, 27Dione 24Dubhe 10dwarf planet 12, 16, 27

E FEarth 5, 8, 13, 14-15,

16-17, 19, 20, 29Einstein, Albert 8-9electric charge 12

Moon 5, 12-13, 15, 18, 20, 21, 28-29

formation of 21phases of 21

moons 12, 23, 24, 25, 28-29

NNebula, Crab 7 Nebula, Horsehead 6nebulae 7, 17Neptune 12, 16-17, 26Nereid 26nitrogen 27, 29nuclear fusion 14nuclear power 7nucleus,cometary 30 galactic 6

OOberon 25Olympus Mons 22Oort Cloud 27Orion 7, 11

PPhobos 22photosphere 14-15planetesimals 17, 20, 31planets 5, 12-13,

16-17, 18-27formation of 17orbits of 12-13, 18relative distances 16-17

relative sizes 16Pleiades 10Pluto 12-13, 16-17, 19,

27, 28Pole Star (Polaris) 10Procyon 10-11prominence, solar 14-15protostars 7Proxima Centauri 4, 16Ptolemy 13

Q Rquasars 8-9radiation 7, 14, 19Relativity, General

Theory of 8Rhea 24Rigel 7, 11rocks 17

SSaturn 12, 16-17, 24,

26, 28shooting stars 30Sirius 11solar wind 12, 17, 20Solar System 5, 12-13,

14, 16-17, 26-27, 28-29, 30

electric current 12Enceladus 24energy 5, 7, 9, 14-15,

20epicycles 13escape velocity 8Europa 23, 28-29flare, solar 14-15

Ggalaxies 4-5, 6, 14GalaxyAndromeda 6, 10Milky Way 4, 6, 9, 14

Galileo Galilei 13, 28Galle, Johann 26Ganymede 23, 28gas giants 16-17, 23, 26gases 5gravity 5, 7, 8-9, 12, 17,

25, 28, 31Great Bear see Ursa

MajorGreat Dark Spot 26 Great Red Spot 23

H IHalley, Edmond 31heat 7, 18-19, 28heliosphere 12helium 5, 14Herschel, Wlliam 25hydrogen 5, 14-15, 23Iapetus 24infrared radiation 19Io 23, 28-29

J KJupiter 13, 14, 16-17,

23, 26, 28, 31Kepler, Johannes 13Kuiper Belt 27

Llife 5, 20light 4, 7, 8light years 4Local Group (of

galaxies) 4

MMagellanic Cloud,

Large 11magnetic field 15, 20Mars 13, 16-17, 22, 31matter 5, 8, 14Merak 11Mercury 12, 13, 15, 16-

17, 18meteorites 18, 31meteoroids 12, 20, 25,

31meteors 30Mimas 24Miranda 25, 29

formation of 17, 31space 4-5, 7space probe,Cassini 16Galileo 12Giotto 31 Pathfinder 22 Pioneer 12-13Viking 22Voyager 1 12Voyager 12, 16, 24, 26, 29

space probes 12, 16, 29spicules 15star,black dwarf 15neutron 7, 8red giant 7, 15 supergiant 7, 8 white dwarf 7, 15 yellow dwarf 14

stars 4-5, 6, 7, 8, 10-11, 14, 16

comparison of sizes 7

life cycle of 7subatomic particle 5, 8,

12sulphur 23sulphur dioxide 28Sun 4-5, 6-7, 8, 12-13,

14-15, 16-17, 18, 19, 20-21, 25, 27, 30, 31

death of 15sunspots 14-15superclusters, galaxy 4supernova 7, 8, 17

Ttectonic plates 20Tethys 24time 5Titan 24, 28Titania 25Tombaugh, Clyde 27Triton 26, 29Tunguska fireball 31

UUmbriel 25Universe 4-5Uranus 13, 16-17, 25,

26, 29Ursa Major (Great

Bear) 10

VValhalla 29Valles Marineris 22Vega 10Venus 13, 15, 16-17, 19volcanoes 22, 28, 29

Wwater 20, 29waves 17

I N D E X

32

INDEXPage numbers in boldrefer to main entries.


150# DTP:48 PAGE:2

Download - children s illustrated encyclopedia Stars and Planets...galaxies. Matter inside the galaxies went on clumping together until stars were created (see page 7). Our own Sun was born in

Top Related