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14 Space

WHAT ARE THE MAIN BODIES IN THE SOLAR SYSTEM?

The Earth is one of eight. PLANETS in the Solar System. Most planets

have natural satellites (moons) circling them. Swarms of mini-planets, called

asteroids, also travel in the Solar System, and much farther out are icy lumps

that grow shining tails and become comets when they near the Sun.

Our local star, the Sun, and everything that circles around it, is known as the

Solar System. The Sun is the largest and most powerful body in this region

of space, occupying more than 99.8 per cent of the Solar System’s entire mass.

Its immense gravity holds planets, asteroids, comets, dust, and other bodies

in oval paths, or orbits, around it. This gravity is so powerful that some objects

are in orbits trillions (thousands of billions) of kilometres out from the Sun.

HOW DID THE SOLAR SYSTEM FORM?

The Sun and planets were born in a huge cloud of cold, swirling gas, called the

solar nebula. The cloud collapsed under its own gravity into a fast-spinning,

ball-shaped mass. The centre part became denser and hotter and eventually began

shining brightly as the Sun. Rocks, dust, and gases circling in a disc around the

Sun began lumping together, eventually forming the planets.

A planet is a world that orbits a star. There

are eight major planets going around the

Sun – Mercury, Venus, Earth, Mars, Jupiter,

Saturn, Uranus, and Neptune. Dwarf planets

are similar to major planets but smaller.

Astronomers now class Pluto as a dwarf

planet rather than a major planet.

WHAT ARE PLANETS MADE OF?

The four planets closest to the Sun are largely made

up of rock, like the Earth, and are called the terrestrial

(Earth-like) planets. The next four larger planets, often

called gas giants, are made mainly from hydrogen

and helium. The dwarf planets beyond Neptune,

such as Pluto, are made of ice and rock.

FIND OUT MORE. Earth 16 • Jupiter 19 • Mars 19 • Mercury 18 • Neptune 21 • Saturn 20 • Uranus 20 • Venus 18 • Dwarf Planets 21

2 CIRCLING THE SUN

Eight major planets travel round

the Sun in huge elliptical (oval)

orbits. The four inner planets

(nearest the Sun) are much hotter,

faster, and closer together than

the four outer planets. The orbits

all lie in roughly the same plane,

or level, in space.

4 PLANET FORMATION

Gas giant planets such as Jupiter

formed in the outer reaches of

the Solar System, where the Sun’s

gravity is weaker, space is colder,

and there were large amounts of

ice and gas. This picture shows

a gas giant just beginning to

form (at the right).

1 GIANT PLANET

Jupiter is mainly hydrogen and helium. Below the

cloudy atmosphere, the pressure is so great that

these gases turn into a great liquid ocean.

1 ROCKY PLANET

A rocky planet, such as Mars, has a thin crust

made up of hard rock. Underneath is another

rocky layer called the mantle. A huge mass of

iron makes up the centre, or core.

PLANETS

Liquid

metallic

hydrogen

Hydrogen

and helium

atmosphere

Liquid

hydrogen

and helium

Hard,

rocky crust

Possible

solid core Heavy

iron core

Rocky

mantle

Mars Venus Mercury Earth

Neptune

Sun

Uranus

Saturn

Jupiter

SOLAR SYSTEM

Solar System

Solar

System