Earth in Space

In Grade 6 you learned that the Sun's gravity holds the planets in orbit. Now let's look at gravity properly.

Gravity is a force that pulls objects with mass towards one another. The more massive an object, and the closer you are, the stronger its pull. The Earth is huge, so it pulls everything towards its centre — that is why things fall down.

Two words are easy to muddle: mass and weight.

• Mass is the amount of matter in an object. It is measured in kilograms and never changes, wherever you go.
• Weight is the pull of gravity on that object. It is a force, measured in newtons, and it changes if gravity changes.

The Moon's gravity is about one-sixth of Earth's. So an astronaut who weighs about 600 N on Earth would weigh only about 100 N on the Moon — but their mass stays 60 kg. Mass stays; weight changes.

In Grade 6 you named the eight planets in order. This year you can describe the solar system in more detail.

The eight planets split into two groups, and the difference is not random:

• Inner planets — Mercury, Venus, Earth, Mars — are small, rocky and close to the Sun. They formed where it was too hot for gas and ice to stay.
• Outer planets — Jupiter, Saturn, Uranus, Neptune — are huge gas giants, far from the Sun where it was cold enough to gather thick layers of gas.

Between Mars and Jupiter lies the asteroid belt — a ring of rocky lumps too small and scattered to form a planet.

Gravity explains the whole layout. The Sun's gravity holds every planet in its orbit. A planet closer to the Sun feels a stronger pull and must travel faster, so its year is short. A distant planet moves slowly on a long orbit, so its year is long — Neptune's year lasts about 165 Earth-years.

Each planet may also have moons held in orbit by the planet's own gravity. Earth has one; Jupiter and Saturn have dozens.

The big idea: gravity is the architect of the solar system — it sets the orbits, the speeds and the years.

Here is a fact that surprises many students: the Sun is a star, just like the points of light you see at night.

A star is a giant ball of hot gas that releases enormous amounts of light and heat energy from deep in its core. The Sun does exactly this — it is our nearest star.

So why does the Sun look so different from the night-time stars? Distance. The Sun is about 150 million kilometres away — close, in space terms. The other stars are trillions of kilometres away, so they look like tiny pinpricks.

If you could travel to a distant star, it would look like a blazing sun, and our Sun would shrink to a faint dot in its sky.

Stars are not all the same:

• They differ in size — some are far larger than the Sun, some smaller.
• They differ in colour — which is a clue to how hot they are. Blue-white stars are hotter; red stars are cooler.
• They differ in brightness as seen from Earth, partly because of their distance.

The Sun matters to us above all else — its energy powers the water cycle, the weather and almost all life on Earth. But across the whole Universe, it is a fairly ordinary star.

Stars are not scattered randomly. Gravity gathers them into huge groups called galaxies.

Our galaxy is the Milky Way. It contains hundreds of billions of stars — and the Sun is just one of them. Beyond the Milky Way are billions of other galaxies, each with billions of stars of its own.

Space is so vast that kilometres become useless for measuring it. Instead, scientists use the light-year — the distance light travels in one year. Light is the fastest thing there is, yet a light-year is a colossal distance.

So the scale builds up like this:

• A planet, such as Earth, orbits a star.
• A star, such as the Sun, is far bigger than any planet.
• A galaxy, such as the Milky Way, holds hundreds of billions of stars.
• The Universe holds billions of galaxies.

The Earth is one planet, orbiting one ordinary star, in one of billions of galaxies. That is the true scale of space.

As well as planets and moons, the solar system holds smaller objects. Three are easy to confuse, so here is the difference.

Asteroids are lumps of rock that orbit the Sun, most of them in the asteroid belt between Mars and Jupiter. They are far too small to be planets.

Comets are balls of ice, dust and rock on long, stretched orbits that take them far from the Sun and then close again. When a comet nears the Sun, its ice warms and releases gas and dust, forming a glowing tail that always points away from the Sun.

Meteors are the streaks of light — 'shooting stars' — you sometimes see at night. A meteor is caused when a small piece of rock or dust enters the Earth's atmosphere at high speed and burns up because of friction with the air.

A handy way to keep them straight:

• Asteroid — rock orbiting the Sun, mostly in the belt.
• Comet — ice and dust, long orbit, grows a tail near the Sun.
• Meteor — a streak of light as a small piece burns up in our atmosphere.

If a piece survives the fall and lands on the ground, it is then called a meteorite.

Understanding Earth in space connects to many later topics:

• Forces and motion — gravity is one of the most important forces; mass and weight come up again and again.
• Energy — the Sun, as a star, is the energy source behind weather, the water cycle and life.
• Light — understanding how starlight travels, and the light-year, builds on what you know about light.
• Space exploration — knowing about gravity, orbits and the scale of space helps you follow real space missions.

The next time you look up at the night sky, you will see it differently — every point of light is a sun, and our whole solar system is a tiny part of one galaxy among billions. Take your time with this topic; it changes how you see your place in the Universe.