Earth in Space

In Grade 6 you learned that gravity holds the planets in orbit. This year you find out how — and the answer is more surprising than it sounds.

Gravity is a force of attraction between any two objects that have mass. The more mass an object has, the stronger its pull. The Sun has enormous mass, so its gravity reaches across the whole solar system.

But if gravity is always pulling the planets towards the Sun, why don't they simply fall in? The answer is motion.

A planet is also moving forwards, sideways through space. Gravity bends that path, pulling the planet round. The planet keeps "falling" towards the Sun, but its forward motion keeps carrying it past — so instead of falling in, it travels round and round. An orbit is that balance: forward motion plus gravity's pull. The same idea explains why the Moon orbits the Earth.

The solar system has not always existed. Scientists think it formed about 4.6 billion years ago from a giant cloud of gas and dust drifting in space.

Here is the story, step by step:

• The cloud began to collapse inwards under its own gravity.
• As it collapsed it started to spin faster and flattened into a disc.
• Most of the material gathered at the centre. It became so hot and dense that a star — our Sun — began to shine.
• The leftover material in the disc clumped together. Small clumps collided and stuck, slowly building up the planets, moons and asteroids.

This is why the planets all orbit the Sun in the same direction and roughly in the same flat plane: they all formed from the same spinning disc.

The rocky inner planets and gassy outer planets formed differently because of temperature. Close to the hot young Sun, only rock and metal could survive, so the inner planets are small and rocky. Far out, where it was cold, light gases could gather too, so the outer planets grew huge. Gravity is the thread running through the whole story — it pulled the cloud together and shaped everything that followed.

A star is not permanent. Like a living thing, it has a life cycle — it is born, it shines, and eventually it dies.

Every star follows the same start:

• A star is born inside a nebula — a vast cloud of gas and dust. Gravity pulls the cloud together until it becomes hot and dense enough to shine.
• It then shines steadily for billions of years, in the long stable part of its life.

What happens at the end depends on the star's mass:

• A medium star (like our Sun) swells into a red giant, then gently sheds its outer layers, leaving a small, dense white dwarf that slowly cools.
• A massive star swells into a red supergiant, then explodes in a brilliant blast called a supernova. What is left becomes an incredibly dense neutron star, or — for the most massive stars — a black hole.

So the same idea, gravity, both creates a star and shapes how it dies.

Our Sun is just one star among an unimaginable number. To make sense of the universe, it helps to think in levels of scale.

• A solar system is a star and everything that orbits it — like our Sun and its planets.
• A galaxy is a huge collection of billions of stars, with their planets, all held together by gravity. Our galaxy is the Milky Way.
• The universe is everything that exists — and it contains billions of galaxies.

The distances are so vast that kilometres become useless. Astronomers use the light-year — the distance light travels in a year. Even light, the fastest thing there is, takes years to cross a single galaxy and billions of years to cross the universe. When you look at a distant galaxy, you are seeing light that set out before the Earth even existed.

One of the most astonishing discoveries in science is that the universe is not still. It is expanding — the galaxies are moving away from one another.

How do we know? Astronomers found that the light from distant galaxies is stretched, and almost every galaxy is moving away from us. The further away a galaxy is, the faster it is moving away.

A useful picture is dots drawn on a balloon. As you blow the balloon up, every dot moves away from every other dot — not because the dots move across the rubber, but because the rubber itself stretches. The universe behaves the same way: space itself is expanding, carrying the galaxies apart.

This leads to a powerful idea. If the galaxies are moving apart now, then in the past they must have been closer together. Wind the clock back far enough and everything was packed into one tiny, incredibly hot point. The universe began from that point in a sudden event scientists call the Big Bang, about 13.8 billion years ago — and it has been expanding ever since.

The expanding universe is the main evidence for the Big Bang. It is a wonderful example of how careful observation — just measuring the light from faraway galaxies — can reveal the story of everything.

Understanding the Earth's place in the universe opens up a lot of later science:

• Forces and energy — gravity is one of the fundamental forces; orbits are forces in action.
• Light — light-years, and the stretched light from distant galaxies, build on how light travels.
• Chemistry of the elements — the elements in your body were made inside stars and scattered by supernovae.
• Space exploration — knowing about orbits and gravity helps you understand satellites, probes and missions.

The scale of the universe can feel overwhelming, but every part of it follows the same patterns you have studied — gravity, motion, energy and change. Take your time with this topic; it changes how you see the night sky forever.