Our Home — Earth
Stop 5 of 12 · Inner Solar System
Our Home — Earth
The third planet from the Sun — a blue world of water, air, and life.
Progress is saved privately in your browser.
Understand why Earth is the way it is — liquid water, a protective atmosphere, and the right distance from the Sun — and reflect on what makes it unique.
Somewhere in the vast, silent Solar System, a small blue marble spins. From space, it glows softly — oceans of deep blue, swirls of white cloud, patches of green and brown.
It is home. It is Earth.
Leela holds up a picture her teacher gave her: a photo of Earth taken from far away. "It looks so peaceful," she whispers. "And so blue."
Why is Earth blue? Is the whole planet made of water?
A beautiful question. What do you see when you look at a big puddle of water under a clear sky? And now imagine the puddle covering most of a world…
About 71% of Earth's surface is covered in liquid water — oceans, seas, and lakes. From space, the sunlight reflecting off all that water is what makes our planet look blue.
And water is not just beautiful. As far as we know, liquid water is one of the key ingredients for life — and Earth is the only place in the universe where we know life exists.
Just the Right Distance
Earth sits in what scientists sometimes call the habitable zone — not so close to the Sun that water boils away, and not so far that it freezes solid. This "Goldilocks" position lets water stay liquid, and liquid water is precious.
Earth is not perfectly round! It bulges slightly at the equator because it spins. And it is moving too — our whole planet orbits the Sun at about 107,000 km per hour.
A Blanket of Air
Earth's atmosphere is a thin, life-giving blanket of gases — mostly nitrogen, with oxygen, water vapor, and a little carbon dioxide. It gives us air to breathe, burns up small space rocks before they reach us, and keeps our temperatures stable.
It is a very different blanket from Venus's. Ours lets heat escape in just the right way.
- Order from Sun
- Third of the eight planets
- Approximate diameter
- 12,742 km
- Known moons
- 1
- Year length
- 365.25 Earth days
That extra quarter-day is why we add a leap day every four years — February 29th keeps our calendar in step with the Sun.
Why Earth Matters
Why Does This Matter?
Earth is the only world we know that supports life. Understanding exactly why — the distance, the air, the water — helps us protect it, and teaches us what to look for when we search for other homes among the stars.
Pause and Reflect
What makes Earth different from the other planets we currently know?
Take a moment. Look out the window, or picture Earth from space. Think about the oceans, the air, the living things, the stories. Then carry that thought to the next stop of the journey.
Quick Check
Why does Earth look blue from space?
Earth is the only world we know that has liquid water on its surface, a life-giving atmosphere, and living things — and it sits at just the right distance from the Sun.
Reflect
What You Discovered
- The only world we know with liquid water and life
- Oceans cover about 71% of the surface
- The right atmosphere and distance make life possible
- It's our home — worth protecting