[ intro ].

Squeaks was just showing me some new pictures of different planets. We have Mars, bright red and rocky … Saturn, with its amazing rings ….

Can you guess which is my favorite to learn about, Squeaks? [Squeaks thinks, then guesses Earth]. You got it! I love learning about planet Earth because it’s so unique.

Here on Earth, there's life everywhere you look. But the seven other planets, all their different moons, other places like asteroids — we've never seen signs of life on any of them. The sun is only one star, though, and the universe is full of stars.

Lots of those stars also have planets orbiting them, just like Earth and Mars and Jupiter and all the other planets in our solar system go around the sun. With so many other planets out there, lots of astronomers — scientists who study space — wonder if some of them are like Earth and living things could survive there. Most of the planets we've seen are very different from Earth — lots of them are boiling hot, or don’t have enough air, or don't have any solid ground.

There's even a planet where it might /rain diamonds!/ But there are a few planets around other stars that scientists think are a little bit like. Earth. That doesn't mean there are living things on them, but at least living things could survive there.

So when scientists are looking to see if a planet is like Earth, what do you think they're looking for? What do living things need to survive? I'll give you a hint: one of them is something you're standing on right now. [Squeaks guesses, the floor?] That's a good guess — it is like the floor, but outside. [Squeaks: the ground!] Yeah, the ground!

Earth has a crust of dirt and rock around the outside; that’s the ground where we walk and what’s at the bottom of the sea. If a planet doesn’t have ground to walk on, then it wouldn’t be as good for most living things. Some planets are just a big ball of gas with nothing solid in them at all!

It would be very hard to live someplace without ground to stand on or oceans to swim in. I think you might be able to guess another thing that we need on a planet to survive,. Squeaks.

It’s all around us… [Squeaks guesses air]. That’s right, we need air to breathe. An Earth-like planet needs to have air for us to breathe.

That air is part of what’s called the atmosphere, a layer all around the planet with all of the air and clouds inside. The atmosphere on a planet has to be just right, with the right amount of air — and it has to be the type of air we can breathe. Knowing that a planet has air to breathe and a solid crust is important, but the planet also has to be the right temperature.

Planets orbit around stars like our sun, but if the planets are too close to their star, they can be horribly, boiling hot. Yikes, I would not want to live there. But if the planet is too far from its star, it’s way too cold.

We’d freeze if we even tried to set foot on that planet. Only planets at that perfect distance away from their star will be at the right temperature to support life like us. We don’t want it too hot or too cold — it has to be just right.

Does that sound familiar, Squeaks? [Squeaks]. It does sound like the story of ‘Goldilocks and the Three Bears.’ In fact, many scientists call that special, perfectly-warm distance away from a star the Goldilocks zone. In the Goldilocks zone, it would also be possible to find liquid water, something that all life on Earth needs.

There are so many different traits that a planet needs to share with Earth for us to live there that most of the planets we’ve found wouldn’t work. Scientists have found some that could be like Earth, though, like a planet called Kepler-452b. It’s much bigger than Earth, but it’s in the Goldilocks zone!

There’s still so much to learn about what it’s like on planets like Kepler-452b. [Squeaks]. I also wish we could visit those planets, Squeaks! But they’re too far away for humans to reach right now.

Even in our fastest rocket ships, it would take thousands of years for us to reach the closest star to the sun and the planets around it. That’s way too long a trip for people to take. Maybe someday, people will invent a way to travel that far quickly, but for now, we can’t reach planets like Kepler-452b.

Since we can’t go visit it ourselves, we’ll just have to make our observations from here on my favorite planet: Earth! We’re so lucky to have such a great planet where we can live, make discoveries, and learn new things about the other planets all around the universe. [Squeaks agrees]. What do you think it would be like to live on another planet?

Do you have an idea to make spaceships move faster, so we can visit other planets? Ask a grown-up to help you leave a comment below, or send us your ideas on our website at patreon.com/scishowkids. We’ll see you next time, here at the Fort! [ outro ].