Could something live on a planet that has water?

Think of a puddle after the rain, cool and shiny under your boots. Soon a bird hops over for a drink, and tiny bugs skate across the top. Here on Earth, living things gather wherever water is. So could something live on another planet that has water too?

Every living thing on Earth needs water, from the biggest whale to the tiniest germ. Inside your body, water carries food and air to every part of you. Without water, nothing inside a living thing could move around and do its job.

But the water has to be runny, the kind that splashes and flows. Water that freezes into hard ice can't carry anything around. Water that boils away into steam floats off and can't carry anything either.

Warmth from a planet's star is what keeps its water runny. Our Sun is a star, and Earth gets the right amount of its warmth. A planet too close to its star gets so hot that its water boils away. A planet too far away gets so cold that its water freezes solid.

Scientists call the right space around a star the Goldilocks zone, which means the place where a planet is not too hot and not too cold. The name comes from the girl in the story who wanted porridge that was just right. Earth sits in the Sun's Goldilocks zone, and that keeps our oceans and rivers runny.

A planet with runny water in its Goldilocks zone has one big thing that life needs. Water alone can't help a living thing grow, though. What else do you think a living thing needs every day, the way you do?

A living thing also needs energy to move, grow and stay alive. You get your energy from food, and plants get theirs straight from sunlight. A planet that gets light and warmth from its star has energy ready for living things to use.

So a planet with runny water in its star's Goldilocks zone, plus energy from that star, is a place where living things could live. Scientists have already found planets around faraway stars that sit in their star's Goldilocks zone.

Nobody knows yet whether anything really lives on those planets. They are so far away that even our biggest telescopes cannot see what is on them.

One word to remember: Goldilocks zone. Something could live on a planet with water if its star keeps that water runny in the Goldilocks zone and gives living things energy.

What a big, wonderful question you asked. Next time you see the stars, you can wonder which of them warms a planet with water of its own.