Curious Kids: What plants could grow in the Goldilocks zone of space?

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Almost every star has planets – so there are more planets in our galaxy than there are stars. NASA Ames/JPL-Caltech/T. Pyle, CC BY-SA
From Curious Kids, a new series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

What plants could grow in the Goldilocks zone of space? – Jesse, 9, Miranda.

Imagine a planet like the Earth, orbiting a distant star. Could that planet have life? Well, life on Earth (the only life we know) needs liquid water. So to find life on another planet, we think that that planet would have to be “just right”.

If the planet is too close to its star, it will be too hot, and any oceans would boil. Too distant, and any oceans would freeze. Somewhere in between lies the “Goldilocks Zone” – not too hot, not too cold, but just right.

We still haven’t discovered life on any other planet, so we can’t say for sure what such life would be like. One thing is certain, though – alien life will be very, very different to anything on Earth.

We can try to imagine plants that could live on other planets based on the facts we do have. Using your imagination like this is a very important part of being a scientist, trying to explore the vast ocean of the unknown.

So many planets

One thing we’ve learned over the last 20 years is that planets are everywhere. Almost every star has planets – so there are more planets in our galaxy than there are stars.

“Goldilocks planets” could be bigger or smaller than the Earth. Smaller ones have weaker gravity, so you would weigh far less there than on the Earth. Plants (and animals!) growing there could easily be much taller than on Earth, since it would be easier for them to grow up!

On a bigger planet, more massive than Earth, plants would probably be much shorter – thanks to the stronger gravity on such a world. We can work out how strong gravity would be on different planets. This cool website lets you work out how much you would weigh on other objects in the Solar system, for example.

Water, wind and light

Also, not all planets are equally wet. Some are likely dry, desert worlds, while other “Earth-like” worlds might have oceans tens, or hundreds of kilometres deep. What kind of plants could grow on those desert or water worlds?

If a “Goldilocks planet” had a thin atmosphere (like Mars), even the strongest winds would push more gently than a soft breeze here on Earth. Any plants probably wouldn’t need to be very strong to protect against bad weather. With a really thick atmosphere, though, winds push harder – and any plants in those conditions would have to evolve to be really tough.

And then we get to light. Plants on Earth have evolved to use the light from the Sun to get their energy, using a chemical called chlorophyll. It absorbs blue and red light, but reflects green light.

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