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What's your favorite movie or cartoon or show, other than SciShow Kids of course? Does it have things like spaceships in it and weird creatures living on other planets? If you've ever seen a movie or show like that, it's probably had you wonder, could aliens be real? Well, one SciShow Kids viewer named Parth wrote to us and asked, "Can you make a video about aliens and where in the universe we might find them?" That's an excellent question!

When people talk about aliens, what they mean are living things that make their home on a planet other than Earth. So, do aliens exist? Well, we don't know for sure, but we do know that so far scientist haven't found any living thing on any other planet. As far as we know, Earth is the only planet that has life on it. So all of those characters that you see in cartoons and movies with green skin or lots of extra eyeballs or octopus legs, they came from our imagination.

But, why haven't we found aliens walking around Mars, or even on the Moon? Well, that has to do with the second part of Parth's question. Where else in space might we find living things? Earth, unlike other planets in our solar system, has three very important things that make it possible for living things to, well, live. The first of those things is water. Most living things that we know about can't survive without water, and no other planet, that we know of, has as much liquid water on it as Earth. In fact, more than half of our planet's surface is covered in water. 

And, speaking of things that are covering out planet, Earth is also surrounded by a layer of gases called the atmosphere, and one of the gases in the atmosphere is the gas called oxygen. Oxygen is the second very important thing that living things on Earth need, which our planet provides. Oxygen doesn't look, or taste, or smell like much of anything, but many things on Earth depend on it to live. It's what we need to breath, and no other planet in our solar system has as much oxygen in its atmosphere as Earth.

And the third thing that Earth has that's really, really, really important is food. The soil on Earth can grow lots of different kinds of food for living things to eat, like grains and fruits and vegetables. Nobody knows much about the soil on other planets, but without liquid water and plenty of oxygen, we think it would be much harder to grow food there than on our home planet. 

So those are the very important things that Earth has that living things need to survive: Water, oxygen, and food. And, no other planets that we know of have very much of any of those things. Some of the planets in our solar system might have one of these three key ingredients, but not a lot. For example, Mars has some water on it, though most of it's ice and not liquid water, and it only has a teeny tiny amount of oxygen in its atmosphere, compared to how much Earth has.

So, no other planet that we've found seems to have all these things put together. Which is probably why we haven't seen any signs of life on any of them, but we can't say for sure that there isn't life some place else in the universe. After all, people haven't visited any other planets just yet, but we're learning more about other planets every day. In fact, we now have telescopes so strong that we can actually see planets outside of our solar system. They're too far away for us to see if they have water or oxygen or anything else, but some scientists believe that there might be more Earth like planets out there just waiting to be discovered.

Some day we might be able to study these planets and find out for sure if life exists on any of them, but for now we'll just have to imagine what aliens in space look like. Who knows, they could look sort of like us. If you'd like us to scan the skies for more information about space or anything else, just let us know by getting help from a grownup and leaving a comment below or emailing us at kids@thescishow.com. Thanks for joining us on SciShow Kids, and we'll see you again soon.