First of all: a star. If we weren't circling the Sun, you and I wouldn't be here, for a couple of reasons. The Sun provides energy; it's the basis of all the world's food chains. It's the fuel that drives almost all living things. The Sun also provides warmth, which allows water to exist in liquid form.

Earth's orbit around the  Sun is in kind of a sweet spot. It is not so cold that all the water is frozen, and it's not so hot that all the water turns to vapor. 

Hm... what's the other thing that life needs? Ah! Food!

Living things need food for the energy that we get from it. And again, it all starts with the Sun. Plants take energy from the Sun and turn it into chemical energy, which can then be transferred to all of the animals that eat those plants or other animals.

So, if life on Earth needs food, water and warmth to survive, then what might alien life be like?

Well, it depends. Just like life on Earth, it would depend on the circumstances of the planet and the resources available to the living things there. 

No water? Maybe animals instead rely on liquid methane instead of liquid water. Too far from a star? Maybe they get their energy and warmth from heat vents in the crust of the planet, like the heat vents at the bottom of our oceans.

Perhaps there is a planet that is always covered in thick clouds close to the ground. Creatures there would have a hard time seeing one another, so they'd  need another way to communicate. Perhaps these aliens use smells and sounds to recognize each other. And perhaps they have hard outer shells to keep them protected in case other bigger aliens come around looking for a snack.

No matter what these aliens look like, one thing is for certain: their food chains would seem very familiar. How so?

Well, the  planet would still need producers to turn energy from starlight into chemical energy. Say, some kind of alien plant-like things. An alien planet would  also have consumers: things that eat the producers. And probably secondary consumers too, the things that eat those first-level consumers.

And, of course, this planet would need  decomposers. Without this part of the food chain, waste would build up on the planet, making it totally unlivable, not to mention, really gross.

All of these organisms would all be on the planet together, helping each other survive by creating complex ecosystems and tangled, interlocking food webs.

Now, I know this all sounds like science fiction, but as we've talked about, the Universe is a crazy big place. Astronomers think that every star in the Milky Way Galaxy has at least one planet, and there are billions of stars in our galaxy alone. That doesn't include the whole rest of the mega-huge Universe. 

So, astronomers think that life on other planets isn't just possible - it's likely. Maybe life on other planets looks like dinosaurs or jelly fish. Maybe it's just tiny bacteria too small to see without a microscope. Or, maybe it's all these things and more, just like on Earth: a big, wild party of diverse, beautiful life.

I, for one, can't wait to find out.