Alex McColgan

We generally understand what we mean when we say a living thing.

We might define it by it moving around, or by it growing.

Generally speaking, scientists define life as any system that is capable of eating, metabolising, excreting, breathing, moving, growing, reproducing and responding to external stimuli.

Essentially, they are aware of their surroundings in some way.

They seek resources.

They take those resources into themselves and they use them to grow or create more of themselves.

and then they get rid of any waste that's left over.

Ew.

Some forms of life are much more active than others, but even things like plants can move to face the sun, open their buds, or spread out their roots over time.

So we look at these things and consider them living.

Even on Earth though, there are some systems like viruses that push the boundary of what it means to be a living thing.

Viruses are so simple that they lack the ability to reproduce by themselves, or to metabolise.

Instead, they get cells they infect to do that work for them.

Are viruses alive?

They certainly have proved devastating to other populations of living things, and we can definitely think of them that way.

but it's a debate that still rages on in the scientific community.

So, although there are certain qualities that are fairly universal for living things here on Earth, we must be careful about how we go about defining life.

For instance, most living things on Earth make use of water to function.

It carries important nutrients around our bodies, and is so vital for all life on Earth that we consider the absence of water to be a serious red flag if another planet doesn't have it.

But if an alien was somehow able to exist by pumping liquid methane through its body instead of water, would that stop it from being a living thing?