Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asking the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and add free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

When we talk about life beyond Earth, we usually start with a familiar picture.

Blue oceans, green plants, a rocky world that could be almost our own.

But the universe is far older and far stranger than that.

And life, if it exists elsewhere, may not play by our rules at all.

We humans are carbon chauvinous.

Every living thing we know is built from the same black skeleton of carbon atoms, swimming

water and breeding oxygen, or something like it.

It's easy to feel that chemistry is vast, billions of combinations of elements,

pressures, any of which might have found its own path to life. It's also easy to get that

we ourselves are not really made mostly of carbon, we're mostly made of water. The majority of

atoms in our body are hydrogen and oxygen. Hydrogen makes about 62% of our atoms,

though only about 10% of our mass. Well, oxygen is about a quarter of our atoms, but 65% of our mass.

Carbon, by comparison, is just 12% of our atoms, and about 18% to 19% of our mass.

Less than a fifth. So when we call our cells carbon-based, it's not about how much carbon we have.

It's about its role.

Carbon is the framework, the skeleton that binds all the rest together.

And that raises an interesting question.

What if something else could play that role?

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