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

we're asked at 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.

We search for life out in the galaxy in what we call the habitable zone, or gaudilocks

zone, not too hot, not too cold, but this presumes alien life is carbon-based or needs water.

Could we be overlooking worlds running on alternative chemistries?

Today we'll be discussing the possibility of life based on ammonia,

probably the most popular alternative chemistry for life that gets discussed,

except for maybe silicon as a carbon alternative,

and not just discussed in fiction, but in science circles too, for many decades.

Indeed, NASA has given this a lot of serious thought, such as the 2003 article from NASA Ames,

searching for alien life having unearthly biochemistry, written by Harry Jones, which we will borrow from today.

The topic of if alien life might exist in the cosmos

inevitably leads us to wondering how it might be different from it having arisen on strange

new worlds unlike our own, and how this should change our methods for seeking it. Right now,

we basically seek out life in what we call the gaudilocks zone, not too cold, not too hot, just right. But unlike

in the story of gaudilocks in the three bears, we're not talking about porridge, but water.

A world covered in ice or in which the oceans would evaporate away, is not a place where

we'd expect to find complex surface life on. In the absence of surface life, we would not expect a rich

ecology powered by sunlight, but life might live deep underground or in the deep ocean, even

the surface of a planet, was frozen over, and we discussed that option before, most recently

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