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 gaudylox 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?

No. 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 gertilocks zone, not too cold, not

too hot, just right.

But unlike in the story of gaudylocks and the three bails, 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

...