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.

While we continue our search for Earth-like planets with oceans and continents and nitrogen-oxygen

atmospheres, we now have reason to believe that there may be super-large planets with oceans

and thick atmospheres, which may be able to support life even without sunlight.

A critical concept in our search for extraterrestrial life is the idea of a Goldilocks

zone, the region distant from a given star, where a planet would not be too hot or too cold,

but just right.

Usually we calculate this off an approximation of how much sunlight a world of around Earth's mass

would need to have, not to have its oceans evaporate off, for the inner boundary, or likewise

freeze solid for the outer boundary.

Needless to say, this presupposes that life is using water rather than something more exotic,

like ammonia or mercury, but the other half of that is that it is assuming a roughly Earth-mass world.

In recent years, we've discovered a much wider variety of larger planets than our solar system

is home to, and we've begun realizing it is possible to have planets potentially 10 times

more massive than Earth that could have oceans and thick

hydrogen atmospheres, and thus might have liquid water at very unexpected distances from stars,

indeed possibly in deep space, floating around as a rogue planet. And this category is being

named Heysian planets, as the blend of the words hydrogen and ocean.

Now while that term is new, the basic concept is not.

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