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.

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Among the billions and billions of planets and moons in this galaxy, slash Isaac Arthur, and use my code, Isaac Arthur.

Among the billions and billions of planets and moons in this galaxy, we may encounter many examples of double planets, but what would they be like and could life arise on them?

When we think of planets out there on the cosmos, we usually envision solitary spheres, opening

a star, or perhaps one with a large moon or two like we do. Sometimes we envision not planets

but large habitable moons, around some even larger gas giant. But out there in the vastness

of space, some planets have formed an extraordinary partnership, locked in a cosmic dance with another planet,

that eventually leads them to looking at each other's face all day, every day, for billions

of years to come.

These are the double planets, a fascinating phenomena that raises even more questions about

their formation, evolution, and unique conditions that might be present in them.

Could we find where neither was a moon, but both were large and habitable like Earth,

and would life have evolved from one of those doubles and spread to the other?

Could life continue to move back and forth between them,

and would such life tend to adapt to natural

...