Here on Earth, the closest world to us is our barren, uninhabited moon. But in many imaginable cases, there could be another inhabited world close by our own, maybe even within our Solar System. How close could one be? (flickr user Kevin Gill)

Ask Ethan: How Close Could Two Alien Civilizations Get To One Another?

Here on Earth, all the right conditions occurred for intelligent life to come about, but the nearest aliens, if they’re on another world, are light years away. But it doesn’t have to be that way at all!

Here on planet Earth, in orbit around the Sun, we’re the only intelligent-life game in town. There might be possibilities for either past life or microbial life elsewhere in the Solar System, but as far as intelligent, complex, differentiated and multicellular life goes, what’s on our world is far more advanced than anything else we could hope to find. Intelligent aliens, if they’re out there inhabiting another world, are at least four light years away. But must that be the case for aliens anywhere in the galaxy? That’s what our Patreon supporter Jason McCampbell wants to know:

What’s [the] closest two, independent intelligent civilizations could be, ignoring interstellar travel and assuming they develop in different star systems and follow roughly what we know as ‘life’? Globular clusters can have a high density of stars, but does too high a density rule out habitability? An astrophysicist in a dense cluster would have a much different view of the universe and the…