> Do those actually qualify as alien, if they're products of our human culture and just the substrate is different?

I'll posit "alien" is a spectrum.

For the sake of the argument, let's assume that some form of panspermia is real and the same tree of life has reached Earth, Enceladus (moon of Saturn) and TRAPPIST-1 (a different solar system in our galaxy). Let's also say there was a second abiogenesis event somewhere in Messier 104 (another galaxy).

Earth to Enceladus would arguably be already "alien", but there might be similarities, maybe there was something there that looked like one of our Archaea, while sharing none of our Eukarya and having its own domains of life.

Earth to TRAPPIST-1 would be distinctly alien, evolved so differently it'd be almost unrecognizable, but they'd likely still be carbon-based lifeforms sharing the same basic building blocks. Maybe something like lipids forming cell walls would also be seen there, but they'd likely be independently evolved.

Earth to M104, any similarity would be at best convergent evolution. Truly unarguably alien.

https://web.archive.org/web/20180423171909/https://cosmosmag...