> I am bothered by how improbable it is that even simple life forms have not been discovered anywhere else in the universe. The obvious reason is that our abil

> I am bothered by how improbable it is that even simple life forms have not been discovered anywhere else in the universe. The obvious reason is that our ability to look is extremely limited. Could there be subterranean life surviving deep in Martian rock? We don't know, we haven't looked yet there yet. Could there be life sustained by thermal vents in Europa's subterranean ocean? We don't know, we haven't looked there yet. We've identified some habitable zone exoplanets. Do any of them have life? We don't know, we don't have the technology to tell. If we are talking about Earth-like multicellular life, you'd expect there would be some atmospheric changes, which we'd eventually be able to detect remotely via space telescopes (even if the current generation of space telescopes can't yet). But if we are talking about subterranean microscopic life – how could we tell without physically sending a probe there? So I don't think we'll know this century. And surely there's many more habitable zone exoplanets out there we haven't identified yet, because our telescopes aren't good enough. And eventually we'll get better telescopes, and we'll identify more of them. But even then there will be many more that even those telescopes aren't powerful enough to identify.