The most- and least-spaceworthy Earth species could provide a starting point. Maybe the cockroach and blue whale.
Ok, whichever scientist decided to take device that can supercool things to 1 degree Kelvin and put some tardigrdes in to see what happens is very nearly as cool as whoever decided it was a good idea to make frogs levitate with magnets.
They're too small to be intelligent enough for space travel.
I wonder how much space-worthiness varies with size. Perhaps you could use that.
The thing is, it gets complicated when you start thinking about hypothetical biomes. How efficient per weight can evolved nervous systems get? Are more efficient nervous systems more likely on lighter and heavier planets?
They're too small to be intelligent enough for space travel.
Are they? What is the limit for how dense intelligence can be concentrated? I would bet that a superintelligence could design something that small that was intelligent enough for mere space travel. Just not out of proteins or dna.
But if we modified tardigrades to be able to support intelligence like ours and sent them into space, would it feel as though we were going out there? What if human minds were imprinted on them?
I might feel as though it were me if hands were included in the deal.
A recent post about the Fermi paradox left me wondering about relative difficulties of getting into space, though I don't think it affects those specific arguments.
People establishing a presence in space is difficult but at least plausible-- I'm talking about biological people as we are now, and being able to live and reproduce indefinitely without returning to Earth.
It would be easier if we were less massive, or our planet was less massive, or if we were more radiation resistant. It would be harder if these qualities were reversed, or if we needed a much denser atmosphere.There might come a point where it just isn't feasible for a species to get itself off its planet.
Is there any reasonable speculation about where we are likely to be on the ease-of-getting-into-space spectrum?