How similar is similar? Mars and Venus are "similar to Earth" in nearly every way that we could detect if they were exoplanets, but neither is capable of sustai

How similar is similar? Mars and Venus are "similar to Earth" in nearly every way that we could detect if they were exoplanets, but neither is capable of sustaining Earthlike life. I think it's far from a solved question how likely life is, given we have exactly one example of it arising.