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Yeah, alien bacteria might be pretty nasty. Unless the alien life was nearly
identical to Earth life we probably wouldn't have to worry about alien viruses,
though, which would be a good thing.
And maybe there's a 50% chance the bacteria wouldn't even be dangerous. All
of our Earth-life amino acids are chiral, meaning their mirror-image isn't
the same as the original. It's an outstanding puzzle how homochirality came
about on Earth, but if the process is random, then there's a 50% chance that
ALL the alien amino acids will be different; I think they all have to be
either right-handed or left-handed.
But there was a recent paper that might be bad news for the random theory;
a meteorite's moledules were recently analzyed for chirality, and a slight
imbalance was found between the right- and left- handed versions. And the
imbalance favored the same versions as Earth life. So maybe it's not a
random process after all, and all alien life will have some duplicate aminos.
It's still an unresolved question, though.
As for the atmosphere on a planet around Tau Ceti... Maybe we have no way of
knowing right now, but is there any way we could make a measurement without
actually going there? I'm not sure what Deep Space Interferometry is
capable of, but anyone know what spectral signatures we might be able to
measure long-distance? Water vapor? Ozone? CO2? I know NASA is batting
around the idea, but I have no idea what the theoretical detection limits
are. Especially with so much light coming from the star...