A study of the potential atmospheric chemistry on TRAPPIST-1e (and to a lesser extent f). For the amount of CO
2 required to maintain habitable conditions, an O
2/CO runaway occurs, leading to an atmosphere dominated by these two molecules. This does not appear to be inhibited by nitrogen photochemistry.
Hu et al. "O
2- and CO-Rich Atmospheres for Potentially Habitable Environments on TRAPPIST-1 Planets"
https://arxiv.org/abs/1912.02313This supports
earlier results that "habitable" M-dwarf planets (if they exist) are going to have lots of carbon monoxide in their atmospheres. So whatever's there probably won't be using haemoglobin (then again, a lot of life on this planet doesn't use haemoglobin either...)