Oxygen has been the most important gas in our search for life among the cosmos thus far. On Earth, we have it in abundance because it is produced by biological synthesis. But that might not be the case on other planets, so even if we do find a very clear high oxygen signal in the atmosphere of an exoplanet, it might not be a clear indication that life exists there. A new paper, available in pre-print on arXiv, from Margaret Turcotte Seavey and a team of researchers from institutions like the NASA Goddard Space Flight Center and Johns Hopkins University, adds some additional context to what else might be going on in those atmospheres. In particular, they note that if there’s even a little bit of water vapor, it can make a big difference in whether a lifeless rock looks like a living, thriving world.