Lin et al. (2104) identify pollutants such as chlorofluorocarbons in the Earth’s atmosphere that would be detectable using JWST on other planets. In particular, they find that the time dedicated to an atmosphere with other potential biosignatures would be sufficient to constrain the levels of these CFCs, so the search can be parasitic, in a sense.
I think this is a great idea! As I come across ideas for artefact SETI, especially parasitic ones, I get more excited about the prospects for the field. Two major issues facing SETI are the lifetime of a technological civilization and funding. Both of these are, if not solved, greatly minimized by artefact SETI.
With that being said, I fear such a search for CFCs will flop. Even though the timescales of CFCs and excess carbon are large (especially compared to the age of human technology), I don’t have much faith in atmospheric measurements. I know that JWST will improve all of our current measurements, but from what I remember it will only be useful for large or close planets. All of the studies on exoplanet atmospheres have returned one result: clouds. I don’t expect this to change by much, but hopefully we’ll get a spectrum that isn’t flat! I do agree with the authors that since this data will already be gathered and analyzed for biosignatures, it might as well be analyzed for CFC absorption. The worst that happens if we find such absorption is that chemists/geologists/biologists publish ways that CFCs can be produced in the absence of intelligent life, which will further our understanding (and maybe will lead us to ways of removing the CFCs in our atmosphere!).
The one issue I have with this paper is the white dwarf thing. In the beginning, the authors say that they will only look at planets around white dwarfs, but say that their results “are generalizable to other telescopes and planetary systems.” I understand their arguments for a white dwarf, in that they provide better contrast, they could be the same temperature as the Sun, and they have very long lifetimes, but as of now (2018), we have yet to find a single planet around a white dwarf. I just feel they should have expanded their discussion to include all stars, and if they wanted the low contrast, then just M dwarfs. I personally don’t have enough background to state whether the absorption features from CFCs in an atmosphere around an Earth-like planet around an FGKM star would be visible. I wish this information had been provided by the authors in this paper.