One of the primary tasks in astrobiology is to detect biomarkers. Lin et al, in a recent paper, argue that industrial pollutants could be used in lieu of biomarkers to detect life on other planets. The unexpected twist in their hypothesis: they propose to observe old white dwarfs. They argue it is possible to look for pollutants from an industrialized society with technology comparable to ours. According to the authors, the ideal pollutant to detect would be specific chlorofluorocarbons (CFCs).
- white dwarfs have long-lived habitable zones as they are at the end of stellar evolution,
- the similarity in size of the white dwarf and an Earth-like planet should give the best contrast between the planet’s atmospheric transmission spectrum and the stellar background, and
- after a few billion years, a white dwarf at the Sun’s effective surface temperature should have a spectrum similar to the Sun, creating a comparable habitable zone, albeit much closer to the star
Lin et al mention that habitable planets could plausibly form debris of the stellar remnant. In short, a white dwarf could host an Earth-like planet at ~0.01 AU. If we further assume there is an anthropogenic civilization on said planet, they could industrialize and produce pollutants. The authors argue the ideal pollutants would be CFC-11 and CFC-14. CFCs have short lifetimes in the atmosphere (at most a few thousand years) and largely produced unnaturally, at least on Earth. There may be volcanic and fumarolic CFCs, but these are appreciably lower in concentration. The estimated time for this measurement would be on the order of day with JWST, which the authors argue could be used to simultaneous detect a spectral edge, be it natural or artificial. The simulations of spectra are shown in Figure 1.
This blogger is in awe at the assumptions going into this paper. The notion of white dwarfs around planets, while not new, invokes various questions of habitability. If we assume the planet existed before the stellar remnant emerged, how could it survive and retain its orbit? If we assume the planet forms from stellar ejecta, would the metallicity allow for the formation of a rocky planet? “Polluted” white dwarfs do exist and have evidence of debris disks, but as of yet no bona fide planet has been detected around a white dwarf. If we ignore that assumption, the issue of detectability persists and the choice of a cultural signal (a pollutant). CFCs are particularly heavy and would require an observation to probe beneath a significant amount of atmosphere. Furthermore, if in the outer atmosphere, UV radiation would readily destroy CFCs. The required resolution of 3000 is the estimate from JWST documentation but it may be much less at these particular windows. Lin et al assume the civilization must develop like Earth and cause an excess (more than ten times our currently level) of pollutants while refusing to control said emissions. Too many assumptions go into this argument for it to be a viable SETI experiment (i.e. a direct JWST proposal).