Technosignatures in exoplanet atmospheres

In this post I shall discuss two articles – Lin et al. 2014, and Lingam and Loeb 2017. The premise of both these articles is the distinction between biosignatures and technosignatures. The two terms are discussed in this paper by Jason Wright. Biosignatures in this case refer to evidence for life found  in the transmission and reflection spectra of exoplanetary atmospheres (review of biosignatures by Schwieterman et al. 2017). Technosignatures refer to signs of intelligent civilization. In the SETI context it can be communication, megastructures, and pollution.

The article by Lin et al 2014 posits the existence of absorption features corresponding to organohalogens, and more specifically chlorofluorocarbons (CFCs). The existence of these chemicals which are not produced naturally would require an intelligent civilization. They posit that  a civilization with 10x our CFC levels would be detectable by the James Webb Space Telescope (JWST) with ~ 1 day of integration time.  I do have objections to their assumptions – an anthropocentric industrial and polluting model; habitable planet around white dwarf. Further how sustainable would 10x CFC levels be. However, despite that I found this interesting since it shows that within an order of magnitude we could detect such a planet with prolonged use of today’s technology (JWST).

The article by Lingam and Loeb, instead of considering polluting features, looks for ‘spectral edges’ from the harvesting of stellar energy by either photovoltaic cells or vegetation. The presence of vegetation on a planet would impart a red edge (infrared) due to expelled heat. Similarly, silicon photovoltaic cells cause a peak  in the extreme UV. This peak if indubitable, and confirmed without any other natural explanation would signify the existence of large photovoltaic farms deployed to harness stellar energy. This would also be an unmistakable technosignature.

The advent of new instruments like JWST and perhaps ARIEL are taking us into the era, where search for technosignatures can be achieved by optical and NIR spectroscopy. Since the instrumentation is being built and developed for finding Earth like planets in their habitable zones, along with characterization of atmospheres; SETI stands to gain in its search for technosignatures by piggybacking on this progress.