Projections of Earth's Technosphere: Strategies for Observing Technosignatures on Terrestrial Exoplanets

Abstract: The search for technosignatures -- remotely detectable evidence of extraterrestrial technology -- draws upon examples from the recent history of Earth as well as projections of Earth's technosphere. Facilities like the Habitable Worlds Observatory (HWO) will significantly advance the feasibility of characterizing the atmospheres of habitable exoplanets at visible and near-infrared wavelengths, while other future mission concepts could extend this search to mid-infrared wavelengths. We draw upon a recently developed set of ten self-consistent scenarios for future Earth technospheres as analogs for extraterrestrial technospheres, which we use to outline a stepwise technosignature search strategy beginning with HWO and followed by other missions. We find that HWO could reveal elevated abundances of a CO$_2$ + NO$_2$ pair on planets with combustion and other large-scale industry, which could be observable in up to in eight of the ten scenarios. Follow-up radio observations could reveal narrow-band directed transmissions, which occurs in two of the scenarios. Further study by direct detection at mid-infrared wavelengths with the Large Interferometer for Exoplanets (LIFE) could reveal spectral features from industry, such as the combination of CO$_2$ + CFC-11/12 in four scenarios and CO$_2$ + CFC-11/12 + CF$_4$ in one scenario; two of these also include the N$_2$O + CH$_4$ + NH$_3$ triple from large-scale agriculture. Other mission concepts such as a Solar Gravitational Lens mission could reveal large-scale surface features in two scenarios that otherwise show no detectable technosignatures, while an interplanetary fly-by or probe mission would be the most conclusive way to reveal the presence of technology on terrestrial exoplanets.