The detectability of nightside city lights on exoplanets

Next-generation missions designed to detect biosignatures on exoplanets will also be capable of placing constraints on technosignatures (evidence for technological life) on these same worlds. Here, I estimate the detectability of nightside city lights on habitable, Earth-like, exoplanets around nearby stars using direct-imaging observations from the proposed LUVOIR and HabEx observatories, assuming these lights come from high-pressure sodium lamps. I consider how the detectability scales with urbanization fraction: from Earth's value of 0.05 per cent, up to the limiting case of an ecumenopolis - or planet-wide city. Though an Earth analogue would not be detectable by LUVOIR or HabEx, planets around M-dwarfs close to the Sun would show detectable signals at 3σ from city lights, using 300 h of observing time, for urbanization levels of 0.4-3 per cent, while city lights on planets around nearby Sun-like stars would be detectable at urbanization levels of ≳10per cent. The known planet Proxima b is a particularly compelling target for LUVOIR A observations, which would be able to detect city lights 12 times that of Earth in 300 h, an urbanization level that is expected to occur on Earth around the mid-22nd century. An ecumenopolis, or planet-wide city, would be detectable around roughly 30-50 nearby stars by both LUVOIR and HabEx, and a survey of these systems would place a 1 σ upper limit of ≲2 to ≲4 per cent, and a 3 σ upper limit ≲10 to ≲15 per cent, on the frequency of ecumenopolis planets in the Solar neighbourhood assuming no detections.