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 3 sigma 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 greater than or similar to 10 per 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 sigma upper limit of less than or similar to 2 to less than or similar to 4 per cent, and a 3 sigma upper limit less than or similar to 10 to less than or similar to 15 per cent, on the frequency of ecumenopolis planets in the Solar neighbourhood assuming no detections.

Automated summary

The study estimates the feasibility of detecting nighttime city lights on habitable Earth-like exoplanets around nearby stars, and finds that under certain conditions, these city lights can be detected.