The Detectability of Nightside City Lights on Exoplanets

TL;DR

This paper evaluates the potential to detect artificial city lights on habitable exoplanets using future space telescopes, finding that certain urbanization levels around M-dwarfs and nearby stars could be observable, informing the search for technological life.

Contribution

It provides the first estimates of the detectability of nightside city lights on exoplanets with upcoming direct-imaging missions like LUVOIR and HabEx, considering various urbanization scenarios.

Findings

Detectable city lights on planets around M-dwarfs at 0.4%–3% urbanization.

Proxima b could show city lights twelve times Earth's level.

Ecumenopolis planets could be detected around 30–50 nearby stars.

Abstract

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%, up to the limiting case of an ecumenopolis -- or planet-wide city. Though an Earth analog 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 hours of observing time, for urbanization levels of 0.4% to 3%, while city lights on planets around nearby Sun-like stars would be detectable at urbanization levels of $\gtrsim10\%$. The known planet Proxima b is a particularly compelling target for LUVOIR A observations, which would be able to detect city lights twelve times that of Earth in 300 hours, 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 to 50 nearby stars by both LUVOIR and HabEx, and a survey of these systems would place a $1\,\sigma$ upper limit of $\lesssim2\%$ to $\lesssim4\%$, and a $3\,\sigma$ upper limit $\lesssim10\%$ to $\lesssim15\%$, on the frequency of ecumenopolis planets in the Solar neighborhood assuming no detections.