http://arxiv.org/abs/2105.09990
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 observatory architectures. I used data from the Soumi National Polar-orbiting Partnership satellite to determine the broadband surface flux from city lights at the top of Earth’s atmosphere, and the spectra of commercially available high-power lamps to model the spectral energy distribution of the emitted flux from city lights. I also 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. I then calculate the minimum detectable urbanization fraction using 100 hours of observing time for generic Earth-analogs around stars with 10 pc of the Sun and for nearby known potentially habitable planets. Though Earth itself would not be detectable by LUVOIR or HabEx, planets around M-dwarfs close to the Sun would show detectable signals from city lights for urbanization levels of $0.4\%$ to $3\%$, while the 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 at an urbanization level ten times that of Earth in 100 hours, a level of urbanization that is expected to occur on Earth around the mid-22nd-century. An ecumenopolis, or planet-wide city, would be detectable around roughly 80 nearby stars by both LUVOIR and HabEx, and a survey of all these systems would be able to place a $1\sigma$ upper limit of $\lesssim1.4\%$ on the frequency of ecumenopolis planets in the Solar neighborhood assuming no detections.
T. Beatty
Mon, 24 May 21
15/41
Comments: Submitted to MNRAS. Comments welcome