Draft version September 6, 2021 Typeset using LATEX twocolumn style in AASTeX63 The Detectability of Nightside City Lights on Exoplanets Thomas G. Beatty1 1Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721; [email protected] ABSTRACT Next-generation missions designed to detect biosignatures on exoplanets will also be capable of plac- ing constraints on the presence of technosignatures (evidence for technological life) on these same worlds. Here, I estimate the detectability of nightside city lights on habitable, Earth-like, exoplan- ets around nearby stars using direct-imaging observations from the proposed LUVOIR and HabEx observatories. I use data from the Soumi National Polar-orbiting Partnership satellite to determine the 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 city lights. 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. I then calculate the minimum detectable urbanization fraction using 300 hours of observing time for generic Earth-analogs around stars within 8 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 city lights on planets around nearby Sun-like stars would be detectable at urbanization levels of & 10%. The known planet Proxima b is a particu- larly 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 50 nearby stars by both LUVOIR and HabEx, and a survey of these systems would place a 1 σ upper limit of . 2% on the frequency of ecumenopolis planets in the Solar neighborhood assuming no detections. Keywords: exoplanets: habitable planets { exoplanets: detection methods: direct imaging { exoplanets: structure: surface characteristics 1. INTRODUCTION technosignatures are observable signals which indicate The search for biosignatures on potentially habitable the presence of technological life on an exoplanet or in exoplanets has driven much of the recent planning in the a star system (Tarter 2007; Wright et al. 2018). An exoplanet community for the next generation of space ideal technosignature would be a long-lived, unambigu- telescopes. In particular, the National Academies' Ex- ous, observable property of another civilization that (for oplanet Science Strategy and Astrobiology Strategy re- simplicity) requires relatively little extrapolation from ports both recommended the development of a large, conditions on Earth (Sheikh 2020). space-based, direct-imaging observatory that would be For the last several decades the main methods of capable of directly detecting emission from habitable searching for technosignatures have been radio (e.g., arXiv:2105.09990v2 [astro-ph.EP] 2 Sep 2021 Earth-like exoplanets. At the moment, the two pri- Worden et al. 2017) or optical (e.g., Howard et al. 2004) mary observatory architectures under consideration are monitoring projects to look for signals actively being the Large UV/Optical/IR Surveyor (LUVOIR, The LU- broadcast by a nearby technological civilization. The VOIR Team 2019) and the Habitable Exoplanet Obser- lack of a technosignature detection from these projects vatory (HabEx, Gaudi et al. 2020). Either would pro- implies that the frequency of other technological life in vide a dramatic increase in our ability to characterize the Solar neighborhood is low, that other technological exoplanet atmospheres and search for biosignatures. life is not strongly broadcasting in the radio or optical, As our ability to characterize exoplanets improves, we that our detection sensitivities are low, or some combi- also move closer to the direct detection of technosigna- nation of these three. tures on exoplanets. Similar in concept to biosignatures, The development over the last fifteen years of the facil- ities and techniques necessary to precisely characterize 2 Beatty Figure 1. The nightside of Earth shows significant emission from city lights in the optical. This is a composite, cloud-free, image of Earth's city lights compiled using Day/Night Band observations taken using the Visible Infrared Imaging Radiometer Suite instrument on the Soumi National Polar-orbiting Partnership satellite (Rom´anet al. 2018). Searching for the emission from city lights is a compelling technosignature because it requires very little extrapolation from current conditions on Earth, should be relatively long-lived presuming an urbanized civilization, and offers a very distinct spectroscopic signature that is difficult to cause via natural processes. This places the emission from nightside city lights high on the list of potential technosignatures to consider (Sheikh 2020). the atmospheres of exoplanets gives us a new way to use of silicon-based photovoltaic arrays, which would search for technosignatures, and to do so in a way that show a significantly different reflection spectrum than does not rely on active broadcasts, by directly examin- vegetation. Berdyugina & Kuhn(2019) considered the ing potentially habitable planets themselves. While this more general problem of identifying artificial surface fea- approach will likely result in narrower searches (in terms tures on Proxima b, which might be possible using next- of number of targets) than the radio and optical broad- generation space telescopes. Socas-Navarro(2018) in- cast surveys, directly searching for technosignatures on vestigated the observational signatures of dense satellite exoplanets is sensitive to a much wider range of possi- constellations in orbit about an Earth-like exoplanet, ble phenomena (e.g., Kipping & Teachey 2016; Wright and found that for Earth-analogs with M-dwarf pri- et al. 2016). Assuming such a direct search finds no maries this may be observable in transit data. Vides significant detections, this also likely means that tighter et al.(2019) determined that relatively low powered (10s constraints could be placed on the last three terms in the of kW) laser pulses would be detectable from nearby Drake Equation (Drake 1962) than for a typical radio or Sun-like stars in direct imaging observations from the optical broadcast survey. Gemini Planet Imager and the Roman Space Telescope. Searching for technosignatures on exoplanets also has A bit more grimly, Stevens et al.(2016) noted that the the virtue of being able to piggy-back on the observa- increased atmospheric dust and clouds caused by a large tions necessary to detect biosignatures on these same nuclear war could be observable on an exoplanet, though planets. For example, the deep spectroscopic obser- it would likely be difficult to distinguish this from the vations needed to successfully detect biosignatures on impact of a large asteroid. an Earth-analog exoplanet using either LUVOIR and More specifically on possible atmospheric technosig- HabEx (e.g., Wang et al. 2018) will also be capable of natures, both Lin et al.(2014) and Kopparapu et al. detecting { or at least meaningfully constraining { cer- (2021) have investigated the detectability of industrial tain technosignatures. In this sense many of these ob- pollution in an Earth-analog's atmosphere. Lin et al. servations will be \free" in terms of observing time, and (2014) looked at chlorofluorocarbons, since these are ex- the expected signals from likely technosignatures should tremely unlikely to be generated via biotic or geologic be looked for in these future data sets. processes, and found concentrations ten times that in Already, significant thought has been put into what Earth's atmosphere would be detectable using 30 to 50 some of these technosignatures might look like. Lingam hours of time on the James Webb Space Telescope for & Loeb(2017) considered the technosignature on the Earth-analogs orbiting white dwarfs. Kopparapu et al. surface of an Earth-like exoplanet caused by large-scale (2021) considered pollution from nitrogen dioxide and Detecting Nightside City Lights 3 ) Latitudinally-Integrated Disk-Integrated 1 0.9 . 0.14 g 0.8 e ) d 2 2 0.12 0.7 m c m c 1 0.6 0.10 1 s s g 0.5 r g 0.08 e r ( e ( x 0.4 u y l 0.06 t i F s 0.3 e n c e 0.04 t a f n 0.2 r I u e S 0.02 c 0.1 a f r u 0.0 0.00 S 150 100 50 0 50 100 150 150 100 50 0 50 100 150 Longitude (deg.) Longitude (deg.) Figure 2. While the latitudinally-integrated surface intensity from Earth (left panel) varies significantly as a function of longitude, the complete disk-integrated surface flux is relatively constant with longitude at 0:071 erg s−1 cm−2. Note too that this average surface flux value is significantly lower than the peak emission in city centers (e.g., the few square kilometers around Times Square in New York City reach 47 erg s−1 cm−2 sr−1). This reflects the fact that the total area covered by cities on Earth is low, at approximately 0.05%. estimated that the 15m LUVOIR A architecture would flux output and the spectrum of the emitted radiation be capable of detecting Earth's level of nitrogen dioxide at the top of the atmosphere. Recent space-based ob- pollution using 400 hours of observing time on a system servations of Earth have generated several high-quality at 10 pc. datasets of Earth's city lights as seen from low Earth The emission from city lights on the nightsides of exo- orbit (Figure1), which I used to make these estimates.