The Detectability of Nightside City Lights on Exoplanets

Brief

Using VIIRS Day/Night Band data to calibrate Earth's disk-integrated nightside flux at 0.071 erg s⁻¹ cm⁻² (cloud-free; ~0.035 with typical cloud cover) and the Planetary Spectrum Generator to model coronagraphic performance of LUVOIR A/B and HabEx, Beatty calculates the minimum urbanization fraction detectable at 3σ in 300 hours of direct imaging. Earth's current 0.05% urbanization sits below every proposed architecture's detection floor. M-dwarf habitable-zone targets within several parsecs become detectable at 0.4%–3% urbanization; Proxima b crosses the LUVOIR A 3σ threshold at 12× Earth's current level. A null-result survey of the ~50 ecumenopolis-accessible stars within 8 pc would cap the frequency of planet-wide cities at ≲2.2% (1σ) and ≲10.7% (3σ), a concrete input to Drake Equation terms for intelligent-civilization prevalence.

Metadata

Category

Search

Venue

Monthly Notices of the Royal Astronomical Society

Type

Peer-reviewed

Year

2022

Authors

Thomas G. Beatty

DOI

10.1093/mnras/stac469

arXiv

2105.09990

Access

Open access

Length

3.8 M

Programs

LUVOIR, HabEx

Instruments

LUVOIR A (ECLIPS), LUVOIR B (ECLIPS), HabEx (HCG), HabEx with starshade, VIIRS Day/Night Band (Soumi NPP satellite), Planetary Spectrum Generator (PSG)

Data sources

VIIRS DNB 2016 annual composite, MERRA-2 atmospheric template, Sylvania 600W SHP-TS Super HPS lamp spectrum (manufacturer datasheet)

Tags

SETI, technosignature, direct imaging, exoplanet, artificial lighting, urbanization

Key points

• Earth's disk-integrated nightside city-light flux, derived from the 2016 VIIRS DNB annual composite, is 0.071 erg s⁻¹ cm⁻² cloud-free and approximately 0.035 erg s⁻¹ cm⁻² under typical cloud cover.p.3
• Only 0.05% of Earth's surface reaches city-core intensity; peak measured emission is 47 erg s⁻¹ cm⁻² sr⁻¹ at Times Square and 35 erg s⁻¹ cm⁻² sr⁻¹ in Tokyo's Shinjuku district.p.3
• An Earth analog at current 0.05% urbanization is undetectable by LUVOIR A, LUVOIR B, or HabEx; M-dwarf planets within ~8 pc cross the LUVOIR A 3σ threshold at 0.4%–3% urbanization in 300 hours, while planets around nearby Sun-like stars require urbanization exceeding 10%.p.1
• Proxima b is the paper's most compelling individual target: LUVOIR A detects city lights at 12× Earth's urbanization in 300 hours, a level Beatty ties to mid-22nd-century Earth projections.p.1
• A modeled ecumenopolis, entire surface at 40 erg s⁻¹ cm⁻² sr⁻¹, yielding 62.8 erg s⁻¹ cm⁻² total, would be detectable around 50 stars with LUVOIR A, 39 with LUVOIR B, 32 with HabEx+starshade, and 15 with baseline HabEx, all within 8 pc.p.6
• A null-detection ecumenopolis survey with LUVOIR A across its 50-star accessible sample would place 1σ and 3σ upper limits of ~2.2% and ~10.7% on the frequency of planet-wide cities in the Solar neighborhood.p.6
• All simulations assume high-pressure sodium lamps; LED street-light replacement already underway on Earth would broaden the spectral signature and reduce detectability, as HPS lamps emit a sharp detectable spike near 6000 Å that LEDs lack.p.5
• The paper models observations at spectroscopic resolution R=140 over 300 hours, with an assumed exozodiacal level of 3 zodis; detections become background-limited quickly, making LUVOIR A the only architecture capable of significant city-light detections beyond ~8 pc.p.5

Verbatim

• “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 𝜎 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 & 10%.”

  p.1

• “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.”

  p.1

• “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.”

  p.2

• “Based on the DNB composite images, only 0.05% of Earth has a surface intensity that is on the order of these city cores, and I therefore considered that only 0.05% of Earth's surface can be considered "heavily" urbanized.”

  p.3

Most interesting

• VIIRS DNB's sensitivity was validated in a field experiment that detected a 30 m² tarp illuminated by a single lamp in rural Puerto Rico, the same instrument underpins the paper's entire Earth flux calibration.
• Even at a phase angle of 60° from nadir, a planet still radiates 75% of its full nightside city-light flux toward an observer, making oblique viewing geometries nearly as sensitive as direct nightside views.
• An ecumenopolis is modeled with every square meter of planetary surface held at city-center intensity (40 erg s⁻¹ cm⁻² sr⁻¹), yielding a total disk-integrated flux of 62.8 erg s⁻¹ cm⁻², roughly 880× Earth's cloud-free nightside output.
• LED replacement of HPS streetlights, actively underway on Earth, would make technologically advancing civilizations progressively harder to detect, inverting the naive assumption that more advanced societies are more visible.
• The Proxima b detection threshold maps directly onto a real demographic projection: Beatty explicitly cites the mid-22nd century as the epoch when Earth's own urbanization fraction could reach that 12× level.
• A detection null result from a LUVOIR A ecumenopolis survey would still be scientifically productive: capping the occurrence rate of planet-wide cities at under 10.7% (3σ) across the Solar neighborhood constitutes a meaningful constraint on Drake Equation terms for civilization frequency and longevity.