Detectability of Solar Panels as a Technosignature

Brief

Kopparapu et al. (2024) simulate the reflected-light spectral signature of silicon photovoltaic arrays on an Earth-analog planet using the Planetary Spectrum Generator paired with a LUVOIR-B/HWO instrument model. The critical detection window is 0.34–0.52 μm, where silicon's reflectance edge is most spectrally distinct, though realistic anti-reflective coatings make that edge considerably weaker than pure-silicon models assumed by Lingam & Loeb (2017). At the most favorable planetary orientation (longitude 315°) and the upper-limit coverage of 23% land area, SNR=5 still demands several hundred hours of integration on an 8-meter telescope observing a Sun-like star at 10 pc. The paper further argues that sustainable civilizations are unlikely ever to deploy panels at the scale needed for detectability, which they frame as a contribution to the 'sustainability solution' of the Fermi paradox.

Metadata

Category

Search

Venue

The Astrophysical Journal

Type

Peer-reviewed

Year

2024

Authors

Ravi Kopparapu, Vincent Kofman, Jacob Haqq-Misra, Vivaswan Kopparapu, Manasvi Lingam

DOI

10.3847/1538-4357/ad43d7

arXiv

2405.04560

Access

Open access

Length

362.5 K

Programs

Habitable Worlds Observatory (HWO), LUVOIR-B

Instruments

Habitable Worlds Observatory (HWO, concept), LUVOIR-B 8-meter concept telescope, Planetary Spectrum Generator (PSG), MODIS Terra/Aqua satellites

Data sources

MODIS-MCD12C1 land-cover dataset, RELAB Reflectance Experiments Laboratory, USGS spectral database, Energy Institute Statistical Review, OECD primary energy supply data, US Energy Information Administration IEO projections

Tags

technosignature, SETI, exoplanet spectroscopy, photovoltaics, Fermi paradox, Kardashev scale

Key points

• Current (2022) global energy demand of 604 exajoules requires only ~2.4% of Earth's land covered in solar panels at a power density of 5.4 W m⁻²; 23% coverage is used as the detectability upper limit, corresponding to 5840 exajoules per year.p.3
• At 23% land coverage and optimal planetary orientation (longitude 315°), SNR=5 requires several hundred hours of integration with an 8-meter HWO-like telescope on a target at 10 pc.p.6
• Realistic solar cells carry anti-reflective coatings (TiO₂ or Si₃N₄) that suppress and redshift the UV absorption edge relative to pure silicon, materially weakening the spectral feature compared to Lingam & Loeb (2017).p.2
• Beyond 0.8 μm, silicon panel signatures blend inseparably with ocean, soil, and vegetation reflectance, confining useful technosignature discrimination to the 0.34–0.52 μm NUV band.p.5
• M-dwarf habitable-zone planets face a compounded disadvantage: spatial resolution limits preclude direct imaging with current technology, and M-dwarf stellar flux in the 0.34–0.52 μm band is substantially lower, reducing the SNR further.p.6
• 3% land coverage suffices to power 10 billion people at a high standard of living (75 GJ per capita per year); the 23% scenario produces 7.8× more energy than a population of 10 billion would consume.p.8
• Kardashev Type I threshold (~5×10²⁴ J) is not reached until 2377 even at the historical 2.6% yr⁻¹ growth rate, and the authors argue sustainable civilizations would stabilize well below it.p.8
• Simulations used the Planetary Spectrum Generator (PSG) with a LUVOIR-B analog: NUV channel 0.2–0.525 μm at R~7 coronagraphic imaging; optical IFS at R=140; planet held at quadrature (orbital phase 270°), inclination 90°.p.6

Verbatim

• “we find that several 100s of hours of observation time is needed to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel coverage of ∼ 23% land coverage of a future Earth.”

  p.1

• “even with the most ambitious land coverage ( ≈ 23%), and with a favorable viewing perspective to the observer (planet longitude of 315 ◦ ), it would take several hundred hours of observation time in reflected light spectra with an 8 m size telescope to reach SNR ∼ 5 (solid purple curve).”

  p.6

• “This suggests that the artificial silicon edge suggested by Lingam & Loeb (2017) may not be detectable.”

  p.6

• “only ∼ 2 . 4% of land coverage by solar panels would be needed to match the world energy consumption in 2022.”

  p.3

• “only 3% land coverage by solar panels would be needed to support a population of 10 billion people at a high standard of living”

  p.8

• “Any extraterrrestrial civilization that likewise achieves sustainable population levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization as imagined in the Fermi paradox may not exist.”

  p.1

Most interesting

• The fiducial deployment site is the Sahara Desert, chosen for equatorial solar flux and low cloud cover, but the authors note dust storm frequency has increased over four decades, averaging ~20 events per year, which would reduce actual energy output.
• Technological efficiency improvements in solar panels would only decrease the land coverage required and therefore decrease detectability, creating an ironic inverse relationship between engineering progress and SETI yield.
• A population of 30 billion people at a high standard of living requires only 8.9% land coverage, roughly the area of China and India combined, still well below the 23% detectability threshold.
• The MODIS-MCD12C1 satellite land-cover dataset was used to construct a realistic planetary surface model at 2.5×2° resolution, with solar-panel reflectance drawn from the RELAB (Reflectance Experiments Laboratory) spectral library.
• The paper calculates that direct thermal heating of Earth's atmosphere becomes a concern at ~3×10²³ J, a threshold reached by 2265 at the 2.6% yr⁻¹ growth rate, suggesting planetary thermodynamics itself may cap energy growth before any technosignature becomes detectable.
• Silicon's fitness for solar cells is grounded in cosmic abundance: silicon is far more common than germanium, gallium, or arsenic used in competing photovoltaic chemistries, making silicon-based panels the authors' best bet for a convergent extraterrestrial technology.