If alien technological civilizations exist, they almost certainly use solar energy. Along with wind, it’s the cleanest, most accessible form of energy, at least here on Earth. Driven by technological advances and mass production, solar energy on Earth is expanding rapidly.
It seems likely that ETIs (Extraterrestrial Intelligence) using widespread solar energy on their planet could make their presence known to us.
If other ETIs exist, they could easily be ahead of us technologically. Silicon solar panels could be widely used on their planetary surfaces. Could their mass implementation constitute a detectable technosignature?
The authors of a new paper examine that question. The paper is “Detectability of Solar Panels as a Technosignature,” and it’ll be published in The Astrophysical Journal. The lead author is Ravi Kopparapu from NASA’s Goddard Space Flight Center.
In their paper, the authors assess the detectability of silicon-based solar panels on an Earth-like habitable zone planet. “Silicon-based photovoltaic cells have high reflectance in the UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept like the Habitable Worlds Observatory (HWO),” the authors write. The HWO would search for and image Earth-like worlds in habitable zones. There’s no timeline for the mission, but the 2020 Decadal Survey recommended the telescope be built. This research looks ahead to the mission or one like it sometime in the future.
Naturally, the authors make a number of assumptions about a hypothetical ETI using solar power. They assume that an ETI is using large-scale photovoltaics (PVs) based on silicon and that their planet orbits a Sun-like star. Silicon PVs are cost-effective to produce, and they are well-suited to harness the energy from a Sun-like star.
Kopparapu and his co-authors aren’t the first to suggest that silicon PVs could constitute a technosignature. In a 2017 paper, Avi Loeb and Manasvi Lingam from the Harvard-Smithsonian Center for Astrophysics wrote that silicon-based PVs create an artificial edge in their spectra. This edge is similar to the ‘red edge‘ detectable in Earth’s vegetation when viewed from space but shifted to shorter wavelengths. “Future observations of reflected light from exoplanets would be able to detect both natural and artificial edges photometrically if a significant fraction of the planet’s surface is covered by vegetation or photovoltaic arrays, respectively,” Lingam and Loeb wrote.
“The “edge” refers to the noticeable increase in the reflectance of the material under consideration when a reflected light spectrum is taken of the planet,” the authors of the new research explain. Satellites monitor the red edge on Earth to observe agricultural crops, and the same could apply to sensing PVs on other worlds.
This figure shows the reflection spectrum of a deciduous leaf (data from Clark et al. 1993). The large sharp rise (between 700 and 800 nm) is known as the red edge and is due to the contrast between the strong absorption of chlorophyll and the otherwise reflective leaf. Image Credit: Seager et al. 2005.While Lingam and Loeb suggested the possibility, Kopparapu and his co-authors dug deeper. They point out that we could generate enough energy for our needs (as of 2022) if only 2.4% of the Earth’s surface was covered in silicon-based PVs. The 2.4% number is only accurate if the chosen location is optimized. For Earth, that means the Sahara Desert, and something similar may be true on an alien world.
The authors explain, “This region is both close to the equator, where a comparatively greater amount of solar energy would be available throughout the year, and has minimal cloud coverage.”
The authors also work with a 23% land coverage number. This number reflects previous research showing that for a projected maximum human population of 10 billion people, 23% land coverage would provide a high standard of living for everyone. They also use it as an upper limit because anything beyond that seems highly unlikely and would have negative consequences. On Earth, the entire continent of Africa is about 23% of the surface.
The authors’ calculations show that an 8-meter telescope similar to the HWO would not detect an Earth-like exoplanet with 2.4% of its surface covered with PVs.
If an ETI covered 23% of its surface with energy-harvesting PVs, would that be detectable? It would be difficult to untangle the planet’s light from the star’s light and would require hundreds of hours of observation time to reach an acceptable Signal-to-Noise (S/N) ratio.
“Because we have chosen the 0.34 ?m–0.52?m range to calculate the impact of silicon panels on the reflectance spectra, the difference between a planet with and without silicon is not markedly different, even with 23% land cover,” the authors explain.
Technological progress adds another wrinkle to these numbers. As PV technology advances, an ETI would cover less of its planet’s surface area to generate the same amount of energy, making detection even more difficult.
This figure from the research shows the planet-star contrast ratio as a function of wavelength for2.4 % land coverage with PVs (blue solid), 23 % PVs (red solid) and 0% (green dashed) land coverage of solar panels. “This suggests that the artificial silicon edge suggested by Lingam & Loeb (2017) may not be detectable,” the authors write. Image Credit: Kopparapu et al. 2024.
Solar energy is expanding rapidly on Earth. Each year, more individual homes, businesses, and institutions implement solar arrays. Those might not constitute technosignatures, but individual installations aren’t the only thing growing.
China built a vast solar power plant called the Gonghe Photovoltaic Project in its sparsely populated Qinghai Province. It generates 3182 MW. India has the Bhadla Solar Park (2,245 MW) in the Thar Desert. Saudi Arabia has built several new solar plants and intends to build more. Other innovative solar projects are announced regularly.
But will we realistically ever cover 2.4% of our planet in solar arrays? Will we need to? There are many questions.
Generating solar power in the heat of the Sahara Desert is challenging. The extreme heat reduces efficiency. Building the infrastructure required to deliver the energy to population centres is also another challenge. Then consider that silicon-based PVs may not be the end point in solar panel development. Perovskite-based PVs hold a lot of promise to outperform silicon. They’re more efficient than silicon, and researchers frequently break energy records with them (in laboratories.) Would perovskite PVs create the same “edge” in a planet’s spectra?
The authors didn’t consider specific technological advances like perovskite because it’s beyond the scope of their paper.
The bottom line is that silicon-based solar arrays on a planetary surface are unlikely to create an easily detectable technosignature. “Assuming an 8-meter HWO-like telescope, focusing on the reflection edge in the UV-VIS, and considering varying land coverage of solar panels on an Earth-like exoplanet that match the present and projected energy needs, we estimate that several hundreds of hours of observation time is needed to reach a SNR of ~5 for a high land coverage of ~23%,” the authors write.
The Bhadla Solar Park is a large PV installation that aims to generate over 2,000 MW of solar energy. Image Credit: (Left) Google Earth. (Right) Contains modified Copernicus Sentinel data 2020, Attribution, https://commons.wikimedia.org/w/index.php?curid=90537462The authors also wonder what this means for the Kardashev Scale and things like Dyson Spheres. In that paradigm, ETIs require more and more energy and eventually build a mega engineering project that harvests all of the energy available from their star. A Dyson Sphere would create a powerful technosignature, and astronomers are already looking for them.
But if the numbers in this research are correct, we may never see one because they’re not needed.
“We find that, even with significant population growth, the energy needs of human civilization would be several orders of magnitude below the energy threshold for a Kardashev Type I civilization or a Dyson sphere/swarm which harnesses the energy of a star,” they conclude. “This line of inquiry reexamines the utility of such concepts and potentially addresses one crucial aspect of the Fermi paradox: We have not discovered any large-scale engineering yet, conceivably because advanced technologies may not need them.”
