Could Solar Panels Be the Next Big Clue in the Search for Extraterrestrial Intelligence?

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1. Ravi Kopparapu
2. Vincent Kofman
3. Jacob Haqq-Misra
4. Vivaswan Kopparapu
5. Manasvi Lingam

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Table of Links

ABSTRACT

1. INTRODUCTION

2. METHODS

3. PHOTOVOLTAIC REQUIREMENTS FOR EARTH

4. DETECTABILITY REQUIREMENTS FOR PHOTOVOLTAICS

5. DISCUSSION

1. CONCLUSION, ACKNOWLEDGMENTS AND REFERENCES

ABSTRACT

In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like exoplanet as a potential technosignature. 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). Assuming that only solar energy is used to provide the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.

\ Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the ultraviolet-to-visible (0.34 − 0.52 µm), 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. We discuss the necessity of concepts like Kardeshev Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy.

\ Even with much larger populations than today, the total energy use of human civilization would be orders of magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev Type I civilization. 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.

Introduction

The search for extraterrestrial life has primarily focused on detecting biosignatures, which are remote observations of atmospheric or ground-based spectral features that indicate signs of life on an exoplanet. More recently, “technosignatures” referring to any observational manifestations of extraterrestrial technology that could be detected or inferred through astronomical searches has received increased attention (Tarter 2007).

\ While the search for extra-terrestrial intelligence (SETI) through radio observations has been popular for decades, recent studies have proposed alternate searches for technosignatures in the UV to mid-infrared part of the spectrum: see NASA Technosignatures Workshop Participants (2018); Lingam & Loeb (2019, 2021); Socas-Navarro et al. (2021); Haqq-Misra et al. (2022c) for a comprehensive description.

\ Specifically, methods to detect technosignatures through spectral signatures from exoplanets have been proposed as a means to utilize existing techniques and telescope facilities. These include nitrogen dioxide (NO2) pollution (Kopparapu et al. 2021), fluorinated compounds such as chloroflorocarbons (CFCs, Owen 1980; Lin et al. 2014; Haqq-Misra et al. 2022b), or nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6) (Seager et al. 2023), night-side city lights (Beatty 2022) and agricultural signatures on exoplanets (Haqq-Misra et al. 2022a).

\ In this work, we focus on another potential technosignature: silicon solar panels. Technological civilizations may harness their host star’s radiation for their energy needs, just like our civilization has commenced with large scale solar photovoltaics. Most solar cells use silicon in different forms. Lingam & Loeb (2017) outlined three primary motivations for employing silicon-based solar panels, which might be broadly applicable.

\ The first is the relatively high cosmic abundance of silicon compared to the elements utilized in other types of photovoltaics such as germanium, gallium, or arsenic. Second, the electronic structure of silicon (specifically its band gap) is wellsuited for harnessing the radiation emitted by Sun-like stars (R¨uhle 2016). Third, silicon is also cost effective in terms of refining, processing and manufacturing solar cells (Bazilian et al. 2013).1

\ Based on these arguments, Lingam & Loeb (2017) suggest that the existence of large-scale silicon solar cells could produce artificial spectral “edges” in some UV wavelength bands when observing the atmosphere of an exoplanet in reflected spectroscopy because of the steep change in the reflectance of silicon. This artificial spectral edge may be similar to the vegetation red “edge” (VRE) seen between 0.70−0.75 µm that can be noticed in the reflected light spectrum of Earth (Sagan et al. 1993; Arnold et al. 2002; Woolf et al. 2002).

\ 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. In the VRE case, the high reflectance arises due to the contrast between chlorophyll absorption at red wavelengths (0.65–0.70 µm) and the scattering properties of the cellular and leaf structures at NIR wavelengths (0.75 − 1.1 µm): see, for example, Seager et al. (2005); Turnbull et al. (2006); Schwieterman et al. (2018); O’Malley-James & Kaltenegger (2018) for more details.

\ Detecting VRE on an exoplanet would provide contextual information about the type of widespread biological life (e.g., autotrophy), and corresponding atmospheric properties relevant for habitability. Lingam & Loeb (2017) suggest that a similar artificial edge, if manifested, could provide some contextual information about the kind of technological activity on a planet. Could we detect surface reflectance features of solar panels on exoplanets as technosignatures? While Lingam & Loeb (2017) suggested this possibility, they did not conduct any quantitative assessment of their detectablity.

\ In this work, we will consider the detectability of solar panels on an Earth-like planet around a Sun-like star with a LUVOIR-B (8 meter) class space telescope. The paper is structured as following: §2 discusses the methods and models used in this work. In §3, we estimate the area of land that would need to be covered to provide human civilization with its energy needs today and in future scenarios. The potential detectability is then assessed in §4. A discussion section and a summary of our findings follows in §5.

Methods

The methods to assess the detectability of photovoltaics as a signature for the presence of advanced civilizations are described here. In order to constrain the spectral signal, the following needs to be assessed: 1) The reflectivity of photovoltaics panels. 2) The panels need to be included in a suitable location on an Earth-ground map 3) The spectroscopic signal from the panels need to be compared to simulations without the panels, and the signal-to-noise that can be attributed to the panels should be computed.

2.1. Silicon and the reflectivity as as spectral signature

Pure silicon is not as well-suited (as a material) to be used in a photovoltaic cell since it is highly reflective in the ultraviolet-to-visible range. As electric energy is generated by absorbing a photon to promote an electron across the PN junction, any light that is reflected leads to a reduction in efficiency. To minimize reflection of light, photovoltaic cells are either subjected to texturing (Campbell & Green 1987; Macdonald et al. 2004; Kim et al. 2020) or coated in anti-reflective coatings, often TiO2 or Si3N4 (Zhao & Green 1991; Raut et al. 2011; Sarkın et al. 2020); the coating results in the typical dark color seen on solar panels.

\ In the scenario with the above anti-reflective coating, the artificial edge is still apparent, but less pronounced and deeper in the ultraviolet (with respect to pure silicon), when realistic materials are assumed for photovoltaic cells. For this work, the reflectance spectrum shown in Fig. 1 is adopted. This explains a major source of divergence between Lingam & Loeb (2017) and our work, because the former emphasized greater surface coverage by pure silicon solar panels that could compensate for reduced efficiency, whereas we consider potentially more realistic photovoltaic cells endowed with higher efficiency, thereby warranting lower coverage.

2.2. Generating the surface model containing solar panels

Based on the estimated surface area required for the current energy use discussed in §3, a ground map is generated that hosts roughly 2.4% of land coverage. The Sahara desert was fiducially chosen to host the solar panels. 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. However, dust storms are also prevalent, and have been increasing in frequency over the past four decades (see Fig.4, Varga 2020). Average events are ∼ 20 per year with varying severity.

\ Such events may reduce the available sunlight, further restricting the energy generated. We recognize and caution that no significant area of the Earth is uninhabited, and even the placement of solar farms in seemingly barren deserts has been contentious due to the destruction of the extremely fragile ecosystems that may consequently arise. However, our goal in this work is to assess the detectability of solar panels on an exoplanet, and as such, the most “optimal” land location in term of solar energy generation is chosen for this purpose. The ground surface model that is used for this study is based on the data products from the moderate resolution imaging spectroradiometer (MODIS), which is hosted on both the Terra and AQUA satellites operated by NASA Goddard Space Flight Center2 . T

\ The MODIS-MCD12C1 maps provide yearly average coverage at high spatial resolution of 18 different land types across the entire planet. For this paper, ground coverage is reduced to 5 different types: ocean, snow/ice, grass, forest, and bare soil. The spatial resolution is binned down to 2.5 by 2 degree (longitude/latitude). The ground albedo, or the fraction of light that is reflected as a function of wavelength for the different surface types, are adopted from the United States Geological Survey database (Kokaly et al. 2017). Subsequently, solar panels are added as a sixth category, which are described with the reflectivity from RELAB (Reflectance Experiments Laboratory (Pieters & Hiroi 2004), and placed on the surface map at the chosen locations.

2.3. Assessing the contribution to the reflectivity from silicon

In order for a spectral feature to be detectable, it needs to satisfy two conditions. It has to be sufficiently strong, and its spectral signal needs to uniquely identifiable with the source molecule or in this case, ground coverage. The detectability of photovoltaic panels was proposed to be in the UV-VIS region of the spectrum, which is the range where the spectral feature of the panels are most uniquely identifiable. The infrared region is not suitable because the difference in the reflectivity here does not correspond to a spectrally unique feature: the features overlap with much stronger signals from the other surface components.

\ This study is constructed so as to focus on the maximum detectability of the technosignature. This does not indicate that the signal may be fully uniquely attributed to the panels, which would require a follow-up study utilizing retrieval methods and an exhaustive search for possible overlapping signals (i.e. Rayleigh scattering, absorption by O3 or hazes). The first step in assessing the detectability is finding the potential signal strength in the most optimistic case, which is pursued here. It should also be noted that the placement of the panels in the desert is fortunate in terms of the detectability as well, because from the ground coverage components shown in Fig.1, the contrast exhibited with soil is the second highest (after snow/ice).

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:::info This paper is available on arxiv under CC BY 4.0 DEED license.

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