In addition to batteries collecting light from stars, telescopes will be able to identify greenhouses if aliens place them on the night hemispheres of their planets.
Scientists from the United States proposed to search for extraterrestrial civilizations by reflecting ultraviolet radiation from their "solar" batteries - more precisely, batteries that use the light of nearby stars. The spectrum of the light they reflect is so specific that it should be visible in terrestrial space telescopes, which will be commissioned in the near future. The text of the corresponding work is available on the Cornell University preprint server.
The authors plotted the reflectance curve of silicon solar cells for different ranges. It turned out that if they absorb visible light and infrared radiation well, then photocells effectively reflect in the ultraviolet part of the spectrum. A natural planetary surface cannot reflect this way, since chemically pure silicon is rare and quickly binds to compounds with other substances that have a completely different reflection curve. According to astronomers' calculations, space telescopes, scheduled for commissioning in the coming years, will make it possible to detect such solar arrays on planets around nearby stars, including Proxima b.
In addition, the researchers believe, extraterrestrial civilizations can give out unnatural illumination of that part of their planets that should remain in eternal darkness. Most of the stars in the universe are red dwarfs like Proxima Centauri. There is a high chance that the planets around them always look at the star with one side, and eternal night reigns on the other. Most likely, when the level of technology of civilization on such a planet reaches a sufficient level, the population will grow along with it, and you will have to master the night side, including growing food on it. Part of the artificial lighting of the greenhouses built there will inevitably be visible from space. It will be easier to see artificial lighting through telescopes on a tidal exoplanet than on Earth, since the operating time of such lighting in one place will be longer.
The job has a number of limitations. It is impossible to say for sure which material is most efficient for solar cell production. It may not be silicon, from which the lion's share of such aggregates is now made. Competitive solar cells of a different composition are actively being developed on Earth. It is also not clear whether extraterrestrial civilizations will choose solar energy or nuclear energy. On our planet at the moment the cost of electricity from both sources, as well as their safety, are quite comparable.
Earlier, another group of scientists suggested looking for alien vegetation by a sharp increase in its reflectivity in the near infrared range. If in the red part of the spectrum (680 nanometers) plants reflect only 5 percent of the light, then already at 730 nanometers - only 5 percent. This approach has its weak point. On Earth, photosynthesizing purple bacteria using a different type of chlorophyll do not see such a jump. Without on-site research, it is difficult to predict whether extraterrestrial plants will have chlorophyll-like compounds of the type that terrestrial plants use or that found in purple bacteria.
