Updated on December 28, 2018.
What are the aliens thinking? That’s always been a problem for the search for extraterrestrial intelligence (SETI). Until recently, SETI’s focus has been on alien “beacons,” signals that somebody somewhere intentionally beamed into space. But this traditional method involves making informed guesses about what the aliens were thinking when they built their beacons, and those guesses may turn out to be laughably wrong.
There’s an entirely new way to do SETI now, and understanding its promise begins with considering the properties of solar panels. Rather than just looking for beacons, SETI researchers now want to also search for unintentional “technosignatures” from alien industrial civilizations. An example of an unintentional technosignature could be pollutants in a distant planet’s atmosphere, or the shadow of a large artificial structure orbiting a planet.
The best way to find a technosignature is to search for the observable byproducts of activities that are necessary for all civilizations. For instance, industrial civilizations, by their very nature, must extract energy from their surroundings to do work and keep themselves running. In a creative paper published last year, the astronomers Manasvi Lingam and Avi Loeb wondered what technosignatures would radiate out from a civilization that powered its world by harvesting solar energy on a planetary scale.
It’s easy to imagine a civilization covering a fraction of its world with solar panels. This is something we might try in, say, the Sahara desert. What Lingam and Loeb did was calculate how the large-scale deployment of solar technology would leave a mark in light that bounces off a planet’s surface.
Technosignatures are similar to biosignatures, which are the means for finding life of any kind on a planet. The chemical makeup of an exoplanet’s atmosphere can, for example, be extracted by catching light passing through the planet’s veil of gas. On Earth, atmospheric oxygen or methane would quickly react away without our planet’s abundant life. That means seeing oxygen and methane in an exoplanet atmosphere might serve as a good signature for a biosphere thriving on that distant world.
But the vegetation covering our planet creates many kinds of signatures that could be seen from a distance. In particular, Earth’s vast array of leaves alters the spectrum of sunlight that we reflect back into space. Chlorophyll, the chemical responsible for photosynthesis, reflects light in the green part of the sun’s spectrum. Its “reflectance” also rises steeply at the boundary between red and infrared light, much of which escapes humans’ visual perception.* That means that sunlight bouncing off Earth has a noticeable “red edge” from all the biosphere’s leaves, grass, etc. If you plotted how much sunlight gets reflected from Earth versus that light’s wavelength, you’d see the planet’s “reflectance” rise sharply as you cross from the green parts to the red parts of the spectrum. The rise is so sharp that astrobiologists have long floated proposals aimed at searching for a photosynthetic red edge from exoplanets. They’ve even calculated the reflecting properties of novel forms of photosynthesis that might evolve on worlds with stars very different from our sun.
Lingam and Loeb saw that large-scale deployments of solar-energy collectors would also change the way sunlight reflects from an exoplanet. Focusing on the properties of silicon, they calculated the wavelengths of light that solar panels would absorb and the wavelengths that they’d bounce back into space. Instead of a red edge, their calculations showed that silicon-based solar panels would produce a sharp change in reflectance at the ultraviolet part of the electromagnetic spectrum. They called this the silicon edge.
A skeptic might argue that silicon is too human-centric a solar-panel construction material to produce a universal technosignature. Lingam and Loeb, however, gave convincing arguments that it’s the element of choice for solar-powered exocivilizations. There are only a few kinds of light-energy harvesting materials you’d find on any planet. Using the cosmic abundances of the elements, Lingam and Loeb showed that most planets are likely to have a lot of silicon lying around, making it an obvious component for building solar collectors.
For good measure, the two scientists also calculated the reflectance properties of gallium arsenide and the mineral perovskite, both of which might be used in solar panels. Each produced its own distinctive edge in the planet’s spectrum. Now, all astronomers have to do is go looking for these signatures. Fortunately, the telescopes they need to do so are starting to come online.
* This article previously misstated the reflective spectrum of chlorophyll.