How large would an extraterrestrial city have to be for present telescopes to see it? Would it require to be a planet-sized metropolitan area like Star Wars Coruscant? Or could we see an alien equivalent of Earths own largest metropolitan locations, like New York City or Tokyo?
A recent preprint by Bhavesh Jaiswal of the Indian Institute of Science recommends that, in fact, we could see cities a mere fraction of that size, utilizing a feature of light understood as specular reflection.
Much of the theoretical musings that professionals have actually taken into detecting alien techno-signatures have envisioned discovering large-scale megastructures well beyond present human abilities, like Dyson spheres or huge orbital rings. These would be similar to huge solar system covering neon indications stating smart life is here! Absolutely nothing of the sort has actually ever been discovered.
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Weve just recently entered an age in which it is possible to straight image exoplanets themselves, opening up opportunities to browse for intelligent life in the Universe on more modest scales.
The light of a star tends to drown out the worlds orbiting it, making exoplanets hard to find, not to mention study. This is why the majority of the exoplanets found so far have actually been found around dim red dwarf stars, which are not as blinding to telescopes as yellow stars like our sun. The exoplanets that we have managed to discover– well over 5000 of them now– are so far away therefore dim that when photographed, the images are barely a pixel in size.
Fortunately, we can discover a lot from one pixel. Spectroscopy, for example, can tell us about the composition of a worlds environment. And methods for blocking or filtering out starlight have actually come a long way: direct imaging of exoplanets is just getting better.
But how could we see something even smaller, like a city?
Specular reflection might be the answer. This phenomenon occurs when light is straight shown at an observer, rather than diffuse light that shines in all directions. Consider the sometimes-blinding glint off an ocean wave, or the bright flash of a carss metal body in the sunshine.
An example of specular reflection glinting off the Heart of the City water feature in Sheffield. Credit: Steve F, Wikimedia Commons.
A reflective surface area on a distant world can release a similar glint of light.
“Future efforts to directly image worlds in reflected light are being strongly pursued for both area telescopes and for exceptionally large ground-based telescopes,” writes Jaiswal. The technique will work for both red dwarfs and sun-like stars.
Of course, specular reflection depends a bit on luck– the light from the exoplanets star would need to bounce off the world at exactly the ideal area and the right angle to reach an observer here in the world. And because of how specular reflection works, there is really a limit on how large an area can reflect light toward an observer. You could not get a specular reflection off of the entirety of the globe-spanning imaginary city of Coruscant: just the part of the megacity placed exactly best to show light at Earth might be seen at any one time.
On an Earth-sized world, the maximum area that might trigger noticeable specular reflection is about 5.4 parts per countless the whole planet, working out to an area with a location of ~ 2800 kilometers squared. Thats relatively small, only about one-sixth of the New York-New Jersey-Conneticut Metro Region, for instance. Its the size of a more common, modest Earthling city.
The exposure of specular reflection relies much more on the materials utilized in such a city or megastructure due to the fact that there is a size limitation. A low reflectivity compound like glass, for example, would cause a visible, however really faint peak in brightness, while an aluminum structure would shine numerous times more brilliantly than the entire planet.
There are several extra factors that might increase the chance of a detection through specular reflection. A slower-spinning world would allow a reflection to be noticeable for longer periods of time. A structure or city that is longitudinally wide would enable it to remain noticeable for longer as the world turns.
Astronomers would likewise have to get really lucky with the alignment of the planets axis. Only structures at specific latitudes would reflect towards the observer, though a planet with an axial tilt, like Earth, would enable observers to scan various latitudes as the planet advanced through its seasons.
While the possibility of stumbling throughout an alien city remains not likely (we dont even understand if they exist, after all), the abilities and innovations required to make such a detection are extremely real, and they are available now.
Look up and wave. Maybe some extraterrestrial observer is viewing the sparkle off your hometown too.
Or, perhaps not.
Check out the paper: Bhavesh Jaiswal, “Specular reflections from artificial surface areas as Technosignature.” ArXiv Preprint, June 2023.
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How large would an extraterrestrial city have to be for existing telescopes to see it? Or could we see an alien equivalent of Earths own biggest metropolitan areas, like New York City or Tokyo?
You could not get a specular reflection off of the entirety of the globe-spanning fictional city of Coruscant: only the part of the megacity positioned precisely best to show light at Earth might be seen at any one time.
Its the size of a more typical, modest Earthling city.
A structure or city that is longitudinally large would allow it to remain noticeable for longer as the planet turns.
