UNIVERSE TODAY
How large would an extraterrestrial city have to be for current telescopes to see it? Would it need to be a planet-sized metropolis like Star Wars’ Coruscant? Or could we see an alien equivalent of Earth’s own largest urban areas, like New York City or Tokyo?
A recent preprint by Bhavesh Jaiswal of the Indian Institute of Science suggests that, in fact, we could see cities a mere fraction of that size, using a feature of light known as specular reflection.
Much of the theoretical musings that experts have put into detecting alien techno-signatures have imagined finding large-scale megastructures well beyond current human capabilities, like Dyson spheres or massive orbital rings. These would be akin to enormous solar system spanning neon signs saying ‘intelligent life is here’! Nothing of the sort has ever been found.
But we’ve recently entered an era in which it is possible to directly image exoplanets themselves, opening up opportunities to search for intelligent life in the Universe on more modest scales.
The light of a star tends to drown out the planets orbiting it, making exoplanets hard to discover, let alone study. This is why most of the exoplanets discovered so far have 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 find – well over 5000 of them now – are so far away and so dim that when photographed, the images are barely a pixel in size.
Luckily, we can learn a lot from one pixel. Spectroscopy, for example, can tell us about the composition of a planet’s atmosphere. And techniques for blocking or filtering out starlight have come a long way: direct imaging of exoplanets is only getting better.
But how could we see something even smaller, like a city?
Specular reflection may be the answer. This phenomenon occurs when light is directly reflected at an observer, rather than diffuse light that shines in all directions. Think of the sometimes-blinding glint off an ocean wave, or the bright flash of a sports car’s metallic body in the sunshine.
A reflective surface on a distant planet can give off a similar glint of light.
“Future efforts to directly image planets in reflected light are being strongly pursued for both space telescopes and for extremely large ground-based telescopes,” writes Jaiswal. The method will work for both red dwarfs and sun-like stars.
