CQ, this is W9GFO.: Looking For Light Pollution But Not From This Earth

Earth at Night
Credit: C. Mayhew & R. Simmon (NASA/GSFC), NAA/NGDC, DMSP Digital Archive

Astronauts orbiting the Earth often look upon our planet illuminated by lights in night zone. Researchers start to think that scientists should be able to detect light from other civilizations too. Many scientific fictions had ever dreamed about a planet filled with illuminated cities, for example, the galaxy's capital like Coruscant from Star Wars.

All of these are based on the assumption that aliens also use light to see in night time. On Earth, synthetic light comes from two ways, heat in circular bulb, and quantum in fluorescent. LEDs' spectrum from those synthetic lights should be different from other natural light sources like volcanic eruptions. Those light might be a beacon calling for us from alien civilizations.

This new strategy is pretty similar to the old one that SETI used by looking for radio signal from aliens. However, from the fact that even radio signal from Earth is diminishing in the last decade because of cable, fable optic, other technological advances. That is, researchers suggest that looking for light pollution from outer space should not be out of reach.

But how practical is this approach after all? Assume that there is a civilization in Kuiper belt which is an area between 30 to 50 AU away from the sun. They have calculated that telescopes on Earth should be able to easily see a Tokyo-sized city which is 50 kilometer wide on an object in Kuiper belt 50 AU away. Moreover, a Tokyo-sized city on an object in Oort's Cloud which is 1000 AU away should be seen by Hubble Ultra Deep Field.

Another question is raised, what is the probability of finding another civilization on outer part of our solar system? Actually, if it does not take too much budget and there are something to look for, we better do it. In 1960s, there was a proposal for a space telescope looking through universe in X-ray spectrum. However, the board saw it as an unnecessary plan since our Sun emits only little X-ray and they thought that we won't be able to see and study much from X-ray. Nowadays, having a bunch of X-ray satellites, we realize that X-ray sky is pretty amazing and filled with unexpected X-ray source such as a accretion disk of a black hole.

Similarly, looking for synthetic lights from outer part of our solar system should give an unexpected result. There might be something else found by this kind of survey. Also, we should be able to learn about shapes, reflecting ability of Kuiper Belt Objects included objects in binary system.

The approach sounds like a bold one but there is still some hope. The night side of the Earth is 100000 times darker than the day side. With telescopes we have today, we should be able to see only cities in the night side on an object approximately 1000 AU away or around the edge of our solar system.

However, the closest star is 100 times further away. In order to see lights with the same brightness as those on Earth on another object in habitable zone of our closest star, we need a telescope with a mirror at least 100 times larger than those on Hubble telescope.

The next generation of ground and space telescopes should be able to detect alien cities on habitable planets from faraway stars by using a filter looking for spectrum of LEDs and other synthetic lights. Nevertheless, in order to do that, the dark side of that planet must be much darker than the bright side (even more than on our Earth.)

Who knows, in the future, we might be able to detect a glimpse of light from a city like Coruscant which is shown in picture below.