About a decade ago, I attended a conference with a colleague from Princeton University, Ed Turner, to inaugurate the New York University campus in Abu Dhabi. The conference included a tour of the neighborhood where the local guide bragged that their city lights can be seen from the moon. Ed and I looked at each other and asked, How far could the Hubble Space Telescope (HST) see the lights of the city?
The following day, we calculated that Hubble Deep Field could detect a city like Tokyo on objects in the Kuiper Belt at 30 to 50 times the Earth-Sun distance. But can we distinguish artificial light from the natural reflection of sunlight if they are of a similar color?
In answering this question, Ed and I came across an important finding about the dependence of the observed flow on the distance of the light source. The flux of reflected sunlight decreases inversely as the square of the reflector’s distance from the sun (for the amount of sunlight it intercepts) times the square of its distance from us (for the light we receive). For sources that are very distant, the product of these factors means that dimming occurs inversely with the distance to the fourth power. On the other hand, an artificial light source that generates its own light acts like a light bulb and only dims in reverse with the square of its distance from us. By testing whether a Kuiper belt object darkens inversely with distance to the second or fourth power as it retreats in its orbit, one can infer whether it is emitting its own light.
Then one of the leading observers of the Kuiper Belt objects happened to visit my office. I insisted on asking him, “Have you ever checked how the brightness of objects in the Kuiper Belt changes with distance along their orbits?” He replied without hesitation, “Why should I look? It has to follow the dependency expected of reflected sunlight. ”To which I could only say:“ Those who are not ready to discover wonderful things will never find them. ”
But I am patient. Education takes time, especially when dealing with scientists. When asked recently how long humans couldn’t know nature, I replied that humans can refuse to consider evidence that has contradicted their beliefs for millennia. This was the case with the idea that we are at the center of the universe or that the outcome of wars is dictated by planets and stars in the sky. The extent of our ignorance is unlimited. We could choose to remain uninformed forever, just like animals.
Granted, a source of light as bright as the city of Tokyo on the edge of the solar system is unlikely to exist unless it is associated with a passing giant spaceship. But we could possibly look for artificial light from habitable planets around other stars.
The closest is Proxima b, a planet in the habitable zone of our closest neighbor, the dwarf star Proxima Centauri, which is 4.25 light years away. Since the planet is 20 times closer to its weak star than the earth is from the bright sun, it is believed that Proxima b is connected to a permanent day and night side (just as the moon is always facing the earth with the same side). A technological civilization on Proxima b might choose to transfer heat and electricity from the warm, illuminated day side to the cold, dark night side. This could be achieved, for example, by coating the day side with photovoltaic cells that generate electricity from starlight. In an article with my former postdoc Manasvi Lingam, we showed that future telescopes can detect substantial coverage of the daytime side by solar panels based on the spectral edge they exhibit in their reflection of starlight.
This raises an interesting hypothetical question. If the night side of Proxima b is illuminated with artificial light, could we detect it with HST’s successor, the James Webb Space Telescope (JWST), due to launch later this year? Because JWST is larger and more sensitive than HST, we could look further into space and extend the search for artificial light from the Kuiper Belt to include habitable exoplanets like Proxima b. I pursued this question in a new paper with Elisa Tabor, a Bachelor student at Stanford University.
We have calculated the curve of light expected from a partially lit Proxima b as it orbits its star. Our calculations showed that JWST will be able to detect light emitting diode (LED) lamps on the night side, which make up 5 percent of the stellar lighting on the day side. But even if the artificial lighting on the night side of the earth is as weak as our civilization currently uses (0.01 percent), JWST could see it as long as it was limited to a frequency band a thousand times narrower than starlight. Future observatories such as the planned Large Ultraviolet Optical Infrared Surveyor (LUVOIR) space telescope will be able to detect even weaker artificial illuminance levels on the night side of Proxima b.
The search for city lights on habitable planets may sound speculative, but it is worthwhile as a potential techno-signature with planned instruments. Proxima b orbits its star every 11.2 days and offers its possible inhabitants 32.6 more opportunities to celebrate their birthday than we have once every 365.2 days on earth. The great demand for bright light at birthday parties on the night side of Proxima b would also be a reason for us to celebrate if the signal were perceived by future telescopes.