Ed and I looked at each other and wondered: from how far away could the Hubble Space Telescope (HST) detect city lights? But can we distinguish artificial lights from natural reflection of sunlight if they have a similar color? But we could potentially search for artificial lights from habitable planets around other stars. A technological civilization on Proxima b might opt to transfer heat and electricity from the warm, illuminated day side to the cold, dark night side. The search for city lights on habitable planets may sound speculative, but it is worth pursuing as a potential technosignature with planned instruments.