Searching for City Lights on Exoplanets with NASA’s Habitable Worlds Observatory
NASA’s future mission “The Habitable Worlds Observatory” was recommended for funding by the Decadal Survey in Astronomy and Astrophysics of the National Academies for the 2020s. The Observatory aims to identify and directly image potentially habitable exoplanets. It would be the first telescope specifically designed to search for life on exoplanets. The telescope will use spectroscopy to search for the molecular fingerprints of primitive life, such as molecular oxygen or methane, which constitute biosignatures.
This proposed infrared/optical/ultraviolet space telescope requires new technologies. NASA recently selected three industry proposals that will facilitate the mission’s needs for a total of $17.5 million. One of the new technologies involves a high-level coronagraph to block light from the host star so that its planets can be imaged. Another requirement involves an extremely stable optical system.
An unrecognized benefit of the Habitable Worlds Observatory is that it can potentially search for city lights on the nightside of habitable exoplanets. Based on a paper I wrote with the student Elisa Tabor, the Habitable Worlds Observatory will not only be sensitive enough to detect artificial nightside illumination at a level comparable to the stellar illumination of the planet’s dayside, but it could also discern the type of lamps used by aliens on their streets and compare them to our own LED lamps, for example. Detection of artificial light does not require imaging of the planet but merely noticing that there is light added from the planet to the emission from the star and the planet’s dayside albedo. The relative contributions of these components vary as the planet changes its orientation relative to the star and our line-of-sight along its orbit.
We should keep in mind that biological creatures on a habitable world near common red-dwarf stars are likely to possess infrared-sensitive eyes, like the Mantis shrimp on Earth, because infrared vision would help their survival on the day-side of their exoplanet. If their civilization advances to a technological phase, they would likely employ infrared lamps to illuminate the night side. Detecting their artificial lights would not only inform us about their technological infrastructure but also about the biology of their vision. It would have been fascinating for me to be a fly on the wall of an ophthalmologist office on their exoplanet.
Obviously, the best chance for imaging city lights outside the Solar system is around the nearest stars. This nearest target is Proxima Centauri, a red dwarf located 4.25 light years away. This star is nearly six hundred times fainter than the Sun, and so a planet needs to be a few tens of times closer to the nuclear furnace of its host star than the Earth-Sun separation, in order for it to support the chemistry of life-as-we-know-it in liquid water. Interestingly, Proxima Centauri hosts a rocky planet with about 1.3 Earth masses at a twentieth of the Earth-Sun separation. Because of its proximity to the star, this planet — Proxima b — is thought to be tidally locked, showing the same side to the star at all times — just like the Moon does relative to Earth.
Proxima b has a permanent day side and a permanent night side. My daughters say that if we ever move there, they want a house on the strip that separates the two sides, where they can watch the sunset forever. If Proxima b is already inhabited by a technological civilization, its dayside may be coated with photovoltaic cells to generate electricity that would illuminate and warm the nightside, which is otherwise cold and dark.
The Habitable Worlds Observatory could potentially detect city lights on the permanent night side of Proxima b. Even if the artificial illumination is as faint as our civilization currently utilizes on the night side of Earth, this Observatory could detect it as long as the illumination is limited to a narrow frequency band that is thousands of times smaller than the full spectral extent of Proxima Centauri.
Proxima b orbits its star every 11.2 days, making birthday celebrations thirty times more frequent than on Earth. The high demand for bright lights during birthday parties on the night side of Proxima b could be an opportunity for us to celebrate as well, if the signal is detectable by the Habitable Worlds Observatory.
In case there is no technological life on Proxima b, we could search for city lights around other nearby stars, such as Alpha Centauri A & B, Barnard’s Star, Luhman 16, WISE 0855–0714, Wolf 359 and Lalande 21185, which are all less than a factor of two farther away than Proxima Centauri.
We could also search for bright lights from gigantic nuclear-powered spaceships moving through interstellar space. A dozen years ago, I attended a conference inaugurating the campus of New York University in Abu Dhabi along with my colleague from Princeton University, Ed Turner. The conference included a tour through the city, during which the local tour guide bragged that their city lights are so bright at night that they can be seen from the Moon. Ed and I looked at each other and wondered: “How far away could the deepest image of the Universe detect a single city?” Subsequently, we published a paper in which calculated that deep exposures by the Hubble Space Telescope, the Webb Telescope and obviously the future Habitable Worlds Observatory, could notice artificial illumination equivalent to the city lights from Tokyo on a spaceship that is 30 times farther away than the Earth-Sun separation. Such lights would be spectroscopically distinct from surface reflectance of the Sun’s spectrum and their flux will scale inversely with distance squared instead of distance to the fourth power — as is the case for objects reflecting sunlight. When I asked the astronomer Mike Brown from Caltech whether he checked if the flux from any Kuiper belt object that changes its distance from the Sun exhibits an unexpected evolution, he responded: “Why would I check? It is obvious that the flux from Kuiper belt objects should decline inversely with distance to the fourth power.”
Here’s hoping that the Habitable Worlds Observatory will open our eyes to “unknown unknowns,” namely worlds that our imagination is currently missing. In a few minutes, I am about to board a flight to Washington DC, where I will be discussing this point in a special event tomorrow evening on Capitol Hill dedicated to my scientific research program.
ABOUT THE AUTHOR
(Image credit: Chris Michel, October 2023)
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
