What Would It Take To See Synthetic Lights at Proxima Centauri B?
Is there an alien civilization next door? It’s…possible(ish). In late 2020, we discovered a signal from the direction of Proxima Centauri (not necessarily from Proxima Centauri), our closest neighbour star. Named BLC- 1 by project Break Through Listen, the signal is still being analyzed to ensure it isn’t simply an echo of our own civilization – typically what they turn out to be. But why not just directly look at planets in Proxima Centauri and see if a civilization is there?
From space, the most obvious sign somebody lives on Earth is the glow from the nightside of our planet. Our cities emit light that’s shed into the Cosmos. Problem is that our current generation of telescopes are not powerful enough to see lights on distant worlds. But several researchers are testing the capabilities of the next generation of telescopes already on the drawing board. The finding? Yes! if advanced enough…or glowy enough…we would be able to see if another civilization has the lights on at Proxima Centauri.
8k compilation of footage taken from the International Space Station orbiting above Earth’s City Lights
There are several ways you could tell alien technology exists on another planet. For example, we may be able to see the light of a distant world waver with the transit of a massive constellation of satellites (a direction we’re headed in). Atmospheric pollution may be detectable from nuclear conflict (ouch). But while these indications of technology could also be caused by natural phenomenon like orbiting debris or a comet impact, artificial illumination is distinct from the natural light of stars. Elisa Tabor of Stanford University and Abraham Loeb of Harvard University took the James Webb Space Telescope (JWST) for a virtual alien light hunting test drive. James Webb hasn’t actually launched yet so the research tests the paper specs of JWST’s capabilities.
The virtual JWST is trained on Proxima b – the one confirmed planet in the Proxima Centauri system that could host a civilization. Located 4.25 light years from Earth, Proxima b is a rocky world in the habitable zone of the M-class red dwarf star Proxima Centauri – a mere 12% the mass of our Sun. Proxima b is more hefty than us at about 1.6 Earth masses and 1.3 times our radius. It orbits Proxima Centauri in just 11.2 days at a distance of a 7 million km – only 5% of the distance at which Earth orbits the Sun.
Confirmed planets at Proxima Centauri! – Video by Fraser Cain – Universe Today
Tabor and Loeb scaled artificial illumination as a fraction of the solar illumination reflecting from the dayside of the planet. 0% on this scale would assume that the nightside of the planet is completely dark, devoid of artificial illumination. 100% means the nightside of the planet is as equally bright as the dayside. The type of light used by the hypothetical civilization on Proxima b is assumed to be similar to LEDs on Earth which have a distinct artificial spectrum. The results? If the artificial nightside illumination of Proxima b reaches 5% of the natural dayside illumination JWST could detect the artificial light with 85% certainty. If artificial illumination were to reach 9% JWST’s detection confidence rises to 95%.
5% illumination doesn’t sound like much, right? Well, we are talking about the light from a star. As faint as Proxima Centauri is compared to our Sun (about 20,000 times dimmer), that’s still a lot of light. By comparison, Earth’s artificial illumination is only 0.001% of reflected stellar illumination. In other words, if Proxima b hosts a civilization as glowy as us, JWST wouldn’t detect it. Those lights would need to be 500 times brighter. That scenario is plausible. Proxima b orbits so closely to its host star that it may be tidally locked – one side of the planet always faces the star while the other is in perpetual night. A civilization on a tidally locked planet may need to focus on illumination infrastructure and could possibly, as Tabor and Loeb hypothesize, use very bright orbital mirrors to reflect sunlight onto the nightside of the planet which could be seen by our telescopes.
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Proxima Centauri is smaller and cooler than the Sun and the planet orbits much closer to its star than Mercury. As a result it lies well within the habitable zone, where liquid water can exist on the planet’s surface.
Next, Next Gen: LUVOIR and HabEx
Tabor and Loeb indicate that other future telescopes such as LUVOIR (Large UV Optical Infrared Surveyor) may be even more capable than JWST at spotting the glow of a distant civilization. Just a few days after their publication, Thomas Beatty of the Department of Astronomy at the University of Tucson, crunched just those numbers. Beatty reviewed both LUVOIR as well as HabEx (Habitable Exoplanet Observatory) to determine the potential of these telescopes to detect city lights not only on Proxima b, but also on planets orbiting stars out to a distance of 30pc (Parsecs. 1pc=3.26 light years). Both LUVOIR and HabEx have missions to catalogue and directly image exoplanets and are scheduled to launch in 2035.
Computer rendering of the LUVOIR Observatory – Credit NASA
Similar to Tabor and Loeb using a virtual JWST, Beatty pointed virtual LUVOIR and HabEx observatories at a number of star systems with known worlds like Proxima b as well as hypothetical Earth-like worlds orbiting G, K, and M class stars. Beatty also scaled the percentage of the planetary surface which was urbanized. The more urbanization, the brighter the planet’s nightside. The type of artificial illumination in this model simulates the most common lights on Earth – high pressure sodium street lights reflecting off concrete surfaces which also feature a spectrum distinguishable from natural starlight. So the variables are a) distance from Earth b) the planet’s level of urbanization, and c) the type of star the planet is orbiting. In each scenario, the virtual scopes are imaging planets for a minimum of 100 hours to collect enough light streaming through the void to resolve the target.
HabEx Observatory using its 52m floating starshade to block out unwanted starlight – Credit NASA
Beatty (2021) Figure 5 See Description below – click to enlarge
The above figure from Beatty’s publication shows the trade offs between the distance from Earth, the level of urbanization, and the class of parent star. The first two variables, level of urbanization and distance from Earth, are self evident. The brighter the artificial lights, the easier they are to see. If the planet resides closer to Earth, its lights are easier to see. But the parent star also plays a role in visibility. Planets orbiting smaller, dimmer stars have better contrast. Their stars are faint enough to not overwhelm artificial illumination and so lower levels of urbanization can be seen on planets in M red dwarf star systems. The figure’s blue colour shade indicates the certainty of detection. 1 sigma (the Greek letter that’s denoted in the figure) is about 67% certainty. 3 sigma is closer to 99%. 10 sigma is virtually 100%.
However, while fainter stars provide better contrast for detecting artificial illumination, their habitable zone radius is very small. Planets orbit so closely to the star that if a given red dwarf is more than 10pc away, we can no longer distinguish its planets from the star. The operative term in the figure is called the IWA or “Inner Working Angle” at which the planet is now visibly too close to the parent star for city lights to be detectable at all. More distant targets would then favour brighter Sun-like stars (G yellow dwarfs (our Sun) and K orange dwarfs) with wider habitable zones where the planets orbit farther, and are more visibly distinguishable, from their stars.
But, in turn, those planets require a higher percentage of urbanization as their brighter parent stars create a less favourable contrast for the nightsides of those planets. Up to 10pc away, a planet with urbanization levels between 0.4 to 3% would have visible city lights if orbiting M stars while planets orbiting G/K stars would need upwards of 10%. Beyond 10pc only planets orbiting G/K stars have visible city lights but need even higher percentages of urbanization. The max range for any significant detection is around 30pc. The 4 scopes used in the figure are two versions of LUVOIR (A/B) and two versions of HabEx. LUVOIR A features a more powerful 15m mirror with LUVOIR B an 8m mirror. HabEx (SS) is the HabEx scope paired with a floating solar shade to filter starlight which aids in the detection of planets.
Ecumenopolis – A City Planet
This composite image, which has become a popular poster, shows a global view of Earth at night, compiled from over 400 satellite images. NASA researchers have used these images of nighttime lights to study weather around urban areas. Credit: NASA/NOAA – click to englarge
For all our own city lights, the Earth’s surface is only 0.05% urbanized. Our telescopes wouldn’t be able to see us were they parked at Proxima Centauri. A greater percentage of urbanization could help us see a distant civilization more clearly like say 100%. But…what exactly is 100% urbanization? An Ecumenopolis.
The glow of an Ecumenopolis world in the space strategy video game Stellaris Habitable planets in the game can evolve into these city worlds as your civilization develops over centuries in a simulated galaxy – Graphics from the video game Stellaris, developed and published by Paradox Interactive. Used with permission – click to enlarge
An Ecumenopolis is a city planet – a world where the entire surface is covered in one giant city. Several examples exist in science fiction such as the Human Empire Capital planet of Trantor in Isaac Asimov’s Foundation Trilogy, the Republic/Empire Capital of Coruscant in Star Wars, or the Ecumenopolis planet type in my favourite space strategy video game Stellaris (which I definitely played for inspiration while writing this article). But more than a geeky sci-fi concept, it’s conceivable that an advanced civilization could completely encase their world in an unending urban landscape. How visible would such a world be?
Figure 6 from Beatty’s article shows the distinct powerful glow of High Pressure Sodium Lights from an Ecumenopolis. These lights peak around the 600nm range in the highlighted area.
Beatty modelled the results and found that future telescopes would be capable of detecting Ecumenopolis worlds around 82 stars in the stellar neighbourhood of the Sun. That’s an astonishing amount of space. So if somebody out there is shining that brightly, we may be able to see them in the coming decade.
Beatty’s work demonstrates that close range red dwarf stars provide the best opportunity for detecting urban worlds meaning Proxima b is the current prime target. Detecting an Ecumenopolis version of Proxima b would be easy for next generation telescopes. In fact, we could detect just 0.5% urbanization on the planet. That’s still 10 times more than Earth’s present urbanization. However, current city growth rates put the Earth at 0.5% within the next hundred years – a blink in stellar time. If an alien civilization does exist, they could have already reached this level of urbanization assuming that technological civilizations are long lived. And that’s the thing – we don’t actually know that technological civilizations are long lived. That’s one of the reasons why we do SETI. Finding someone else shining in the void means there’s more hope that we can keep the lights on too.
Feature Image Description: Ecumenopolis Planet orbiting Proxima Centauri-like Red Dwarf Star – Graphics from the video game Stellaris, developed and published by Paradox Interactive. HUGE thanks to the Paradox Interactive and Stellaris team for allowing usage of their cool Ecumenopolis screenshots in this article. Build your own advanced civilization with Ring Worlds, Dyson Spheres, and Ecumenopolis planets by finding Stellaris on Steam.
More to Explore
[2105.08081] Detectability of Artificial Lights from Proxima b (arxiv.org)
[2105.09990] The Detectability of Nightside City Lights on Exoplanets (arxiv.org)
Stellaris Grand Strategy Science Fiction Video Game
James Webb Space Telescope – Webb/NASA
A Very Interesting Radio Signal was Just Detected Coming from Proxima Centauri – Universe Today
Habitable Exoplanet Observatory (HabEx) (nasa.gov)
Potentially Habitable Exoplanet Confirmed Around Nearest Star! – Universe Today
The Color of Habitable Worlds – Universe Today