Astronomers May Have a New Technique to Spot Distant Alien Cities
We may be just a decade away from being able to spot alien construction from afar.
A momentary gleam of starlight could reveal alien construction to the next generation of telescopes, a recent paper suggests.
We’ve all experienced it: the glint of sunlight bouncing off a car passenger’s cell phone screen, nearly blinding us as we try to keep our eyes on the road. That phenomenon, in which a shiny object reflects light in one direction, is called specular reflection. It’s what makes those old-school signal mirrors a moderately useful tool if you’re lost in the wilderness — and it’s how we might spot alien cities on distant planets in the next few decades.
Imagining A Future Alien Discovery
The year is 2037, and astronomers are poring through data from the Habitable Exoplanet Observatory (HabEx) when they notice something –—a sudden bright flash in the light curve of a distant, Earth-sized planet as it rotates from day to night. Immediately, the astronomers ask for more time on the telescope and scrutinize their data for details: how much of the planet’s surface area does the light come from? Are the light waves polarized (all oriented in the same direction)? Will the flash happen again?
On the planet’s next rotation, the flash repeats itself. The light is very polarized, which suggests starlight reflecting off something very smooth, very reflective, and about the size of a major city. In fact, the light is too polarized to come from a lake or an ice field; it’s probably something made of metal, glass, or both: maybe an alien solar farm or a huge industrial complex with reflective metal rooftops.
That sounds like the start of a science fiction novel, but Indian Institute of Science astronomer Bhavesh Jaiswal says it could be a reality in the next decade thanks to an upcoming generation of telescopes like HabEx and the Nancy Grace Roman Space Telescope. In a recent paper in the journal Astrobiology, Jaiswal calculated how much of a planet’s surface area would need to be covered in reflective material in order for telescopes on Earth to be able to spot its specular reflection (see the cell phone analogy above).
“They could constitute other types of constructions as well that lie beyond our present knowledge and understanding,” writes Jaiswal in his recent paper.
Solar farms like this one in Thailand, if they’re large enough, could create the kind of specular reflection Jaiswal suggests future astronomers should look out for.
What’s Coming Next
Today, the James Webb Space Telescope can study the atmosphere of an exoplanet passing in front of its star by measuring how the starlight changes as it filters through the alien atmosphere. But in 10 or 15 years, a new generation of telescopes might be capable of what astronomers call direct imaging — looking for starlight reflecting from the surface of planets. As with Webb’s exoplanet studies, the first direct imaging targets of HabEx or the Roman telescope are likely to be gas giants, because they’re bigger and reflect more light, making them easier to see.
“So once that technology is demonstrated for larger planets, then it can be extended to a mission for habitable planets, which are more Earth-like,” says Jaiswal. “And once we have that, that would be a good time to look for such transient signatures on these planets.”
In other words, if aliens have built anything big and shiny on their planet’s surface, it might reflect sunlight like a mirror (or that annoying passenger’s phone screen in your car), bouncing a beam of bright light straight at telescopes here on Earth.
The Cosmic Version of a Signal Mirror
Here’s how it works. Nearly everything reflects some light, but usually it’s diffuse — meaning the reflected light is scattered in all directions. (If you’re a fan of technical jargon, that’s called Lambertian reflection. If you’re not a fan of technical jargon, that’s still what it’s called, but you won’t need to remember it.)
Some astronomers hope that future telescopes could spot differences in albedo (how much light a surface absorbs or reflects) that would reveal the presence of oceans, continent-spanning rainforests — or large-scale, building-obsessed alien civilizations. Earlier studies have suggested using those telescopes to look for alien megastructures: construction projects that span whole continents — or more. Small details like cities would be lost to distance, however.
But seeing a specular reflection (like the cell phone analogy earlier) requires the light, the object, and the observer all to be at exactly the right angle at the right time. Specular reflection can be “almost a million times stronger” than the more diffuse light reflected from the rest of the planet, according to Jaiswal.
“The reflection from a metal like aluminum would be seen to be several times brighter than the planet,” he writes. “The reflected flux from a specularly reflecting structure can be comparable or even larger than the planetary flux despite its much smaller area.”
In other words, if the angles are right and the hypothetical aliens really like big, shiny-topped buildings, astronomers in the 2030s could spot targets the size of cities. And while megastructures would be evidence of a civilization much more technologically advanced than ours, specular reflections from smaller cities or solar farms could point to aliens whose technology is on par with our own.
A city with its buildings roofed in solar panels or other reflective material could also catch its star’s light and create a dazzling reflection — but future astronomers will have to work out how to tell construction apart from nature.
False Positives and Unknown Unknowns?
That doesn’t mean that any bright specular reflection from the surface of a distant world is an alien city, however.
“A possible false positive is my biggest concern with any bio/technosignature, and it’s what the community are increasingly focusing on at the moment,” Columbia University astronomer David Kipping tells Inverse. “At a basic level, what else might use specular reflection besides from artificial structures (e.g. oceans and icy surfaces)?”
Jaiswal says he’s given that some thought, and he thinks a property of light called polarization might help. Light waves vibrate in two directions, but not always the same two directions. The light from a lamp contains some waves that wiggle up and down, and others that move left to right, and so on. But if all the waves coming from a light source are lined up the same way, the light is polarized. You’ve probably seen polarization at work in computer screens that can only be read from a certain angle.
And in the case of reflected light from an alien world, Jaiswal says polarization is likely to suggest a smoother, more reflective surface — polished metal rather than the wind-ruffled surface of a lake or even a glacier, for example.
Normally, astronomers learn more about what they’re looking at by studying the spectrum of light from the object. But Jaiswal worries that specular reflections from alien cities may not contain much spectral information. However, Florida Institute of Technology astrobiologist Manasvi Lingam, who commented on the paper, tells Inverse that we should be prepared for anything – and not rule anything out.
“With that being said, we should be cautious nevertheless since we cannot dismiss the possibility of unknown materials on other worlds that resemble the spectra of buildings,” Lingam says. “In other words, we should be aware that ‘unknown unknowns’ may potentially exist on other worlds.”
That’s part of the importance of thinking now about what future astronomers might be able to do.
“We need to prepare now for the possible science goals and engineering requirements of these future missions,” says Kipping.
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