So far astronomers have discovered nearly 6,000 exoplanets—worlds that orbit stars other than our sun. If that number already feels ridiculously large, you’d better brace yourself: extrapolation of that total suggests there could be hundreds of billions of planets in our galaxy alone. Some fraction of them will be like Earth, although at the moment we don’t know what that fraction is. Still, with an amount that huge, even a small percentage of Earth-like orbs could mean a lot of habitable planets.
That’s why most scientists take the idea of life on other worlds seriously. Life arose here pretty rapidly—practically as soon as Earth had cooled enough to harbor oceans—which implies that it’s easy to get started once conditions are clement. The timing of the advent of hazily defined, higher-order features such as intelligence and technology, however, is a different question and one about which we are mostly restricted to speculation (although there have been some interesting investigations). But let’s say that right now there are intelligent aliens and technological civilizations out there somewhere in the Milky Way. Could they detect us?
With the question phrased that way, in the most general sense the answer is yes. By this I mean there’s no physical reason you couldn’t build an immense telescope, one far, far larger than any currently in existence, that would be capable of taking a detailed image of a planet from a great many light-years away. The engineering task may be considerable, but it’s not technically impossible. Then it might only be a matter of seeing city lights at night, for example, to confirm that aliens—that is, humans, because we’d be alien to them—exist.
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In fact, it may be even easier than that. A much smaller telescope need not resolve the planet; just observing it well enough to watch it get brighter and dimmer as nighttime cities rotate into and out of view might be good enough. And that “smaller” telescope would have to be, oh, let’s say, only ridiculously huge instead of overwhelmingly so.
The reason to wonder about any of this is that it flips the script on what’s usually asked, which is how we can detect aliens given our current level of technology. We can’t know their level in advance, but we do know our own—so it makes sense to assume their tech is equivalent to ours and then ask from what distance they could spot us.
That task is actually extremely difficult. Space is big, and vast distances dim even the mightiest of civilizations. But we can use our own as a template and work backward to estimate the outer limits of any interstellar eavesdropping on our noisy little world by extraterrestrials that use similar technology.
Altering our planet’s climate is not great, to say the least, but it does make a signature detectable from space.
A team of astronomers headed by Sofia Sheikh of the SETI Institute has run the numbers on this question and published its results in the Astronomical Journal. (“SETI” stands for “search for extraterrestrial intelligence.”) The researchers looked at various methods of detecting our various so-called technosignatures and found that the answer, unsurprisingly, depends on which specific one any extraterrestrials would be looking for. Many such ideas have been investigated individually before, but this recent analysis examines them collectively and consistently to arrive at some fresh insights.
One example of a technosignature is radio. Since its inception in the mid-20th century, SETI has focused on detecting artificial radio signals from space. Radio waves are easy to make and detect, and they can pass at the speed of light through interstellar space, scarcely impeded by any gas or dust that might be in the way. That makes radio a nearly ideal carrier for galactic-range communication.
The astronomers divided radio signals into four categories: first, pointed but intermittent broadcasts to space, essentially “we are here” messages; second, intentional and persistent targeted signals sent to our planetary probes in deep space that continue on into the galaxy; third, persistent omnidirectional signals, such as “leakage” emissions from cell-phone towers, as well as radio and television stations; and fourth, signals from artifacts, such as low-power downlinks from our interplanetary probes.
Signals in the first category can be detected from the farthest away because the power involved in their transmission is highest. Sheikh and her colleagues estimate that these waves can be spotted at a staggering 12,000 light-years from Earth! That’s a maximum distance, but several billon stars lie within that range. If you want to be found, this is probably the way to go.
The other methods don’t fare as well. For the second category, the maximum distance is more like 65 light-years, which still includes thousands of stars. The third category gets out to only four light-years, which isn’t even as far as the closest star to the sun. (That star, Proxima Centauri, is 4.25 light-years away.) The fourth one, which would include signals from our spacecraft, such as the Voyager 1 probe, has a detection limit of just under one light-year away. That surprised me, given how weak the signal is now, when the spacecraft is “only” about 25 billion kilometers away. Voyager 1’s 23-watt transmitter is already dimmed to less than a billionth of a billionth of a watt as seen from our world.
Clearly, radio is the method of choice for aliens looking for Earth. But there are other signatures.
One outcome of our modern civilization is an imprint on our atmosphere. Besides carbon dioxide, quite a few other chemicals have been dumped into our air by industry and other anthropogenic sources. Altering our planet’s climate is not great, to say the least, but it does make a signature detectable from space. And that signature could be especially obvious for interstellar observers located along our solar system’s ecliptic, the plane of Earth’s orbit around the sun. From that perspective, they would see our planet pass directly in front of our star once every year, slightly dimming its light. Called a transit, this has been the most successful method so far for discovering exoplanets.
Such transits can also be used to remotely analyze a world’s air. As starlight (or, in our case, sunlight) passes through a planet’s upper atmosphere, certain wavelengths of light will be absorbed by molecules there, creating a kind of fingerprint that can be measured. We already use this method to study some transiting exoplanets with the James Webb Space Telescope (JWST). And proposed future telescopes such as NASA’s Habitable Worlds Observatory are meant to scan the atmospheres of dozens of potentially Earth-like exoplanets that may exist around nearby stars (even if they don’t transit as seen from our solar system).
In their study, the SETI Institute astronomers focused on the remote detection of nitrogen dioxide, or NO2, a conspicuous by-product of fossil-fuel burning. Given the current levels in our polluted air, they find that we could detect such a signature from a distance of 5.7 light-years. Only the Alpha Centauri system is within that range, which limits any aliens’ options for uncovering us. Still, it’s an impressive technological achievement to be able to do this kind of search at all.
Most other types of technosignatures aren’t as helpful. A JWST clone perched somewhere in the vicinity of Neptune’s orbit could detect the infrared glow of heat emanating from our cities, but farther out that trail grows cold. At about 100 times that distance, the optical gleam of Earth’s city lights would fade to black—better but still far short of even our next-nearest star.
Lasers are easier to detect and are already being tested by NASA and the European Space Agency for in-space satellite communication. Still, under reasonable assumptions, a laser’s beam of focused light would be too dim to detect from a distance of just under six light-years, which is not enough to be spotted even at Barnard’s star, the second-closest star system to our own.
The worst case involves searches for our off-world technological artifacts. Earth’s swarms of artificial satellites, for example, slightly change the amount of sunlight our planet blocks during a transit but not enough to be detectable even from Mars. Suffice to say that if aliens were close enough to see such things, there would be far easier ways for them to spot us.
All these numbers come with the pretty big caveat that the extraterrestrials are no more advanced technologically than we are. This assumption may be very conservative because, after all, we get more advanced all the time. We continue to build bigger telescopes, limited only by budget and the laws of physics, and we are still finding and developing new ways to investigate the cosmos, such as detecting neutrinos and gravitational waves. We’ve been doing what might be called “modern” astronomy for only a century or so, and it’s difficult to predict where we might be 100 years hence.
The galaxy has been around for billions of years. No one can say yet who else shares it with us or what they’re using to explore it. The truth is out there, and turning our search for aliens inside out—by looking from the outside in—may best inform us on how to find it.