Are aliens watching us right now?

One of the most popular tropes in science fiction is that aliens are spying on us. From classic UFO flicks like Close Encounters With the Third Kind to Star Trek's Vulcans, who observe Earth’s technological progression in secret for years, humans have long been fascinated by the idea that our cosmic brethren are aware of our existence, watching and waiting for the right moment to make contact.

But if an advanced extraterrestrial species really wanted to keep an eye on humanity, sending a cloaked ship to hang out in orbit is a pretty high-effort way to do so: It’d be much easier for aliens to watch us from the comfort of their planet using telescopes. Human astronomers, after all, are already scanning the cosmos for radio waves transmitted by advanced lifeforms. At the same time, exoplanet researchers are developing powerful new telescopes that can detect “bio-signatures” like methane in the atmospheres of distant worlds.

According to a new study, there are a number of places in our interstellar backyard where intelligent aliens — should they exist — might be doing the same things.

In fact, there are 1,715 star systems within about 300 lightyears of the Sun where alien astronomers would have been able to find and study Earth at some point over the past 5,000 years, according to research published Wednesday in the journal Nature. Astronomers within this “Earth transit zone” might be employing the same methods used by human astronomers today to determine that our planet is not only habitable but populated by advanced life forms capable of blasting out energy signatures and altering their planet's climate.

“What we wanted to do in some ways was hold a mirror up to how we are approaching exoplanet searches” by thinking about them from an outsider's perspective, Jackie Faherty, an astrophysicist at the American Museum of Natural History in New York and co-author on the study, told The Science of Fiction.

To detect planets outside our Solar System, astronomers look for flickering stars. When a planet passes in front of, or transits, its star, it temporarily blocks a small amount of starlight. But just as you can only spy on neighbors who live in your line of sight, only those exoplanets whose orbits align with Earth’s line of sight produce a flicker we can see. The same holds true for planet hunters on other worlds.

In earlier research, study co-author Lisa Kaltenegger, an astronomer at Cornell University, as well as other astronomers, identified nearby star systems with the best cosmic stadium seats to watch Earth transit the Sun today. In the new study, Kaltenegger and Faherty used the European Space Agency’s Gaia Catalog of Nearby Stars to explore which planets have been, or will be, in Earth’s transit zone over a 10,000 year period — from 5,000 years in the past to 5,000 years in the future — as stars move around in the sky relative to one another.

All told, the astronomers identified 2034 stars within 300 light-years of the Sun that have entered, or will enter, Earth’s transit zone sometime in that ten millennia interval. 313 of those stars were in Earth’s transit zone in the past, but have since moved out of it. Those include Ross 128, a system just 11 light-years away that has a confirmed Earth-sized exoplanet in the habitable zone of its star (where liquid water can form). People living on this planet, the unimaginatively named Ross 128b, could have watched Earth transit the Sun for more than 2,000 years, from the time of the Egyptian Pharaohs to when Henry the First was crowned King of England in the 12th century.

At that point, alien astronomers might have detected all sorts of bio-signatures in Earth’s atmosphere, but humans would have been tough to spot. Aliens on any of the 1,424 stars that have seen Earth transit the Sun since the invention of the steam engine might have had more luck collecting clues to our presence, such as spikes in industrial pollutants or greenhouse gases in Earth's atmosphere. Kaltenegger notes, however, that alien astronomers would need "really big telescopes" to notice the (still cosmically small) ways in which humanity has altered Earth's atmosphere, and that any changes they have documented are unlikely to be a "smoking gun" for intelligent life since natural processes like volcanic eruptions can also transform an atmosphere over time.

The star systems most likely to have spotted a rapidly advancing human civilization, Kaltenegger says, are the 75 that are close enough for our radio broadcasts to have washed over them in the past 100 years. One of those stars is Wolf 359, made famous by the eponymous battle against the Borg featured in Star Trek: The Next Generation’s Season 3 finale. The real Wolf 359 does not appear to have any exoplanets capable of supporting life, although the jury is still out on whether any ships full of hostile cyborgs are hanging out there.

In fact, of the more than 2,000 stars that could see Earth in the past, present, or future, just seven have confirmed exoplanets. These include the aforementioned Ross 128, as well as Trappist-1, a nearby red dwarf star that will start to see Earth transit the Sun in 1,642 years. Assuming we're still around at that point, perhaps somebody on one of the system’s seven Earth-sized exoplanets will catch wind of our existence.

Faherty says that the small number of stars with confirmed exoplanets that have the ability to see us doesn't mean that our cosmic neighborhood is devoid of neighbors. "If we haven’t found exoplanets around our nearby sample it’s not because they aren’t there, it’s because we just don’t have the right position to catch them,” she said. In other words, there may be plenty more planets that can see us, but which we can't see at all.

Kaltenegger and Faherty's catalog of star systems that can watch Earth transits offers "a trove of new good targets" to scan for signs of intelligent life, said Penn State astronomer Jason Wright, who wasn't involved with the paper. Seth Shostak, a senior astronomer with the SETI Institute, agrees. The list of stars in the new paper, he says, "is somewhat like a 'map' of a South Sea island with circles on it showing where treasure might be buried.  If you’re hoping to strike it rich, using the map is a lot better than trying to dig up the entire island."

Not only will the new catalog help astronomers focus their search for alien signals, it creates a shortlist of places where we could try sending a message. As Faherty was compiling the data, she says she “kept thinking we might as well take a chance and focus on [broadcasting a message to] the stars that are about to enter or are in the Earth transit zone now” just in case someone there is already watching us.

Kaltenegger points out that even astronomers on a planet like Ross 128b, which can't see Earth transit the Sun anymore, might be intrigued by radio waves emanating from our region of space if they had centuries-old data indicating that Earth harbored life. "I think they'd put two and two together," she said.

While less dramatic than a UFO de-cloaking over Washington, D.C, it's strangely comforting to think that first contact might look like two faraway worlds flickering at each other in the dark.