Thousands of alien worlds have revealed their presence to astronomers by briefly blotting out a bit of starlight as they slip across their home stars’ faces—a simple consequence of celestial geometry. By observing these transits over the last decade, we’ve learned that planets outnumber stars in the Milky Way—and that the galaxy is stuffed with worlds where the conditions could be right for life to thrive.
Now, astronomers are considering the other side of that twinkling equation.
“Which stars could see us as the aliens, as the transiting planet where the Earth blocks out light from the star?” asks Cornell University’s Lisa Kaltenegger, who worked out an answer to that question in a new study published today in the journal Nature.
The majority of planets beyond the solar system have been detected by watching for worlds that traipse across their stars’ faces. Although this transit method has produced prodigious results, it misses the countless planets that don’t pass in front of their stars from Earth’s perspective. Likewise, alien observers need to be in the right spot to watch as Earth periodically blots out a fraction of sunlight—and that is subject to change as stars shift their relative positions.
“The cosmos is dynamic, so that vantage point should change through time—and I wanted to know, how much time do you actually have to find a planet?” Kaltenegger says.
With Jackie Faherty, a senior scientist at the American Museum of Natural History in New York, Kaltenegger calculated that any aliens orbiting 2,034 nearby stars could see Earth marching across the sun’s face during a 10,000-year period of time extending 5,000 years into the past and future.
The pair also calculated that an estimated 29 potentially habitable planets can see Earth transit and are near enough to detect human-made radio transmissions. Studies like this therefore provide a set of stars we could target in our own search for extraterrestrial intelligence, or SETI.
“These newly identified stars should be prime targets for our own SETI because they could be sources of deliberate interstellar messages to us,” René Heller of the Max Planck Institute for Solar System Research muses in an email. If alien observers know we’re here, they may “send us greetings.”
A constantly swirling set of stars
To identify stars with a view of Earth transiting the sun, Kaltenegger and Faherty sifted through data from the European Space Agency’s Gaia spacecraft, which keeps close tabs on the movements of more than a billion stars.
All the worlds that could see Earth orbit stars that are precisely aligned with the plane in which we circle the sun—a small sliver of space generally known as the ecliptic, and in this study, as the Earth transit zone. A little too far above or below the ecliptic, and Earth’s footprint wouldn’t be visible. The pair identified 1,402 stars that are currently in the ecliptic and within about 300 light-years of Earth. Then they ran the sky on fast-forward and rewind, studying how the stars shift over time to find the ones that fortuitously slide into position to enable Earth-gazing.
Although the stars in our sky might not seem to move all that much, they are constantly shifting relative to one another. In 2,000 years, for example, Polaris will no longer be the north star—just as it was not the pole star when ancient Egyptian, Babylonian, and Chinese observers charted the sky thousands of years ago.
That’s why adding that element of time “is crucial to this idea of looking at the Earth as a transiting planet because of the large distances involved,” Heller, who did a similar calculation, writes in an email. “One must consider the appearance of the sky literally as a movie, not as a picture.”
Over the past 5,000 years, the team found an additional 313 stars could have watched Earth march across the sun. Over the next 5,000 years, another 319 will have that same view.
“It was interesting to figure out how long that cosmic front-row seat lasts,” Kaltenegger says. Many stars have at least a thousand years to find Earth. “And a lot of them have more than 10,000 years,” she says. “So that’s quite some time.”
Seven of these stars host known exoplanets. Some even host worlds that are suspected to be rocky. Using what we know about the occurrence of rocky planets, Kaltenegger and Faherty estimated that at least 508 habitable worlds are in their sample, with 29 close enough to detect Earth’s radio transmissions.
For the last century or so, we’ve been leaking radio signals into space. Some, such as our TV broadcasts, are too faint to be easily discernible over cosmic distances. But others, such as concentrated blasts of radio waves emitted by powerful radar instruments, are bright enough to be easily detectable.
Today our most powerful radio transmissions are in the form of planetary radar—used by astronomers to study solar system objects such as asteroids by bouncing radio waves off of them. Until it collapsed in December, the Arecibo Observatory was the planet’s most powerful planetary radar, and pings from its transmitter—aimed largely at objects in the ecliptic—were effectively spamming any alien worlds that were also in its field of view.
“If you’re in the Earth transit zone, you’re going to preferentially see huge bursts of radio emission as we study our own solar system, because everything is in the same plane,” says Sofia Sheikh of the University of California, Berkeley’s SETI Research Center, who has performed SETI searches on a similar set of stars. “So, stars that will be able to see us transit will most likely, by chance, pick up the spillover from radar astronomy.”
With the right instruments, alien observers in the Earth transit zone could have even watched as humans slowly altered the composition of the planet’s atmosphere—starting most dramatically about 200 years ago with the Industrial Revolution and continuing through today. That, Sheikh says, is an example of a technosignature, or a sign that something artificial is affecting the natural composition of the gases sheathing a planet.
Ships passing in the night
Yet as Kaltenegger and Faherty point out, some of the alien worlds we think may be ripe for life cannot yet see Earth as a transiting planet—even though we can see them. It’s kind of like gazing through a cosmic two-way mirror.
These worlds include four of the seven Earth-size planets orbiting a star called TRAPPIST-1, which won’t be able to see Earth for another 1,642 years. Two Earth-mass worlds orbiting Teegarden’s star, about 12 light-years away, won’t be able to see us until the year 2050. And Ross128, which hosts an Earth-mass planet about 11 light-years away, could have watched Earth’s transits for 2,158 years—up until 900 years ago, when we slid out of view during the High Middle Ages.
“Some hypothetical observers around stars close to the ecliptic might not see us in transit today, but they might have discovered Earth as a living planet millennia ago—or they will only discover our transits in thousands of years,” Heller writes.
Would anyone on Ross128’s planet have recognized an inhabited Earth nearly a thousand years ago? Did they miss their chance to spot signs of an evolving biosphere on this pale blue dot? And what will life on Earth even look like when the TRAPPIST-1 worlds have their chance to find our planet? How might its fingerprints change?
“We need to think beyond the terms of the here and now,” Sheikh says. “We really limit our search if we’re looking for things that are in the same stage of evolution. Whether biological or technological, I think it’s necessary that we think about the far future and the far past.”