Image: OpenSpace/ American Museum of Natural History
Astronomers has discovered thousands of planets outside of our solar system over the past few decades, revealing a multitude of captivating worlds within our galaxy. This golden age of exoplanet discovery raises an alluring question: If we can spot worlds orbiting stars from Earth, could speculative alien civilizations spot Earth from their own vantage points?Most exoplanets are discovered as they pass in front of their host star from our perspective, causing a very slight dip in stellar brightness that can be detected by telescopes on Earth. This technique, known as the transit method, has enabled scientists to spot and even characterize basic details about these worlds, including whether they might be potentially habitable.
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Now, a pair of exoplanet researchers have inverted this process by cataloging which star systems within roughly 300 light years of our solar system are in the perfect spot to witness Earth crossing in front of the Sun. As it turns out, a whopping 1,715 stars “are in the right position to have spotted life on a transiting Earth since early human civilization (about 5,000 years ago), with an additional 319 stars entering this special vantage point in the next 5,000 years,” according to a study published on Wednesday in Nature.“Everything moves in the universe,” said Lisa Kaltenegger, an associate professor of astronomy and director of the Carl Sagan Institute at Cornell University who co-authored the new study, in a call. “The cosmos is dynamic—we are moving around the Sun, the Sun moves around the center of the galaxy—so this vantage point or this cosmic front seat, so to say, to see the Earth backlit or as a transiting planet, has to be at a point that is both gained and also lost.” “It’s a little bit like ships in the night passing each other, and some see each other and some don’t,” she added. To explore these relationships between exoplanets, Kaltenegger teamed up with Jaqueline Faherty, a senior scientist at the American Museum of Natural History, who is well-versed in utilizing the immense observations collected by the European Space Agency’s Gaia satellite. Gaia, launched in 2013, is currently building the most comprehensive map of space objects in the Milky Way.
The sheer magnitude of Gaia’s catalogue, which tracks millions of stars and their movements, allowed the researchers to pinpoint systems in what they called the Earth transit zone (ETZ) over the past 5,000 years, along with those that will enter it during the next 5,000 years. The list of those systems is compiled at this link.While past studies have identified star systems that can spot Earth transits in the present, Kaltenegger and Faherty are the first researchers to expand this temporal aperture out to a range of 10,000 years spanning the past and (fingers crossed) future of human civilization. The results revealed an abundance of systems that have occupied the right position to watch Earth transits for millennia at a time.In other words, if there are any hypothetical aliens that live in the star systems identified in the study, they would have had ample opportunity to spot Earth in front of the Sun and perhaps even identify signs of life and intelligence on our world, such as by identifying radio signals. “What we showed in our paper is that most stars have this vantage point [to see Earth transits] for at least 1,000 years, and a lot of stars actually have it for more than 10,000 years,” Kaltenegger said. “We couldn't say anything more than that because our timeline is 10,000 years, but it was interesting that this vantage point holds for generations of astronomers, or generations of alien astronomers” that “could develop technology to find us.”
The new study also spotlights subpopulations within that list that might be particularly interesting in the search for extraterrestrial intelligence (SETI). For instance, Kaltenegger and Faherty found that human-made radio waves have already reached 75 of the closest stars on the list, some of which host potentially habitable exoplanets. By combining these observations with a likely rate of rocky planets in the habitable zone of star systems, the team concluded that “an estimated 29 potentially habitable worlds that could have seen Earth transit and could also detect radio waves from our planet,” according to the study.This subcategory is especially important for SETI because it hints at the star systems that might be good candidates for attempts at two-way interstellar communication, assuming they are inhabited. Some of the most promising locations highlighted in the study include the Trappist-1 system, which contains seven Earth-sized planets. This system is close enough to Earth to have received human-made radio waves and will be in the right position to witness Earth transits in 1,642 years. Ross-128, a system located just 10 light years away, exited the ETZ about 900 years ago, while Teegarden’s Star, located 12 light years away, will enter this zone in 2050.Kaltenegger and Faherty point out that there is a lively debate about whether humans should attempt to contact speculative alien civilizations on potentially habitable exoplanets, given that we know nothing about their technological capabilities, motives, or indeed, if they exist at all.
However, the researchers note that this point may be moot to some extent, because biological activity on Earth has been visible for eons and technological activity has been evident for at least a century.“There are so many exoplanets that could have found us already as an interesting life-bearing planet,” Kaltenegger said. “They wouldn't know—and we wouldn't know if we found oxygen and methane somewhere else—which stage that life is in. But the ones within 100 light years would know that there's a technologically advanced civilization, if they could actually find radio waves.” Nobody knows if any extraterrestrial lifeforms exist within the ETZ, but the new research reveals that our biological and technological footprints are exposed to any prospective aliens that might have the wherewithal to search for them. Regardless of whether you find that to be a comfort or a concern, it’s good to know exactly where in space we should be looking to see signs of those looking back at us.Given the long distances between star systems and the corresponding delays in potential communication between humans and intelligent aliens, it’s also useful to think about these interactions on intergenerational timescales.“When you think of the evolution of our planet through time, how the Sun changes through time, and how we have a certain amount of time in the habitable zone, I think the whole universe becomes much more interesting because you can glimpse the past, present, and future of what's happening around you even though we have this tiny, tiny life,” Kaltenegger said.
For Kaltenegger, imagining how aliens might view Earth from afar is an extension of the legacy of Carl Sagan, who pioneered interdisciplinary research into these ideas at Cornell for decades. One of Sagan’s most famous reveries, entitled “Pale Blue Dot,” was inspired by an image of Earth captured in the outer solar system by the Voyager mission. Our planet from that distance looks like “a mote of dust suspended in a sunbeam,” he said. That description could also apply to observations of Earth transits from the star systems catalogued in the new study, adding a new scale to the classic image of our pale blue dot.“I wish I could talk to Carl Sagan,” Kaltenegger said. “I wish I had met him when he was alive. I sit in his office, so sometimes when I look out of the window, I think: ‘this is what he would have seen. I wonder what he thought about when he was standing here.’”“We stand on the shoulders of giants,” she concluded. “All these people that came before us have put small puzzle pieces into place that let us understand what is around us.”