“Perhaps,” says Harvard astrophysicist Avi Loeb told The Daily Galaxy: “We developed our modern science and technology (including transit astronomy and radio communication capabilities) only over a century out of the 4.5 billion years of the Earth’s lifetime. The window of opportunity for anyone to notice us at a random time in Earth’s history is roughly one part in 45 million (4.5 billion divided by 100 years).
Technological “Window of Discovery”
“We know that most stars formed billions of years before the Sun, and we are sampling them at different times in the history of their habitable planets. If our circumstances on Earth are typical (which is a good assumption since it is the only example we have), then one needs to search through tens of millions of stars before finding one that will host a civilization that developed transit or radio technology.” Loeb commented. “This number is tens of thousands of times larger than the size of the sample of stars considered by scientists at Cornell University and the American Museum of Natural History who, in a recent paper, “have identified 2,034 nearby star-systems—within the small cosmic distance of 326 light-years—that could find Earth merely by watching our pale blue dot cross our sun.”
1,715 Possible Star-Systems Since Human Civilization Blossomed
That’s 1,715 star-systems that could have spotted Earth since human civilization blossomed about 5,000 years ago, and 319 more star-systems that will be added over the next 5,000 years, say Lisa Kaltenegger, professor of astronomy and director of Cornell’s Carl Sagan Institute and astrophysicist Jackie Faherty, a senior scientist at the American Museum of Natural History’. They authored “Past, Present and Future Stars That Can See Earth As A Transiting Exoplanet,” where they used positions and motions from the European Space Agency’s Gaia eDR3 catalog to determine which stars enter and exit the Earth Transit Zone—and for how long.
The Thought Experiment
Replying to Avi Loeb, Faherty told The Daily Galaxy “It was a thought experiment to see what worlds could detect us the way we want to detect others.”
“As for the sample size,” Faherty points out, “this is only the closest stars to us. We limited the sample to a volume of ~300 light years which only contains around 330k stars. So the really intriguing part of this study isn’t the bulk quantity results, but really the focus on your closest neighbors and what you might be able to say about them. A big take away I have from this is how important timing is in finding each other. Close-by stars that can see Earth as a transiting planet zip in and out of that zone because their relative speeds across the sky are so much faster than more distant objects.”
If you are lucky enough to be on an exoplanet in orbit around a star that enters the Earth transit zone then you have a cosmic front-row seat to see if Earth holds life, Kaltenegger and Faherty said in research published June 23 in Nature.
Exoplanets’ Point-of-View–“We are theAliens”
“From the exoplanets’ point-of-view, we are the aliens,” said Lisa Kaltenegger, professor of astronomy and director of Cornell’s Carl Sagan Institute, in the College of Arts and Sciences. “We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun’s light,” she said. “And because stars move in our dynamic cosmos, this vantage point is gained and lost.”
Gaia Map of Our Galaxy
“Gaia has provided us with a precise map of the Milky Way galaxy,” Faherty said, “allowing us to look backward and forward in time, and to see where stars had been located and where they are going.”
Of the 2,034 star-systems passing through the Earth Transit Zone over the 10,000-year period examined, 117 objects lie within about 100 light-years of the sun and 75 of these objects have been in the Earth Transit Zone since commercial radio stations on Earth began broadcasting into space about a century ago.”
Our solar neighborhood is a dynamic place where stars enter and exit that perfect vantage point to see Earth transit the Sun at a rapid pace,” Faherty said.
Included in the catalog of 2,034 star-systems are seven known to host exoplanets. Each one of these worlds has had or will have an opportunity to detect Earth, just as Earth’s scientists have found thousands of worlds orbiting other stars through the transit technique.
Ross 128 System
The Ross 128 system, with a red dwarf host star located in the Virgo constellation, is about 11 light-years away and is the second-closest system with an Earth-size exoplanet (about 1.8 times the size of our planet). Any inhabitants of this exoworld could have seen Earth transit our own sun for 2,158 years, starting about 3,057 years ago; they lost their vantage point about 900 years ago.
The Trappist-1 system, at 45 light-years from Earth, hosts seven transiting Earth-size planets—four of them in the temperate, habitable zone of that star. While we have discovered the exoplanets around Trappist-1, they won’t be able to spot us until their motion takes them into the Earth Transit Zone in 1,642 years. Potential Trappist-1 system observers will remain in the cosmic Earth transit stadium seats for 2,371 years.
A 1,000 Year Window
“Our analysis shows that even the closest stars generally spend more than 1,000 years at a vantage point where they can see Earth transit,” Kaltenegger said. “If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.”
The James Webb Space telescope—expected to launch later this year—is set to take a detailed look at several transiting worlds to characterize their atmospheres and ultimately search for signs of life.
The Breakthrough Starshot initiative is an ambitious project underway that is looking to launch a nano-sized spacecraft toward the closest exoplanet detected around Proxima Centauri—4.2 light-years from us—and fully characterize that world.
Worlds That May Have Already Detected Us
“One might imagine that worlds beyond Earth that have already detected us, are making the same plans for our planet and solar system,” said Faherty. “This catalog is an intriguing thought experiment for which one of our neighbors might be able to find us.”
Should we want to make contact with our alien neighbors however, Harvard’s Loeb told The Daily Galaxy: “In order for us to receive a response from another civilization, the signals must go both ways. This means that we can converse across a distance that is half the distance where our farthest radio signals reached. Half the distance means 1/8 of the volume. Therefore, there are only 8 stars rather than 75 from where we can potentially get a response.”
[Editor’s Note: Jackie Faherty is an editor and science advisor for The Daily Galaxy. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. She received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognizes scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments]
Image top of page: Shutterstock License
Your free daily fix of stories of space and science –a random journey from Planet Earth through the Cosmos– that has the capacity to provide clues to our existence and add a much needed cosmic perspective in our Anthropocene epoch.