(CN) — Since early human civilization, dozens of potentially habitable planets and thousands of their stars could’ve been within earshot of radio waves and within view of astronomical instruments --- assuming the worlds are inhabited and the inhabitants have developed sufficiently advanced technology.
One way to spot exoplanets — planets outside our solar system — is to scan for their tracks as they pass across a star. With this concept in mind, researchers from Cornell University in Ithaca, New York, proposed that Earth could also be detectable by other planets using this method.
The idea has been partially explored, with previous studies examining the space where nearby stars may have had a view of Earth crossing the sun.
The Cornell University study released Wednesday and published in the journal Nature is the first to consider the various vantage points the nearby stars and exoplanets may have had over time.
For the study, researchers Lisa Kaltenegger and Jaqueline Faherty tapped into the European Space Agency's Gaia database, which contains a catalogue of nearby space objects within 100 parsecs --- about 300 light-years of the sun. They determined that 1,715 stars have been in a position to see Earth in the last 5,000 years.
“The universe is dynamic, changing the vantage point of other stars, from which they could find us over thousands of years — as well as ours, from which we could detect planets transiting other stars,” the study authors wrote. “During Earth’s Anthropocene period — 237 years so far, starting with the steam engine in 1784, when humans started to influence Earth’s climate — 1,424 stars have seen Earth transit the sun. For a further 1,000- and 5,000-year period, that number would increase to 1,489 and 1,743, respectively.”
Researchers note an additional 319 stars will enter that vantage point in the next 5,000 years and that 75 stars are close enough now — within 100 light-years of Earth — to receive human-made radio signals.
“Here we assume that any nominal civilization on an exoplanet would have astronomical instrumentation comparable to what we have now,” the study authors wrote. “Recent technosignatures — like radio waves — can, in addition, indicate a technological civilization on Earth. Even though radio waves have been emitted by humans only for a comparably short time (about 100 years), they have already reached 75 stars in the past and present [Earth transit zone] in our neighborhood and are still travelling farther out.”
The study found that 29 potentially habitable planets orbiting some of these stars could have both received radio waves and observed Earth from their vantage points. The estimate is based on the distance radio waves have traveled, which is about 100 light-years.
Stars in a position to observe the Earth pass across the sun are considered priority targets in the search for potentially habitable planets, the study suggests.
“Seven of the 2,034 stars are known exoplanet host stars,” the researchers wrote. “The Trappist-1 system at 12.467 parsecs hosts seven transiting Earth-size planets, with four of them in the temperate habitable zone. This system will enter the [Earth transit zone] in 1,642 years and remain there for 2,371 years.”
Researchers did not immediately respond to a request for comment by press time.
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