An estimated 1,715 nearby stars may have been in a position to see Earth in the past 5,000 years, according to a paper in Nature. The study goes on to propose that 29 potentially habitable worlds orbiting some of these stars could have both seen Earth and received human-made radio waves. The work indicates that signatures of life from Earth could be detected, assuming that observers would have astronomical instruments comparable to those that we currently use.
One method for detecting exoplanets is to look for signs of them transiting across a star; Earth could also be detectable from other exoplanets using this method. The zone from which nearby stars might have a view of Earth transiting across the Sun has been explored, but previous studies have not considered changing vantage points over time.
Using the Gaia database, which includes a catalogue of nearby astronomical objects within 100 parsecs (around 300 light years) of the Sun, Lisa Kaltenegger and Jaqueline Faherty explore how that view point has changed over time. They determine that 1,715 stars are in the right position to have seen Earth since early human civilization developed (around 5,000 years ago), with an additional 319 stars entering this vantage point in the next 5,000 years. In addition, 75 stars are close enough (within 100 light years) for human-made radio waves to have reached them.
Seven of the stars that lie in the zone from which Earth is visible in the past, present and future are known hosts of exoplanets. For example, the Trappist-1 system — home to seven Earth-sized planets — will enter this zone in 1,642 years and remain there for 2,371 years. Stars with a vantage point from which they could see Earth transit the Sun could be priority targets for searches for potentially habitable planets, the authors infer.
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