For possible aliens: Earth’s transits visible from more than 2000 stars

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Transits of the earth in front of the sun could be observed from more than 2000 comparatively close star systems and thus traces of humanity could be found. Some of these systems have even received radio waves from us. That comes from a list that two astronomers from the USA made.

Thanks to the data from ESA’s revolutionary Gaia space telescope, they were able to incorporate the stars’ own movements into the calculations for the first time. Transits of the earth in front of the sun have been visible from more than 1700 stars at a maximum distance of 100 parsecs in the past 5000 years; this will be possible for over 300 more in the next 5000 years. Systems with known exoplanets are also included.

For us humans, the so-called transit method is currently by far the most productive in the search for exoplanets, i.e. planets in other star systems: of more than 4,400 confirmed exoplanets, more than 3,300 were found using the minimal darkening that they cause during transit in front of their star . So if possible extraterrestrial civilizations are looking for intelligent life on other worlds with comparable means, it is reasonable to assume that they act in a similar way to ours. Lisa Kaltenegger and Jackie Faherty wanted their work now determine where possible aliens would be sitting in the first row when it comes to observing our earth and us.

That is why they determined the Earth Transit Zone, in which the earth can be seen at least temporarily in front of the sun, they explain. In the next step, thanks to the Gaia data, they determined which stars, at most 100 parsecs (326 light-years) away, lie in this zone in the past and next five millennia. As a result, they have now presented a list of 1715 stars from whose system the earth was visible from the sun after the formation of the first civilizations 5000 years ago. From there you could partly see how the CO2-Content in our atmosphere increases, they explain what possible extraterrestrials with a level of development comparable to us could see there. In the next 5000 years, 319 more star systems will come into that zone.

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Since we started to emit radio waves, these in turn have already reached 75 of these star systems from which transits of the earth can be seen, the two write. In addition, exoplanets have already been found in seven of the more than 2000 stars listed, for example the star Ross 128, which is only 11 light-years away. Strange signals that seemed to come from the system caused a stir in 2017. From the Trappist-1 system, where seven exoplanets have already been discovered, earth transits will be seen for the first time in 1642 years and then for more than 2000 years.

Overall, it has been shown that transits of the earth can be seen for more than 1000 years, even from the closest stars. So extraterrestrial civilizations have enough time to discover us, says Kaltenegger. You could even do thought experiments where possible aliens have already found us and could possibly work to explore us, adds Faherty. We too could consider whether we should pay special attention to the systems that have been identified. The scientific work is published in the journal Nature.


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