Who could see and hear the Earth in outer space
The director of the Carl Sagan Institute at Cornell University (USA) and her colleague Jackie Faherty from the American Museum of Natural History in New York have virtually dared to look back at Earth from the perspective of aliens. "We wanted to know which stars have the right angle of view of Earth to see it when it blocks light from the Sun. But because stars move in our dynamic cosmos, that viewing angle can come and go," Kaltenegger explained. When a planet passes by its central star on its orbit, its light is reduced a little - provided the observer is in the plane containing the sun and its planets.
2,034 solar systems possible observation points
If such a "transit" occurs, clues about the composition of the planet's atmosphere and conclusions about possible life can also be gathered. Such events have enabled science to detect more and more potentially habitable extrasolar planets since the 1990s. Kaltenegger and Faherty used data from the European Space Agency's "Gaia" space telescope to determine the conditions for such a discovery of us "aliens" from the perspective of other intelligent beings. The telescope has been measuring our cosmic environment since 2014.
Within a radius of 326 light-years, which is narrow by astronomical standards, the scientists found exactly 2,034 solar systems from which our home planet could theoretically be observed over a period of 10,000 years. Since human civilization began to develop about 5,000 years ago, a total of 1,715 solar systems have had at least the chance to look in our direction - were thus at least temporarily in the transit zone of the Earth. In the next 5,000 years, in turn, 319 more would be added, according to the researchers.
Estimated 29 planets in habitable zone
Within a radius of about 100 light-years there are 117 stars, which fit into the narrow scheme. For about 100 years, mankind has been delighting its environment with radio waves sent into space by commercial radio stations. According to the new analysis, the listening pleasure could have reached 75 stars in a 100 light-year radius since then, Kaltenegger and Faherty calculated.
Seven of the 2,034 stars identified are already known to have exoplanets orbiting them. Each of these worlds has had, or will have in the reasonably foreseeable future, the opportunity to detect Earth by the transit method, just as researchers have detected thousands of distant planets on Earth.
Among the 117 stars in the near vicinity, the scientists estimate that there should be 29 planets in the habitable zone. It is to be assumed that nearly each star is accompanied at least by a planet, in approximately each fourth is also in the habitable zone, explained Kaltenegger to the APA.
Little knowledge about closer environment
Especially in the nearer environment, little is still known about any companions of the central stars, he said: this is because light from relatively many, relatively nearby stars makes it difficult to even detect the only minimal lowering of radiation during planetary transits. "But now we have a good reason to search these 2,034 stars for planets. That's why we wrote this article and created this list of stars, so that observers can now target these stars that might see Earth transits," Kaltenegger said.
Most interesting to the scientist are already known exoplanets in habitable zones: There is one of these in the Ross 128 system about eleven light-years away. From there, the Earth was visible for 2,158 years, but 900 years ago it disappeared from view for any Ross 128 inhabitants. The Trappist-1 system, 45 light-years away, is also promising. Here there are even seven discovered planets, of which four are in the moderate range. But it will take another 1,642 years for Earth to reach their field of view. Altogether there are four systems with such exoplanets, which already receive radio waves and had or will have sight on the Blue Planet in the investigation period.