Many exoplanets are betrayed by the fact that they pass directly in front of their sun and thus darken it for a short time. Two astronomers have now turned the tables and investigated the stars from which a transit of the earth across the sun is visible. Because that would be the most likely method by which extraterrestrial observers would discover our planet and its traces of life. The result: 1402 stars currently have a clear view of the earth’s transit. 46 stars are so close to us that the inhabitants of potential exoplanets could also capture our radio waves, which have been radiating into space for around 100 years, in these systems.
Are we alone in the Universe? Or is there life on other planets and maybe even extraterrestrial civilizations? We can still only speculate about it, because neither traces of life nor signals from inhabitants of strange worlds have been discovered so far. After all, astronomers have already discovered several thousand extrasolar planets and many of them orbit their star in the habitable zone. Most of these planets are tracked down using the transit method: telescopes detect the slight dimming of the star caused by a planet passing in front of it. For nearby planets, scientists can also use highly sensitive spectroscopes to determine the composition of their atmosphere. It leaves lines in the star’s light spectrum that are specific to the elements and molecules. Astronomers hope to use this technology to soon find traces of life on such planets.
When the earth passes in front of the sun
Conversely, extraterrestrial astronomers could also use these methods to search for inhabited planets in their vicinity. “From the perspective of an exoplanet, we are the aliens,” explains Lisa Kaltenegger from Cornell University in Ithaca. Together with astrophysicist Jackie Faherty from the American Museum of Natural History, she investigated where our earth could be found using the transit method. “We wanted to know which stars have the right vantage point to observe the earth as the sun is hidden,” explains Kaltenegger. In contrast to previous studies of this type, the two researchers also take into account when and for how long this “front row seat” will be retained for other stars and planetary systems. “Because the stars move in our dynamic cosmos, they can move into the line of sight, but they can also leave it again,” says the astronomer.
For their study, the researchers evaluated the latest catalog from the European Gaia satellite, which has mapped the positions and movements of 1.8 billion celestial bodies in the Milky Way. “Gaia gives us a precise map of the Milky Way and allows us to go back and forth in time to see where stars were and where they are going,” explains Faherty. The evaluation showed that a total of 2034 stars will pass the earth transit zone between 5000 years ago and 5000 years from now – the zone from which the earth’s transit in front of the sun can be seen. Of these, 313 objects have already moved out of the zone, 319 will enter them in the future and 1402 stars currently have a clear view of the earth’s transit. Among these celestial bodies with currently free view are 128 sun-like stars and 1050 stars of the spectral class M – red dwarfs and red giants, as determined by the astronomers.
Free view and reception of our radio waves
Of all these stars on “observation posts”, however, those that are close enough to possibly learn more about the earth during a transit are of particular interest. As Kaltenegger and Faherty found, there are currently 46 stars both in the Earth’s transit zone and within a radius of up to 100 light years. “These 46 objects could observe the passage of the earth in front of the sun and at the same time would be able to capture radio waves from the earth,” report the researchers. Because the radio messages and radio broadcasts that marked the beginning of the communication age around 100 years ago have since spread to around 100 light-years away. Based on estimates of the abundance of planets around different types of stars, this could mean that around 29 potentially life-friendly planets are currently in the right position to detect both the earth and human radio waves, according to the astronomers.
So far, only seven systems with planets are known of the 2034 stars that will pass through the Earth’s transit zone in the entire 10,000-year period. Closest to us is the red dwarf Ross 128, only eleven light-years away, with an earth-like exoplanet. Its inhabitants – if they existed – could have seen the earth transit for 2158 years – from the time 3057 years ago to around 900 years ago. The two planets around Teegarden’s star, which is twelve light-years away, will come into line of sight in 29 years. On the other hand, it takes a little longer until the TRAPPIST-1 system with its seven exoplanets, which is a good 40 light-years away, has a clear view of the earth transit: You will enter the earth transit zone in 1642 years, but then remain in the “front row” for 2371 years “.
“Our analyzes show that even the nearby stars remain in a position for more than 1000 years in which they can see the earth’s transit,” says Kaltenegger. “This means that possible civilizations on planets of these systems have enough time to discover the earth as a potentially interesting planet.” However, whether these civilizations exist and whether extraterrestrial astronomers have already pointed their telescopes at the earth is unknown.
Source: Lisa Kaltenegger (Cornell University, Ithaca) et al., Nature, doi: 10.1038 / s41586-021-03596-y