How far can human civilization be seen from anywhere else in the universe? That is the question astronomers Lisa Kaltenegger of Cornell University and Jacqueline Ferti of the American Museum of Natural History ask themselves. Therefore, they turned the search for extraterrestrial life. They did not look for signs of life around the stars other than the sun. They calculated how many stars have been able to pick up signs of terrestrial life over the past 5,000 years – assuming that civilizations around those stars use the same search technology as astronomers here on Earth.
The technique astronomers often use to search for life on exoplanets (planets outside the solar system) is called the transit method. When a planet passes in front of its star, the brightness of this star decreases slightly. By measuring fluctuations in brightness, astronomers can find exoplanets and learn about their properties. The amount of light from the star obscuring the planet reveals the size of the planet. The regularity in brightness fluctuation tells us something about its distance from the star. Sometimes a little light also escapes through the planet’s atmosphere. From this light, if the signal is strong enough, astronomers can read the composition of the atmosphere and look for materials that indicate life, such as oxygen.
Human radio signals have now reached 75 stars
The idea of reversing the transit method is not new. Some stars are in a spot, a strip where they can see the Earth moving in front of the sun. Astronomers have previously calculated how many stars are in this transit bar. But those studies only looked at the stars that are there now. But the stars change their position over time.
To calculate which stars modern humanity may have glimpsed, which have been orbiting the Earth for nearly 5,000 years, American astronomers looked at which stars are now in the transit sector and which stars have done so at all in the past 5,000 years. They searched for the current and past locations of stars in the Gaia Database, a catalog of astronomical objects within a radius of about 300 light-years from the Sun. Scientists compiled this catalog using the European Space Agency’s Gaia space telescope.
1,715. Among the many solar power systems, humanity may have embezzled one day, is over astronomers in nature. If you add the stars that will pass through the transit strip over the next 5,000 years, it comes to 2034. Human radio signals, which were broadcast about a hundred years ago, now have reached 75 of those stars. Remarkably, of all the planets orbiting those 2034 stars, 508 are estimated to be habitable. Habitable planets are neither too far nor too close to their star, which makes liquid water possible. And they are rocky. Of the 508 potentially habitable planets, 29 may have received human radio signals.
Ignas Snellen, an astronomer at Leiden University who was not involved in the study, calls it the academic game. Astronomers hypothesize that the transit method can be used to detect signs of life on Earth-like planets around other suns, but this is not the case. This is only possible with planets whose atmospheres provide an adequate signal; In planets that orbit near their star, which often passes through it, astronomers can observe the atmosphere several times. An Earth-like planet wouldn’t do that, it’s too far from its star. So civilizations around 1,715 stars can see Earth as an exoplanet, but humanity cannot be seen from the atmosphere using the transit method.”
Astronomers are building a method that will allow them to pick up a signal from the atmospheres of Earth-like exoplanets; a high contrast photography-telescope. “But even if other civilizations already had telescopes at their disposal, the calculations of American astronomers say nothing of reality,” Snellen continues. „to study a planet with a high contrast photographyThe telescope does not require a civilization to be in the transit corridor; This telescope points you at the planet itself rather than its star. This greatly increases the number of stars that may spy on Earth.”
A version of this article also appeared on NRC on the morning of June 24, 2021