These possibly habitable exoplanets can see Earth in its evolution

(CNN) – When human civilization began to flourish on Earth about 5,000 years ago, 1,715 star systems within 326 light years of Earth could have seen our planet during that time. And in the next 5,000 years, another 319 star systems will be able to see Earth.

If there are exoplanets orbiting these nearby stars, they could witness our planet crossing in front of the sun. Observing the passage of a planet in front of its host star is called a transit, and it is one of the main methods used by astronomers to detect exoplanets using ground and space telescopes. And it’s the way that life on other planets, if it exists, could observe Earth.

Some of the planets that orbit these stars could potentially be habitable. When planets orbit at a certain distance from their stars, those planets could harbor liquid water on their surfaces. Astronomers call this distance the habitable zone.

In the last hundred years, Earth has also sent messages revealing the intelligent life it sustains in the form of man-made radio waves, and 75 stars are less than 100 light-years away, meaning they are close enough. as for these waves to reach them.

Lisa Kaltenegger, associate professor of astronomy and director of the Carl Sagan Institute at Cornell University, explores the idea of ​​how Earth can be seen on other planets.

His previous research, published in October, “I was exploring who might see us as a planet in transit right now, the way we look for other worlds,” he said.

In his latest research, published Wednesday in the journal Nature, Kaltenegger and astrophysicist Jackie Faherty, principal scientist at the American Museum of Natural History, wanted to explore the idea of ​​Earth’s changing views over time, from the perspective of nearby star systems.

The stars are not immobile. They move, just as our sun revolves around the center of our galaxy, prompting Kaltenegger to wonder how this “affects who might see us as ‘aliens’.”

“The universe is dynamic and everything is moving,” Kaltenegger wrote by email. “Therefore, this cosmic front seat to view the Earth as a planet in transit, blocking the sunlight is fleeting. You can win and lose. We wanted to know how long the stars retain that vantage point and also which stars had it and which stars will have it, within a 5,000-year timeline. 5,000 years in the past, when civilizations began to flourish, and within the next 5,000 years, as I am optimistic that we will discover how to survive.

The researchers used data from the European Space Agency’s Gaia database, a catalog of nearby astronomical objects found about 300 light-years from our sun. The team wanted to know when the stars enter the so-called Terrestrial Transit Zone, where the Earth could be detected through transit, how long they stay in this zone, and when they exit.

“Gaia provided us with an accurate map of the Milky Way, allowing us to look back and forward in time, and see where the stars have positioned themselves and where they are going,” Faherty said in a statement.

The scientists determined that 2,034 star systems passed through this area during 10,000 years.

“Our solar neighborhood is a dynamic place where the stars move in and out of that vantage point perfect for watching Earth transit the Sun at a rapid rate,” Faherty said.

Within star systems that are at the right distance and vantage point to observe Earth, past, present and future, scientists know that seven are home to exoplanets. If there is life on these exoplanets and they have the ability to detect other planets in the same way that astronomers do on Earth, they could determine that Earth can support life.

When a planet crosses in front of its star, the planet’s atmosphere is essentially backlit. Earth’s atmosphere would reveal that it has chemical signatures of life.

The researchers determined that the Ross 128 system, the second closest to include an Earth-size exoplanet orbiting a red dwarf star 11 light-years away, could have seen Earth transit the sun for 2,158 years. Its perspective window opened about 3,057 years ago and closed 900 years ago.

In the future, the system Trappist-1home to seven Earth-sized planets, including four potentially habitable, will be within Earth’s transit zone in 1,642 years. These planets will be able to see Earth for 2,371 years.

About a hundred years ago, humans began to generate radio waves.

Like vehicles, light needs time to travel. So the radio waves that humans created in the beginning have only traveled about 100 light years from Earth, Kaltenegger said.

«In our list of stars there are 75 stars that could have seen or seen the Earth right now passing through the Sun, which also would have received radio waves from us at the same time. And they would know our taste in music, for better or for worse, “he said.

As for why we haven’t gotten a signal back, there could be many reasons, Kaltenegger said. One hundred years is a short time on the astronomical timescale, and we become less and less dependent on radio as we develop other technologies.

“And any message, if it is sent, is sent with that future technology that we cannot find yet,” he said.

When NASA’s James Webb Space Telescope is launched in October, it will be used to observe the atmospheres of exoplanets, and astronomers will be able to use that data to help characterize the atmospheres of exoplanets.

The permanent search for exoplanets has rarely explored the plane of the ecliptic, or the plane of Earth’s orbit around the sun, of the Milky Way because it is heavily populated with stars, making it more difficult to detect planets, Kaltenegger said.

“But with this new list and the added motivation that these are the star systems that might have seen us already, that changes,” he said. “Our research provides the best list of targets for the search for extraterrestrial intelligence.”

Kaltenegger is on the science team for NASA’s TESS planet-hunting mission, which launched in 2018 and searches for exoplanets orbiting nearby stars. The Transiting Exoplanet Survey Satellite began looking for rocky exoplanets in this region in April, and she hopes it will find many more intriguing planets now that they have this list of stars.

“Once NASA’s TESS mission and ground watchers find new planets around these stars, I want to model them to find out if they could be like Earth and then ask for time (from the James Webb Space Telescope) to observe them and find out if there are signs of life in its atmosphere. That would be fascinating, signs of life on a planet that could have seen us too, ”he said.