While the search for exoplanets has revealed more than 4,000 planets beyond our solar system over the last couple of decades, it begs the question of what may be looking back.
That’s the question posed by two astronomers in a study published Wednesday in the Monthly Notices of the Royal Astronomical Society.
“In our search for life in the universe, we ask a little bit of a different question in this research,” said Lisa Kaltenegger, associate professor of astronomy in Cornell University’s College of Arts and Sciences and director of Cornell’s Carl Sagan Institute, in a video shared by the institution.
“We ask, who could have actually spotted us? Who could have found out that Earth is teeming with life from their vantage point?”
Kaltenegger and Joshua Pepper, an associate professor of physics at Lehigh University, have identified more than a thousand stars similar to our sun that may have Earth-like planets orbiting at a distance from those stars where they could support liquid water on their surfaces. This distance is referred to by astronomers as the habitable zone.
To be clear, such planets have not yet been detected or confirmed around these stars.
And those potential planets, all within 300 light-years from Earth, could have a direct view of Earth and the life that thrives on it.
“It takes a specific location to be able to see the Earth go in front of its star, the sun. And then once a year, if you see the Earth go in front of the sun from your point of view, the sun would be just a little bit less bright,” Kaltenegger said.
“And so you would know a planet orbits it. And you would also know it’s at the right distance so it could have liquid water, one of the key ingredients for life.
“So we identified the thousand closest stars within 300 light-years, roughly, that could have spotted us already. Maybe there’s life out there in the universe. Maybe they already spotted us. What would they think?”
If life exists outside of Earth and has us in sight, they could use the light of the sun as Earth passes in front of it to look through our atmosphere and understand more about our planet.
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“If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot,” she said, “And we can even see some of the brightest of these stars in our night sky without binoculars or telescopes.”
“Pale Blue Dot” is the name of an iconic image that Cornell astronomer Carl Sagan suggested that NASA’s Voyager 1 probe take of Earth from 3.7 billion miles away as the probe moved to the edge of the solar system. The image was taken on February 14, 1990.
The search for life
The observation of a passing of a planet in front of its host star is called a transit, and it’s one of the main methods used by astronomers to detect exoplanets using ground and space-based telescopes.
When NASA’s James Webb Space Telescope launches next year, it will be used to peer into the atmospheres of exoplanets using this method, and astronomers can use that data to help characterize the atmospheres of exoplanets.
And NASA’s planet-hunting Transiting Exoplanet Survey Satellite (TESS) mission, which has been observing the brightest nearby stars since launching in 2018, will begin a new phase of its mission in 2021. The mission will search for exoplanets in the ecliptic, which is the plane of Earth’s orbit around the sun. The TESS spacecraft will essentially turn on its side to observe more of the sky.
The list of stars compiled by the researchers, which was created from TESS’ star catalog, could be used as targets to search for transiting exoplanets.
The ecliptic is also essentially the location where exoplanets that have Earth in sight could be found because from their vantage point, they could see Earth as it crosses in front of the sun.
“Only a very small fraction of exoplanets will just happen to be randomly aligned with our line of sight so we can see them transit,” Pepper said.
The star systems that could have seen Earth when life first began on our planet are different from the ones that can see the signs of life on our planet now — as well as the star systems that could see Earth in the future, the researchers wrote in their paper.
“If we found a planet with a vibrant biosphere, we would get curious about whether or not someone is there looking at us too,” Kaltenegger said. “If we’re looking for intelligent life in the universe, that could find us and might want to get in touch, we’ve just created the star map of where we should look first.”