Over the last few decades, we have found over 4,000 confirmed exoplanets. Some are similar to our Earth, most are wildly different. A simple but obvious question when it comes to these worlds is, “If we can see them, can they see us?” Two researchers have found an interesting answer.
Within 100 parsecs (326 light-years) of Earth, they have identified 1,004 stars that may contain Earth-like planets from which Earth may be detectable using the transit method. The transit method is our most successful technique to date to discover exoplanets. A transiting planet can be spotted as it crosses its star, creating a dip in the starlight observed by a telescope. This also allows us to glean information about the planet’s atmosphere by observing how it absorbs different kinds of light, which in turn gives us clues about what elements are present in the atmosphere.
For us to detect a planet in this way, its orbit needs to be more or less aligned with the line of sight between us and the star. Now, researchers have discussed a reverse scenario, looking at potential candidates that may be able to detect Earth in this way. According to their study published in the Monthly Notices of the Royal Astronomical Society, there are plenty of stars from which our planet and its atmosphere would be clearly detectable.
Professor Lisa Kaltenegger, director of Cornell’s Carl Sagan Institute, and Professor Joshua Pepper, from Lehigh University, focused on stars similar to our Sun. They found just over 1,000 such stars, 508 of which guarantee an Earth transit observation that would last at least 10 hours.
“If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot,” Professor Kaltenegger said in a statement. “And we can even see some of the brightest of these stars in our night sky without binoculars or telescopes.”
“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.” Professor Pepper added. ”But all of the thousand stars we identified in our paper in the solar neighborhood could see our Earth transit the Sun, calling their attention.”
This study provides some interesting targets for NASA's exoplanet-hunting telescope TESS and the SETI program to look at. Will we find Earth-like planets in the habitable zone around these stars? Could there be somebody already looking at us?
“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.”