The hunt for alien life has so far concentrated on our nearest neighbours in the solar system, and planets orbiting stars far beyond our reach. Since the first exoplanet was discovered in 1992, astronomers have found there are more planets out there than the stars dotting our night skies. Missions like NASA’s Kepler and TESS have uncovered thousands of these worlds in hopes we can catch a glimpse of their make up for possible biosignatures – chemistry that could be indicative of life.
Now, a pair of astronomers in the US has proposed that if we have the means to see these worlds, potential alien civilisations could have the means to see us.
And if advanced life exists somewhere out there among the stars, chances are we may have already been spotted.
Lisa Kaltenegger, an associate professor and director of Cornell University’s Carl Sagan Insitute, and Joshua Pepper, associate professor of physics at Lehigh University in Pennsylvania, have identified 1,004 close stars similar to our Sun.
These stars might be orbited by Earth-like planets within the so-called habitable zone where conditions are ripe for liquid water to exists on the surface.
All of these stars are found within 300 light-years of Earth, meaning they are close enough for us to scan their potential planets for biosignatures.
Planets within this catalogue will also have a direct line of sight to Earth, which implies aliens could be scanning our world for signs of life as well.
Professor Kaltenegger said: “Let’s reverse the viewpoint to that of other stars and ask from which vantage point other observers could find Earth as a transiting planet.”
Transiting planets are worlds that pass in front of a star, through the observer’s line of sight.
Finely-tuned space telescopes like NASA’s Transiting Exoplanet Survey Satellite (TESS) can see these transits by detecting the dips in brightness they cause.
Transits also allow astronomers to catch a glimpse of a planet’s atmosphere, which could yield clues about what is happening on the surface.
Professor Kaltenegger said: “If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot.
“And we can even see some of the brightest of these stars in our night sky without binoculars or telescopes.”
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The astronomers presented their findings this week in the Monthly Notices of the Royal Astronomical Society.
The study identifies the thousand closest stars to Earth, using NASA’s TESS catalogue.
Professor Kaltenegger said: “In our search for life in the Universe, we ask a little bit of a different question in this research.
“We ask who could have actually spotted us? Who could have found out that Earth is teeming with life from their vantage point?
“Because 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 less bright. 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.
“And 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 Universe. Maybe they already spotted us. What would they think?”
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