A pair of scientists published a paper in the Monthly Notices of the Royal Astronomical Society this week that asks the question “which stars can see Earth as a transiting exoplanet?” Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) star catalog and per the press release, they “identified 1,004 main-sequence stars (similar to our sun) that might contain Earth-like planets in their own habitable zones – all within about 300 light-years of Earth – and which should be able to detect Earth’s chemical traces of life.”

Lead author Lisa Kaltenegger goes on to explain: 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.

Here’s where the geometry comes in: Earth’s ecliptic plane, or the plane in which we orbit the Sun, has to line up just right for those exoplanets to see our planet transit our star. It’s one of the main ways we find exoplanets ourselves, so it makes sense that a technologically advanced civilization would do the same.

Co-author Joshua Pepper noted: 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. 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.

So perhaps while we are looking for a vibrant biosphere across the stars, someone out there is looking back. It gives us another good place to focus a search for bio- and technosignatures, and you all know I am for that.

In the meantime, look up and give a wave to any potential friends that might be looking back at us.

More Information

Cornell University press release 

“Which Stars Can See Earth as a Transiting Exoplanet?” Lisa Kaltenegger &
Joshua Pepper, 2020, to appear in Monthly Notices of the Royal Astronomical
Society (preprint on arxiv.org)