Earthly astronomers are busy scanning distant star systems for planets. There is one limitation: With the popular transit method, we can only detect planets if they move in front of their star from our point of view and change its brightness. Of course, this limits the selection quite a bit, it is a big coincidence if the orbital plane of an exoplanet is roughly parallel to our viewing direction to the star.

Now you can also ask different questions. Let’s assume that aliens were looking for other planets that harbor life, just like us. Where would they have to live in order to discover the Earth? This question was asked some time ago by researchers at the Max Planck Institute for Solar System Research in Garching. They found only nine of the 3600 exoplanets known at that time with a direct view of the Earth.

Lisa Kaltenegger of the College of Arts and Sciences and Director of the Carl Sagan Institute in Cornell and Joshua Pepper of Lehigh University have now related this question to stars. They have identified 1,004 main-sequence stars (similar to our Sun), which could contain Earth-like planets in their own habitable zones – all within a radius of about 300 light-years from Earth – and which should be able to detect the chemical traces of life on Earth.

The paper «Which stars can see the Earth as a transiting exoplanet?» was published in the Monthly Notices of the Royal Astronomical Society on 21 October. «If observers were out there searching, they could see signs of a biosphere in the atmosphere of our pale blue dot,» says Kaltenegger, «and we can even see some of the brightest of these stars in our night sky without binoculars or a telescope.

Pepper and Kaltenegger compiled the list of the thousand nearest stars using NASA’s Transiting Exoplanet Survey Satellite (TESS) star catalog. «Only a very small fraction of the exoplanets will be randomly aligned with our line of sight so that we can see their transit», says Pepper. «But all of the thousands of stars we have identified in the solar neighborhood could see our Earth moving in front of the Sun, attracting their attention».