Any aliens living around 1,004 nearby stars should be able to see the signs of life on Earth, a study says

• Astronomers have identified 1,004 sun-like stars that could have Earth-like planets in their orbits.
• Any intelligent aliens on those exoplanets would be able to detect the signs of life on Earth.
• The researchers say these stars could be prime targets in the search for alien life: If we know about the aliens and they know about us, there might be a better chance of communicating.

If there are aliens in our galaxy, they might already know about us.

In a new study, two astronomers have identified 1,004 sun-like stars that could all have habitable Earth-like planets in their orbits. Any intelligent aliens on those exoplanets should be able to see Earth and spot the chemical signs of life here.

That's because, from those planets' perspectives, Earth passes in front of the sun each time it orbits — a tiny, dark spot in front of the blazing star.

This appearance is called a transit; astronomers on Earth use these tiny drops in the brightness of other stars to identify planets passing in front of them. In just 11 years, scientists have discovered more than 3,000 planets using this method.

By measuring how the light from a star changes when a transiting planet passes in front of it, scientists can determine the exoplanet's size and, sometimes, the composition of its atmosphere. So it follows that intelligent aliens could do the same about Earth. If they did, they would spot signs of life: Plants on our planet fill the atmosphere with oxygen, and bacteria produce nitrous oxide, a gas which is unlikely to appear without biological processes.

"If we found a planet with a vibrant biosphere, we would get curious about whether or not someone is there looking at us too," Lisa Kaltenegger, a co-author of the new study, said in a press release. She and her co-author, Joshua Pepper, an associate professor of physics at Lehigh University, published their research Wednesday in the journal Monthly Notices of the Royal Astronomical Society.

All the stars identified in the study are within 326 light-years of Earth. That makes them prime targets in the search for intelligent alien life. If they can see us and we can see them, there'd be a better chance of communication.

"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," Kaltenegger, who directs Cornell University's Carl Sagan Institute, added.

Kaltenegger and Pepper selected the stars they analyzed from a catalogue that NASA uses to identify targets for its planet-searching telescope, the Transiting Exoplanet Survey Satellite (TESS). TESS has scanned about 75% of the sky and found dozens of new planets, as well as thousands of possible planets that astronomers are still working to confirm.

The scientists excluded stars for which we don't have reliable data, so there may be far more than the 1,004 nearby stars they found from which aliens could see Earth. Scientists have only observed planets around two of the stars on Kaltenegger and Pepper's list, but astronomers think that most stars have planets, and billions of stars in our galaxy have planets that could be habitable.

"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 in the release. "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."