Earth could be visible to alien civilisations on planets outside our solar system, scientists have found.

One of the major projects of astronomers in recent years is looking for exoplanets, or other worlds beyond our solar system. When they are found, the key question is whether they might support life, and so whether we might be able to spot habitable planets at this distance.

Now astronomers have reversed that question, and explored whether we might also be visible to other civilisations that are outside of our solar system. They found that they were – and that we could be seen in all of the required detail.

In the study, astronomers took more than 1,000 main-sequence stars, those similar to our Sun, which might have Earth-like planets orbiting in their habitable zone, neither close enough to be too hot or far away enough to be too cold. All of the planets were within about 300 lightyears of Earth, and were spotted as part of Nasa’s TESS catalogue, which lists exoplanets.

They found that many of them would be able to see us – and, if they had the right technology, be able to see the chemical traces that life leaves on our planet.

An article describing the research, ‘Which Stars Can See Earth as a Transiting Exoplanet?’ is published in the Monthly Notices of the Royal Astronomical Society. Transiting exoplanets are those worlds we can see from Earth, spotting them when they move in front of their star and register as a dip in the light that makes it to Earth.

“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,” said Lisa Kaltenegger, associate professor of astronomy at Cornell University and director of Cornell’s Carl Sagan Institute, in a statement.

“If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot,” she said, “And we can even see some of the brightest of these stars in our night sky without binoculars or telescopes.”

Professor Kaltenegger led the research alongside Joshua Pepper, associate professor of physics at Lehigh University.

The key question to answer when understanding how visible we might be is the Earth’s ecliptic, or the plane on which it orbits around the Sun. To be able to see our world – and explore its atmosphere and signals of life – Earth, our Sun and the other planet would have to line up correctly.

The researchers found that many of them were, and that any civilisations should therefore be able to get a good view of our Earth.

“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. “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.”

“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” she said, “we’ve just created the star map of where we should look first.”