If life exists on other worlds, would they even be able to see us?

"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," one of the researchers said.

NGC 4866, a lenticular galaxy, is shown in this NASA handout provided on July 19, 2013. Situated about 80 million light-years from earth, this image was captured by the Advanced Camera for Surveys, an instrument on the NASA/ESA Hubble Space Telescope (photo credit: REUTERS/EUROPEAN SPACE AGENCY/NASA/ESA HUBBLE SPACE TELESCOPE/HANDOUT VIA REUTERS)
NGC 4866, a lenticular galaxy, is shown in this NASA handout provided on July 19, 2013. Situated about 80 million light-years from earth, this image was captured by the Advanced Camera for Surveys, an instrument on the NASA/ESA Hubble Space Telescope
(photo credit: REUTERS/EUROPEAN SPACE AGENCY/NASA/ESA HUBBLE SPACE TELESCOPE/HANDOUT VIA REUTERS)
Scientists at two American universities are trying to determine if exoplanets, planets found outside our Solar System, are able to see us with the same visibility that we see them.
To date, over 3,000 exoplanets have been found, with "dozens" orbiting the habitable zone of their host stars. The discovery of these exoplanets came within NASA’s Transiting Exoplanet Survey Satellite (TESS) program, which has already surveyed 74% of the sky as part of its two-year mission to find transiting extra-solar planets, including potentially habitable worlds.
"In our search for life in the universe, we ask a little bit of a different question in this research," said Lisa Kaltenegger, associate professor of astronomy at Cornell University and director of it's Carl Sagan Institute, according to CNN. "We ask: 'who could have actually spotted us? Who could have found out that Earth is teeming with life from their vantage point?'"
Kaltenegger and her fellow researcher, associate prof. Joshua Pepper from Lehigh University, found over a thousand stars resembling the sun that could host planets, in their orbit, with water on their surface.
While the stars are a bit easier to detect, the Earth-like planets are not, and have not yet been confirmed to exist. But if they do exist, can they even see us?
"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 bit less bright – and so you would know a planet orbits it," Kaltenegger said, according to CNN. "And you would also know it's at the right distance so it could have liquid water, one of the key ingredients for life."
She added: "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 the universe. Maybe they already spotted us. What would they think?"

IF SO, and life exists outside of our tiny planet, then alien scientists could use the light from the sun to study Earth's atmosphere when it passes in front of the sun - called a transit - and learn about the planet the same way we do now.
"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."
"Pale Blue Dot" refers to the image taken of Earth in 1990 by NASA's Voyager 1 from 3.7 billion miles away as it departed our Solar System. The name was popularized by Carl Sagan in his 1994 book of the same name. "Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light," he wrote.
In 2021, NASA's TESS program will start searching the ecliptic, the plane on which the Earth orbits the Sun, after it finishes its current two-year mission. Planets found along the ecliptic would have a perfect vantage point for the transit research astronomers perform today.
"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, according to CNN.
The researchers note that as the universe expands it also changes, and that the solar systems that had the vantage point to see Earth when life first began here would be completely different from those that can spot us now - and that goes for solar systems of the future.
"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, according to CNN. "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."
Their findings were published in the Monthly Notices of the Royal Astronomical Society.