Looking at Earth like an alien planet

Twenty-nine potentially habitable planets orbiting relatively nearby stars were in in a position to spot Earth in the past 5,000 years and possibly detect radio waves from our planet, according to a new study.

Why it matters: If intelligent life is out there, chances are it's searching for us too and any theoretical astronomers on these worlds would have been in a position to observe our planet in much the same way as Earthlings study distant stars and planets today.

What they found: The new study, in the journal Nature, used a database of 331,312 stars within 300 light-years to show that 1,715 stars have been in a position to see Earth in the last 5,000 years, with 319 other stars expected to be able to see our world in the next 5,000 years.

• If there are any alien astronomers out there on these potentially habitable worlds, they — in theory — could have seen the small dips in the Sun's light created when the Earth passes between the distant planet and our star; a method for finding exoplanets used here on Earth.
• The researchers also found about 75 stars are close enough to Earth that any radio waves sent out from our world could have reached them and possibly been detected, the same method used by SETI researchers to search for signs of intelligent life.
• "We can't search everywhere, and so this is the best input target list now for anyone interested in potentially habitable worlds that can see us as a transiting planet," Cornell University astronomer Lisa Kaltenegger, an author of the new study, told Axios via email. "If someone had found us already, I wonder what they would think about us?"

Yes, but: Just because these theoretical alien astronomers might have Earth in a database of potentially habitable planets doesn't mean they would know for sure that we're here or that they could reach us.

• Astronomers don't currently have the technology to confirm a truly Earth-like world somewhere out there in the universe, but future space telescopes being proposed now could allow researchers to detect habitable exoplanets in the future.
• Some star systems with known potentially habitable worlds aren't yet able to see our planet, or our solar system has already moved out of view. Trappist-1, for example — which plays host to multiple potentially habitable planets — won't be able to see our Earth transit the Sun for another 1,642 years.