Dozens of planets might have civilisations that know about Earth - study

Don't be surprised if ET decides to phone our home sometime soon.

New research has found there are at least 29 potentially habitable planets orbiting other stars that could have both seen Earth and are close enough to have received radio waves from our planet in the past 5000 years. 

Scientists in the US looked at thousands of star systems located within 300 light years of Earth, and how their vantage points have changed since human civilisation began. 

They found 1715 stars that have had a clear view of the Earth passing in front of the sun - that's how our scientists find planets orbiting other stars. Another 319 will have the chance to see us in the next 5000 years, containing another known 42 planets.  

"Among these stars are seven known exoplanet hosts, including Ross-128, which saw Earth transit the Sun in the past, and Teegarden's Star and Trappist-1, which will start to see it in 29 and 1642 years, respectively," wrote Lisa Kaltenegger of Cornell University and ‪Jacqueline Faherty‬ of the Department of Astrophysics at the American Museum of Natural History.

While the Earth's transit across the sun is only temporary, we've been pumping out radio signals revealing our existence for about a century now. 

"We found that human-made radio waves have already swept over 75 of the closest stars on our list," the pair wrote. 

And long before that, life began changing the atmosphere in ways sufficiently advanced alien astronomers could likely detect. The steam engine's invention in the late 1700s kickstarted human-made global warming, but there have been other significant changes to climate in the past.

"The discussion on whether or not we should send out an active signal or try to hide our presence is ongoing. However, our biosphere has modified our planet's atmosphere for billions of years, something that we hope to find on other Earth-like planets soon. 

"Thus, observing Earth as a transiting planet could have classified it as a living world since the Great Oxidation Event, for a billion years already, through the buildup of oxygen and ozone in the presence of a reducing gas... That provides a long timeline for nominal civilisations to identify Earth as an interesting planet."

The furthest space probe we've sent to date is Voyager 1, launched in 1977 and now in interstellar space, about 19 light-hours from the sun. 

The research was published Thursday in journal Nature