Earth's transit zone would be the best spot from which to listen out for alien messages, astrophysicists have said. René Heller and Ralph Pudritz say intelligent extraterrestrial life should be able to see our planet as it moves in front of the Sun – just like how on Earth we use this technique to detect far away exoplanets.
As a result, they think this would be a good place to listen out for messages and a strong candidate for the Breakthrough Listen Initiative, a ten-year $100m (£72m) initiative to search for extraterrestrial intelligence announced by Stephen Hawking and Yuri Milner last year.
On Earth, scientists look for exoplanets (planets outside of our solar system) by tracking their shadows as they pass in front of their host stars. By doing this, they can work out all sorts of information, like surface temperatures and potential habitability.
Heller and Purditz's study, which is due to be published in the journal Astrobiology, says we should assume an intelligent alien civilisation is using the same methods to search for life as we are: "Our planet may be seen by distant observers against our Sun's light, allowing them to detect us. As an ultimate consequence, even if our species chose to remain radio-quiet to eschew interstellar contact, we cannot hide from observers located in Earth's solar transit zone, if they exist."
In the study, the researchers work out how many planets and moons would be able to see Earth's transit zone – the area where Earth's shadow would be visible to other planets. They find there are around 100,000 stars that fall within this range, each with planets and moons where life could exist.
"If any of these planets host intelligent observers, they could have identified Earth as a habitable, or even as a living, world long ago, and we could be receiving their broadcasts today ... Ultimately, the ETZ [Earth transit zone] would be an ideal region to be monitored by the Breakthrough Listen Initiatives, an upcoming survey that will constitute the most comprehensive search for extraterrestrial intelligence so far."
Heller said: "It's impossible to predict whether extraterrestrials use the same observational techniques as we do. But they will have to deal with the same physical principles as we do, and Earth's solar transits are an obvious method to detect us."
The Breakthrough Listen Initiative has access to two of the world's most powerful telescopes - the Green Bank Telescope in West Virginia and the Parkes Telescope in New South Wales, Australia. The programme will cover 10 times more sky than previous projects and will be 50 times more sensitive than other SETI initiatives.
On its announcement, SETI found Frank Drake said: "Right now there could be messages from the stars flying right through the room, through us all. That still sends a shiver down my spine. The search for intelligent life is a great adventure. And Breakthrough Listen is giving it a huge lift.
"We've learned a lot in the last fifty years about how to look for signals from space. With the Breakthrough Initiatives, the learning curve is likely to bend upward significantly."