If there are other civilisations out there on Earth-like planets orbiting Sun-like stars, we're likely to waste a lot of time filtering out billions of prospects.
That's led McMaster University (located in Hamilton, Ontario, Canada) boffins to suggest we thin out the field, by looking at planets that are in a position to discover Earth.
In their paper in Astrobiology (PDF here), they note that a civilisation as developed as our own probably has its own Kepler-like exoplanet hunt – and that might suggest where we should look first.
The indomitable Kepler space telescope's search for exoplanets looks for tiny fluctuations in light as planets transit their suns, and that means we only see planets if they pass between their sun and us – in other words, the orbital plane of the planet has to be oriented just right.
As soon as it's said, it's obvious: Earth would be similarly detectable to other civilisations if (from their point of view) it transits the sun.
"We explore Earth's transit zone (ETZ), the projection of a band around Earth's ecliptic onto the celestial plane, where observers can detect Earth transits across the Sun. ETZ is between 0.520° and 0.537° wide due to the noncircular Earth orbit. The restricted Earth transit zone (rETZ), where Earth transits the Sun less than 0.5 solar radii from its centre, is about 0.262° wide," they write.
"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."
Even then, the list could be long: the researchers, René Heller and Ralph Pudritz, estimate that when upcoming surveys start cataloguing possible targets, we might end up with 100,000 possible targets.
Such missions include Kepler's K2, the Allen Telescope Array, the Square Kilometre Array, or the Green Bank Telescope. They note that the Earth transit zone would also offer a good survey field for the US$100 million Yuri Milner and Stephen Hawking-backed Breakthrough Listen Initiative.
In the university press release, Heller notes that "It's impossible to predict whether extraterrestrials use the same observational techniques as we do," says Heller. "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."
Which sounds rather better than trying to listen for the faintest of signals from the billions of possible stars out there. ®