Despite humanity's best efforts to find evidence of intelligent alien life out there somewhere in the Universe, we've turned up squat so far. Is Earth really so unique, or is it just that any extraterrestrials that once existed gave up the ghost long ago?
A new Canadian study suggests there's an alternative possibility. Aliens may very well be out there and looking for us too, and, if so, our best chance of finding one another is to presume that their ability to see across the chasm of space is effectively the same as our own.
One of the ways our Earthly scientists study faraway planets is by examining their shadows as they pass in front of their host stars. The authors of the new study – to be published in Astrobiology – say this is the most likely way alien observers would know anything of Earth too.
"It's impossible to predict whether extraterrestrials use the same observational techniques as we do," said astrophysicist René Heller, who worked on the research while at McMaster University in Canada, and is now at the Institute for Astrophysics in Göttingen, Germany. "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."
When a planet performs a transit in front of a star – as seen with Venus passing over the Sun in the NASA image above – it's the closest astronomers get to shining a light on planets that are so far away, they're difficult to study in other ways.
Transit observation not only allows scientists to visually see planets using telescopes, but it also helps them to collect other kinds of data, such as atmospheric conditions and surface temperature, giving them an idea of whether the world they're looking at might be habitable.
It stands to reason that intelligent extraterrestrials with the same level of technology could find Earth the same way – and in fact, they might already have done so.
"If any of these planets host intelligent observers, they could have identified Earth as a habitable, even as a living world long ago and we could be receiving their broadcasts today," the authors write in their paper.
The scientists propose that we should focus our alien-hunting efforts on Earth's own transit zone - a thin slice of space, about half a degree wide, but potentially less for atmospheric detection – from which our planet's passage in front of the Sun can be detected from elsewhere in the Milky Way.
That slender window in space is where we're most visible to anybody out there, so that's where we should be looking – or listening – for any kinds of extraterrestrial broadcasts or communications.
"As an ultimate consequence," the researchers write, "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."