From the Washington Post:
"The hunt for extraterrestrial life—of any kind, including lowly, long-dead microbes—is lofty enough. But the hunt for intelligent civilizations that could be looking for us in return? It's even more of a long shot. In a new paper published in the journal Astrobiology, researchers present one possible strategy for finding these theoretical beings: Assume that they're searching for us in exactly the same way we're searching for them.
Humans detect exoplanets (planets beyond our solar system) by observing their transits in front of their host stars. In essence, space telescopes, such as the Kepler, can watch the way a star twinkles and blinks and determine whether a planet is regularly passing in front of it.
That brightening and dimming can be used to calculate the size of the planet and its distance from the star. Scientists can also figure out what kind of atmosphere the planet has based on the way the molecules surrounding it scatter the light of its sun. Based on these factors (and the kind of star in the system) scientists can make educated guesses about what sort of body the planet is—and whether it could hold liquid water.
NASA now estimates that there are more than 1 billion 'Earth-like' planets in our galaxy alone. It's true that we have absolutely no idea what intelligent life on another planet might look like, but looking for life that evolved on a planet like our own seems like a safe start. After all, we know it happened at least once.
The new study suggests expanding that approach: What if aliens didn't just evolve on an Earth-like planet, but evolved into the sort of beings who would use planetary transits to go looking for other Earth-like planets?
In other words, what if the aliens have their own Kepler?
If that's the case, then those aliens would be within Earth's own 'transit zone'—the thin sliver of space from which an observer could see our planet's passage in front of the sun.
"It's impossible to predict whether extraterrestrials use the same observational techniques as we do," study co-author René Heller of the Institute for Astrophysics in Göttingen, Germany, said in a statement. '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.'
In theory, we may be able to catch a planet that had already sent us some kind of message long, long ago. And once we knew what direction to listen in, we'd stand a better chance of capturing it.
So how much does that narrow down our potential search? A lot, but probably not enough: There are likely at least 10,000 star systems with planets worth checking out in that region. And in 2010, researchers turned a telescope array on the transit zone for a few days just to check for any obvious alien signals. Like most proposed techniques in the hunt for intelligent life, this idea—while intriguing—is unlikely to drop an alien civilization into our lap.
But it doesn't hurt to look."