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takyon writes:

The Search for Extraterrestrial Intelligence could be more successful if a smaller number of targets with a higher probability of success are observed. In a new paper, two astrophysicists propose looking at the thin region of space where aliens could observe the Earth transiting the Sun, the same technique that the Kepler space observatory uses:

In a paper to published in the journal Astrobiology, and available now online, Heller and Pudritz turn the telescope around to ask, what if extraterrestrial observers discover the Earth as it transits the sun?

If such observers are using the same search methods that scientists are using on Earth, the researchers propose that humanity should turn its collective ear to Earth's "transit zone", the thin slice of space from which our planet's passage in front of the sun can be detected. "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."

The transit zone is rich in host stars for planetary systems, offering approximately 100,000 potential targets, each potentially orbited by habitable planets and moons, the scientists say – and that's just the number we can see with today's radio telescope technologies. "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," write Heller and Pudritz.

[...] Heller and Pudritz propose that the Breakthrough Listen Initiative, part of the most comprehensive search for extraterrestrial life ever conducted, can maximize its chances of success by concentrating its search on Earth's transit zone.

From The Register .

[Continues.]

The Search for Extraterrestrial Intelligence in Earth's Solar Transit Zone (open, DOI: 10.1089/ast.2015.1358)

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 center, is about 0.262° wide. We first compile a target list of 45 K and 37 G dwarf stars inside the rETZ and within 1 kpc (about 3260 light-years) using the Hipparcos catalogue. We then greatly enlarge the number of potential targets by constructing an analytic galactic disk model and find that about 10⁵ K and G dwarf stars should reside within the rETZ. The ongoing Gaia space mission can potentially discover all G dwarfs among them (several 10⁴) within the next 5 years. Many more potentially habitable planets orbit dim, unknown M stars in ETZ and other stars that traversed ETZ thousands of years ago. 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.

Original Submission