Abstract
Over the past few years, astronomers have detected thousands of planets and
planet candidates by observing their periodic transits in front of their host
stars. A related transit method, called transit spectroscopy, might soon allow
studies of the chemical imprints of life in extrasolar planetary atmospheres.
We here address the reciprocal question, namely, from where is Earth detectable
by extrasolar observers using similar methods. Thus, we explore the Earth's
transit zone (ETZ), the projection of a band around the Earth's ecliptic onto
the celestial plane, where observers can detect Earth transits across the Sun.
The ETZ is between \$0.520^\circ\$ and \$0.537^\circ\$ wide due to the non-circular
Earth orbit. The restricted ETZ (rETZ), where the Earth transits the Sun less
than 0.5 solar radii from its center, is about \$0.262^\circ\$ wide. We compile a
target list of 45 K and 37 G dwarf stars inside the rETZ and within 1
kiloparsec (about 3260 lightyears). We construct an analytic galactic disk
model and find that about \$10^5\$ K and G dwarf stars should reside within the
rETZ. The ongoing GAIA space mission can potentially discover all G dwarfs
among them (several \$10^4\$) within the next five years. Many more potentially
habitable planets orbit dim, unknown M stars in the ETZ and other stars that
traversed the 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.
The K2 mission, the Allen Telescope Array, and the upcoming Square Kilometer
Array might detect such deliberate extraterrestrial messages.
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