Abstract
The question of whether intelligent life exists elsewhere is one of the fundamental
unknowns about our Universe. Over the past decade 
extra-solar planets have been discovered, providing new urgency for addressing this
question in these or other planetary systems. Independently of this perspective, new radio
observatories for cosmology are currently being constructed with the goal of detecting
21 cm emission from cosmic hydrogen in the redshift range 
. The radio frequency band covered by these experiments overlaps with the range of
frequencies used for telecommunication on Earth, a regime that was never explored with
high sensitivity before. For example, the low-frequency demonstrator (LFD) of the
Mileura Wide-Field Array (MWA), will cover in 8 kHz bins the entire frequency
range of 80–300 MHz, which is perfectly matched to the band over which our
civilization emits most of its radio power. We show that this and other low-frequency
observatories (culminating with the Square Kilometer Array (SKA)) will be able to
detect radio broadcast leakage from an Earth-like civilization out to a distance of
∼101−2.7 pc, within a spherical
volume containing 10(3−8) × (Ωb/4π)α stars,
where α = 1 (or 1.5) for a
radar beam of solid angle Ωb
that remains steady (or sweeps) across the sky. Such a radio signal will show up as a series
of narrow spectral lines that do not coincide with known atomic or molecular lines. The
high spectral resolution attainable with the upcoming observatories will allow us to
monitor the periodic Doppler shift of the broadcast lines over the planet's orbital period
around the parent star. Determination of the parent star mass through observations of its
spectrum could then be used to infer the inclination, semi-major axis and eccentricity of
the planet's orbit. This, in turn, will allow us to estimate the temperature on the planet's
surface and to assess whether it can support liquid water or life as we know it.