Eavesdropping on radio broadcasts from galactic civilizations with upcoming observatories for redshifted 21 cm radiation

The question of whether intelligent life exists elsewhere is one of the fundamental unknowns about our Universe. Over the past decade {\gtrsim } 200 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 6\lesssim z\lesssim

15 . 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 ~10^1-2.7 pc, within a spherical volume containing 10^(3-8) × (Ω_b/4π)^α stars, where α = 1 (or 1.5) for a radar beam of solid angle Ω_b 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.