succeeds, we could send one into deep space, perhaps programmed into an interstellar probe. Or if we can find a way to tell receivers how our software works, we could even send avatars to them via a radio beam.

Whatever we decide, the next problem is where to aim our message. Broadcasting loudly to the whole sky would use ludicrous amounts of power, far beyond our capabilities today. Instead, active SETI would target promising star systems. A sun-like star is a start; planets are better. Space telescopes such as Kepler should be able to detect planets that are Earth-size, and future telescopes should eventually be able to pick out those with liquid water on the surface and oxygen in their atmospheres.

This still might not be a well-targeted address list, however, because civilisations may be very thinly spread. "Even if you found a list of 10,000 Earth-like worlds, that might not be good enough," says Shostak. "We've had biology for 4 billion years, and radio telescopes for 40 years; that's 1 in 100 million." If the technological window for aliens is as short as ours then we might have to transmit to 100 million Earths before anyone hears us.

For practical reasons, Shostak thinks we should wait and listen. "The bottom line is that for the foreseeable future, the only decent targets are people that have contacted you. Let them do the heavy lifting."

In the meantime, planning our own message could help to focus the minds of SETI experts on what kind of communication we should be looking out for. If Earth's efforts are anything to go by, we can expect a basic maths lesson and some pictures of naked aliens.

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