Hence the chosen name of SETI. They are not searching just for alien life, but for alien intelligence — life forms that could communicate with us.
SETI not only looks for planets that could be Earthlike and habitable, it listens out for things like radio waves, produced only by advanced technological civilisations. Like ours.
Aye, there’s the rub. It’s one thing to listen out for signals from the cosmos that might tell us They’re Out There.
But we are also sending out radio signals of our own, and have been for the past 100 years. One of the first public radio broadcasts was from the Metropolitan Opera House in 1910: Cavalleria Rusticana.
Such broadcasts weren’t necessarily received only by the handful who owned a wireless set. They also started to travel outwards from the Earth, like ripples from a stone thrown into a pond — but moving at the speed of light.
They are now more than 100 light years away — one light year is approximately 5,878,499,810,000 miles — and SETI reckons the nearest inhabited planet is just 12 light years away.