The search for extraterrestrial intelligence (SETI) could be taking the wrong approach. Instead of listening for alien radio broadcasts, a better strategy may be to look for giant structures placed in orbit around nearby stars by alien civilisations.

“Artificial structures may be the best way for an advanced extraterrestrial civilisation to signal its presence to an emerging technology like ours,” says Luc Arnold of the Observatory of Haute-Provence in France. And he believes that the generation of space-based telescopes now being designed will be able to spot them.

Arnold has determined the characteristic transit signal that differently shaped objects would produce, including a Jupiter-sized equilateral triangle and a louvre – parallel slats with gaps between them. Corot and Kepler will be capable of distinguishing these objects from most planets, though they could still be confused with a ringed planet like Saturn, he says.

To ensure the signal is unambiguous, an alien civilisation would have to launch a number of objects into orbit around a star. As an example, Arnold imagines 11 objects orbiting a star in groups of one, two, three and five – the first prime numbers…