Beaming signals into space to find ET could potentially be risky for Earth and its inhabitants. So researchers are developing a Richter-like scale to assess the chance that extraterrestrials could detect – and potentially react to – such signals.

Decades of passively monitoring microwave frequencies have failed to find any evidence of signals from extraterrestrial civilisations. Frustrated by the long silence, some researchers want to start transmitting signals towards nearby stars with possible habitable planets in a plan called "active SETI".

However, others warn that this would be the equivalent of "shouting in the jungle", and that it is better to keep quiet for the time being. "Concerns range from worries about potential existential danger all the way to a desire for consensus about what should be said in such messages," says astrophysicist and science fiction writer David Brin, a leading voice of caution on an International Academy of Astronautics committee considering the issue.

To give the debate a "modest analytical basis", Iván Almár of Konkoly Observatory in Hungary and Paul Shuch of the SETI League in Little Ferry, New Jersey, US, have written a study proposing a way to rate the chance that aliens might detect terrestrial signals.

Called the San Marino Scale because it was first proposed in Europe's tiny Republic of San Marino, it rates the chance of a detection from 1 to 10 based on the strength and type of the transmission. The scale is split into two halves, each worth between 1 and 5. One part is based on signal intensity (compared to solar noise), the is other based on the type of signal.