The search for extraterrestrial intelligence needs all the help it can get. Depending on who you listen to, the chances of us spotting an intelligent technological society vary from an almost certainty to practically zero.

The trouble is the sheer size of the search. The Milky Way contains around 10^10 sun-like stars, any one of which may have a planet whose citizens are at this very moment pointing their beady eyes or antennae in our direction.

But if we want to peer back, in which direction should we look?

Shmuel Nussinov at Tel Aviv University in Israel makes a thoroughly sensible suggestion of narrowing the search: why not look only towards stars that have a reasonable chance of having seen Earth?

We know of several ways to detect planets aroudn other stars but only one that might reveal an Earth-like body and that is to look for changes in brightness that are the signature of a transiting planet.

Earth passes in front of the sun for 13 hours once a year, dimming it by 77 parts per million. Venus transits for 11 hours every 7 months with even less dimming. Mars gives three-fold weaker eclipse every 1.9 years and Mercury dimming is ten times weaker than Earth’s but occurs four times a year.

Only stars within a narrow angle of the ecliptic will be able to detect these transits. And so only civilisations on planets around these stars could possibly be aware of Earth might be broadcasting our way.

Common sense really.

Ref: arxiv.org/abs/0903.1628: Some Comments on Possible Preferred Directions for the SETI Search