Despite our best efforts, we have thus far failed to find any signal from an alien civilization. There have been a few odd detections that are hard to explain, but nothing definitively alien. Of course to be successful, we not only have to be listening, but the aliens have to be transmitting. It turns out that transmitting a message across light years is extremely challenging, and would take a great deal of power to succeed. Is it really likely that an alien civilization would try to transmit a message in all directions? Perhaps they might just focus on stars where they know life exists.
It turns out that detecting life in another star system isn’t too challenging for an advanced civilization. In fact we are reaching the point with our own technology that we’ll soon be up to the task. The easiest method would be to observe a planet as it transits its star. In this way we can not only determine the planet’s size, we can also see how starlight is absorbed by the planet’s atmosphere. From this we can determine the composition of the atmosphere. Alien astronomers could see Earth’s atmosphere is rich in oxygen and water vapor, and might conclude that life is probable on our world. Knowing that, they might send a signal in our direction. So perhaps we should focus our SETI efforts on stars that could see Earth transit the Sun. It’s an interesting idea that’s worth considering.
Even now we can imagine ways of observing exolife that doesn’t require a planetary transit. One could imagine that a truly advanced civilization would know we exist even if Earth doesn’t transit the Sun from their vantage point. But with the universe so strangely quiet up to this point, what have we got to lose?
Paper: Shmuel Nussinov. Some Comments on Possible Preferred Directions for the SETI Search. arXiv:0903.1628 [astro-ph.EP] (2009)