How Far is ET?

Aug. 16, 2001

by Seth Shostak - Senior Astronomer

How many light-years is it to the nearest alien civilization? In 1960, when Frank Drake made the first modern effort to eavesdrop on radio signals from ET, he trained his antenna on two relatively close stars, Epsilon Eridani and Tau Ceti: respectively 10 and 12 light-years from Earth. He picked these backyard buddies for several reasons. For one thing, they are Sun-like stars, the type most likely to have planets suitable for the dirty chemistry we call life. But of great importance, they are nearby, and signals from close-in transmitters are likely to be stronger. Such signals will suffer less from the inevitable dilution of distance. In addition, it would be more interesting to find aliens in our neighborhood, as opposed to halfway across the Galaxy. If the aliens are really close say less than 100 light-years then two-way communication, while admittedly tedious, would at least be thinkable.

Drake didnt hear anything from these solar siblings. Nor have subsequent SETI searches, including the SETI Institutes Project Phoenix, a scrutiny of a thousand nearby stars. What this tells us is that the universe is not laced with strong, persistent signals from advanced societies. Evidence for extraterrestrials is not trivial to find, and the signals we search for are likely to be weak. Of course, this wasnt known in 1960. After all, it was possible that galactic civilizations are so rampant that even the stars next door might house aliens. But the SETI experiments of the last decades have shown thats not the case. We have to sift through more than a spoonful of hay to find a needle.

Fair enough. But how much grain do we have to paw through? In other words, just how far is it to the nearest active alien civilization?

Obviously, we still dont know. It depends on how many societies populate the Galaxy. This is the number "N" in the famous Drake Equation. The pessimists would say that N is 1: in other words, were the only game in town. In that case, the nearest aliens are well, there arent any nearest aliens! Carl Sagan, on the other hand, was a lot more optimistic. He occasionally suggested that N might be a million. In that case, its easy to work out that the average distance to the nearest galactic civilization is roughly 100 light-years. Thats comparable to the amount of space weve actually searched.

Drake himself is inclined to a more modest estimate for N, namely that there are approximately 10 thousand technological civilizations sprinkled through the Milky Way. If hes right, then our nearest neighbors will be 500 to 1,000 light-years distant. Thats a fair piece, although its only one percent of the distance across the Galaxy.

The plain and simple fact is that we wont know the mileage to ETs home turf until we find a signal. But unless youre of the opinion that the Galaxy fizzes with more than a million inhabited worlds, the distance will be at least 100 light-years. Conversation should we ever attempt it will be mighty slow. Two centuries for a query and a reply, minimum.

But thats OK. In the 16th century, communication between Europe and the newly discovered Americas was also poky. However, the slow speed of information exchange didnt make the discovery less important. Just knowing there were societies on the other side of a vast emptiness was reason enough to stop and wonder. We may never meet the aliens face to face, but well know we have company.