Would Aliens Visit?

Jun. 27, 2002

by Seth Shostak - Senior Astronomer

When it comes to alien activities, visiting Earth seems to be pretty high on the "to do" list. But does that make sense?

Approximately half the U.S. population suspects that extraterrestrials have come to our planet. This is such a controversial (and emotional) topic that its mere mention in one of these articles is usually sufficient to guarantee a storm of Web chat and high-voltage e-mails. In the end, of course, the matter of alien visitation will be decided by the evidence, not by the intensity of opinion. While I certainly expect that the Galaxy is home to many advanced societies, the quality of the evidence has so far failed to convince me that any of them have emissaries on our planet.

But lets back off to our neutral corners for a moment and consider an intimately related question: why would aliens be visiting now? According to the most popular view of this matter, extraterrestrial craft have been flitting across our skies since 1947. Thats 55 years in a planetary history of 4,600,000,000 years. If we assume for the moment that these claims are real, this chronology tells us immediately that either (1) we are the beneficiaries of an enormously rare event (one chance in 100 million, or if you want to argue that no aliens would visit until they detected oxygen in our atmosphere, one chance in 40 million), (2) the aliens routinely visit Earth, or (3) our activities (nuclear tests, environmental degradation, etc.) have attracted the aliens attention, and encouraged them to drop by.

The first possibility, that we just happened to luck out (being around for the first and only alien encounter), is less probable than that you not someone, but you will win next months lottery jackpot. It strains credulity, to use polite vernacular.

The second possibility, that Earth hosts extraterrestrials on a routine basis, and therefore a visit during your lifetime is not particularly improbable, deserves a bit more scrutiny. The question is, how often do they visit? If its only once in a few tens of millions of years, were back to the first possibility, and the odds are highly stacked against you being one of the lucky visitees. But some folk claim that aliens have glissaded to Earth in historical times (five millennia ago, when the pyramids were built, or one millennium ago, when the Nazca Indians elected to decorate the Peruvian desert floor with glyphs of turkeys and other of their favorite fauna). If any of this is true, it argues for visits at least once every 1,000 years or so. The problem with this is that barring some reason for them to visit humans in particular (a possibility we consider below) it implies that there have been millions of expeditions to Earth! We may send the occasional anthropological research team to Borneo, but we dont send millions. And its a lot easier to get to Borneo than to traverse hundreds or thousands of light-years. This, too, seems to be an unlikely explanation for visitors now.

Finally, we consider door number three we have enticed the aliens with human activity. Lets set aside the question of whether advanced galactic societies would have the slightest interest in our wars, our pollution problems, or our reproductive systems. The real question is, how would they know about us at all?

In fact, theres only one clear and persistent "signal" that Homo sapiens has ever sent to the stars: our high-frequency radio transmissions, including television and radar. The Victorians (let alone the Egyptians or the Nazca Indians), despite all their technical sophistication, could never have been spotted from light-years away. Humans have been making their presence known to the universe only for the last 70 years or so.

And thats a problem. It means that even if, after receiving an earthly transmission, the aliens can immediately scramble their spacecraft and fly to Earth at the speed of light, they cant be farther than 8 light-years away to have arrived by 1947. There are four star systems within this distance. Count em, four. Were back to winning the lottery.

What about warp drive? Maybe the aliens can create wormholes and get here in essentially no time. It doesnt matter. Our signals travel at the speed of light, and this means that even with infinitely fast spacecraft, the aliens cant be farther off than 15 light-years to have reached our lovely planet by 1947. The number of star systems within 15 light-years is about three dozen. There would have to be 10 billion technically sophisticated societies in the Galaxy to have a reasonable chance of finding one camped out among the nearest three dozen stars. Thats optimism of a high level indeed.

Its nice to think that either Earth or its human inhabitants have not only attracted the attention of galactic neighbors, but encouraged them to visit. But frankly, the numbers dont give much support to this somewhat self-indulgent idea.