Summary (Jul 29, 2003): What are the chances that an advanced civilization might be periodically visiting Earth? Surveys consistently show that many people are not only intrigued by this possiblity, but moreover are convinced of its veracity.
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Earthly Emissaries by Seth Shostak, Senior Astronomer, Project Phoenix
When it comes to alien activities, visiting Earth seems to be pretty high on the "to do" list. But does that make sense?
 | Water (Dihydrogen Oxide, H2O) is a truly remarkable chemical compound and is fundamental to life on Earth.
Credit: NASA |
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 let's 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. That's 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 month's 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?
 | "Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck." -David Grinspoon
Image Credit: NASA |
If it's only once in a few tens of millions of years, we're 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!
 | The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
Credit: NASA |
We may send the occasional anthropological research team to Borneo, but we don't send millions. And it's 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.
Let's 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?
Related Web Pages SETI Institute
NSF Life in Extreme Environments (LEXEN) Program
Introduction to the Archaea - Life's extremists
The Envelope of Life? Please
Defining Life
Life from Scratch?
Biodiversity: Interview with Andrew Knoll Part I
Extinctions: Interview with Andrew Knoll Part II
Genetic Engineering and Human Intervention: Part III
Note: Messages
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