I doubt very much that extraterrestrials visit our planet often, if ever. Interstellar (beyond a solar system) travel is generally prohibitively expensive as far as our physics can project. But signals or emissions patterns leak out of possibly billions of advanced civilizations into the universe. From us, through T.V. signals, Hitler's unfortunate 1936 Olympics Games speech, "I Love Lucy," and "Howdy Doody" have already broadcasted their electromagnetic patterns many light years beyond Earth, possibly within detection-range of a listening civilization in orbit around another star. So, if they do visit us, don't be too surprised by an enthusiastic, "Lucy!! I'm home!" as their greeting. E.T. might presume it customary.
In 1960, long before we dreamed of the Hubble Telescope or the Kepler Mission, a scientist named Frank Drake created a mathematical model to determine the most likely number of "technologically advanced" civilizations in our galaxy. His theory is centered on the eponymous "Drake Equation" which would provide a mathematically definite answer, if only we knew the required pieces of information. Some of this information is how many stars there are, how many stars have planets, and how many of those planets are suitable for life to evolve. For most of five decades, the calculations and solutions to Drake's "probability of aliens" have been mostly speculative without much data. Estimates based on rational probability have suggested "optimism" that the sky is a-twitter with noisy who-knows-how-little-and-why-green-men transmissions. (See: http://en.wikipedia.org/wiki/Drake_equation )
SETI (the Search for Extra-Terrestrial Intelligence) is a project to detect those "alien" signals. ( http://en.wikipedia.org/wiki/SETI ). You might not know this, but it is true, that we are actually dedicating some of our most powerful supercomputing grids to looking for unnatural patterns in the radio telescope data from beyond our galaxy. We could "hear" them any day.
Now, 49 years after Drake asked the question, NASA's Kepler Mission is poised to provide some answers. For the first time ever, we will be able to look out into a distance into the Milky Way Galaxy and find planets that look a lot like our own Earth. A recent NASA press release states:
"This early result shows the Kepler detection system is performing right on the mark," said David Koch, deputy principal investigator of NASA's Ames Research Center at Moffett Field, Calif. "It bodes well for Kepler's prospects to be able to detect Earth-size planets."
SETI and astrobiology expect at least two areas of important results from this exciting project.
"Optimists" who anticipate many galactic civilizations have successfully argued from a probability standpoint that they are correct. Kepler will make the guessers obsolete with hard, cold data. Watch for "Loads of Earth-like Planets" headlines quite soon, unless we "optimists" are wrong.
And secondly, scientists have been forced to listen at the sky for E.T. signals at more or less random directions because they didn't really know where to look. When Kepler zooms it's "sharp eye" on several relatively near Earth-like planets, we'll know right where to listen.
My gut wager: headlines of "Scientists Detect Definitive Intelligent Extra-Terrestrial Signal" will be confirmed before the kids get much older.
Hat tip to Carl Sagan. He has shaped my thinking on our place in the Cosmos with his brilliance. He shined so brightly among we Star-Stuff.
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