Unfortunately, the debate has taken place on a very slanted playing field, at least in the United States.
Among astronomers, the pro-ETI forces have gained the power to effectively squash dissenting opinion in recent years. This power is exemplified by the "gatekeepers" at popular and even some refereed science journals, and by the successful public relations of the SETI institute (see, for example, the movie Contact).
While I was a "believer" in ETI most of my life, I changed my mind about 10 years ago; after I thought carefully about the astronomical and geophysical requirements for advanced life in the universe. I found that ETI proponents were ignoring basic constraints such as high levels of radiation in many regions of the universe.
These proponents use the Drake Equation (invented by astronomer Frank Drake) to estimate the number of civilizations in the galaxy. But the equation is laden with optimistic assumptions and virtually useless. In the early 1970s, astronomers led by Drake sent a radio message to the M 13 star cluster--not recognizing that the cluster's shortage of heavy elements makes it an unlikely abode for radio listeners.
I kept my opinions about all this to myself until July 16, 1997, when a number of news events relating to ETI came together in a cosmic (or is that comic?) conspiracy: the enormous popularity of two alien flicks, Men in Black and Independence Day; the August 1996 "discovery" of remnants of life in a Martian meteorite and the 50th anniversary of the "discovery" of an alien spacecraft near Roswell, New Mexico.
Having reached the breaking point, I fired off a letter to the Wall Street Journal--something I have never previously, nor since attempted. To my surprise, they published it on their op-ed page, with the title "Nobody Here but Us Earthlings."
While I was a 'believer' in extraterrestrial intelligence most of my life, I changed my mind about 10 years ago.
My Journal article led to two radio interviews and an invitation to publish a longer article in Society, a sociology journal. What I learned from the experience is that it's extremely difficult to get valid dissenting scientific opinion published in the usual places, especially if it goes against public opinion as well.
I've also found that the debate on ETI is not so strongly one-sided overseas. The British semi-technical journal Astronomy & Geophysics published a piece I wrote in 1998 critical of the pro-ETI position. They have a long history of giving a balanced platform (originally known as the Quarterly Journal of the Royal Astronomical Society) for intellectual debate on the ETI question; this has no counterpart in the U.S.
Perhaps the single most influential criticism of the pro-ETI position in the U.S. was just published in book form--Rare Earth. It is by two of my colleagues at the University of Washington, Peter Ward and Donald Brownlee; we met often during the last couple of years while the book was being put together to discuss astronomical constraints on advanced life.
The book's thesis is that advanced life may be extremely rare due to: 1) the many potential hazards in the universe and 2) the stringent requirements for its existence. The book's growing success shows that a significant fraction of the public wants a real debate on the subject, not one-sided propaganda from the astronomy establishment.
Guillermo Gonzalez is Assistant Professor of Astronomy at Iowa State University and Fellow at Discovery Institute. His current research in astrobiology focuses on the "Galactic Habitable Zone" and captured the October 2001 cover story of Scientific American. Another focus of his research analyzes and interprets ground-based photometric and spectroscopic observations of low and intermediate mass stars in relation to current theories concerning the late stages of stellar evolution and the formation and evolution of planetary systems. Dr. Gonzalez has published over sixty peer-reviewed astronomy and astrophysics articles. He has co-authored The Privileged Planet with cosmologist Jay Richards.