About 2,000 planets have been discovered to date, making the possibility of the existence of intelligent alien life seem more probable than ever before. Should we just listen and see if we hear evidence of other civilizations or should we wave our arms and shout, "We're here!"? This is the debate going on in the astronomy community.
We have tried the first approach since 1960. Known as passive SETI (Search for Extraterrestrial Life) it's been a big bust. We haven't heard a peep from aliens. Focusing on planets around sun-like stars, passive SETI involves searching for radio or optical signals coming from them. It means we have to know all the natural sources of these emissions, so we can identify any that seem to come from technology-based sources. Active SETI uses radio transmitters and lasers to send information to where we know there are exoplanets or to stars we suspect have orbiting planets, hoping that an alien civilization will notice us.
Active SETI is also referred to as METI (Messaging to Extraterrestrial Intelligence) and would include such efforts as the plaque that was placed on the Pioneer spacecraft with the hope that it would someday be found by intelligent beings outside our solar system.
In the 1960's, the Soviet Union dominated SETI efforts. The United States became more involved during the 70's. SETI programs were established at NASA's Ames Research Center and at the Jet Propulsion Laboratory (JPL) in Pasadena, California. Each took a different approach, with Ames examining sun-like stars, and JPL sweeping the sky in all directions. In 1988, after a decade of study NASA formally adopted this strategy, and funded the program, with observations beginning four years later. But, within a year, Congress ended the funding. However, there are some long-term SETI projects still going on, including one operated by the University of California, Berkeley and one by the SETI Australia Center at the University of Western Sydney.
Arguments against passive SETI, of course, exist. Some people simply think it's a waste of time and money, others thing some of the approaches used are wrong or that's in exercise in futility because even if evidence of intelligent life is found, the tremendous distances involved make communication or travel between civilizations impractical.
Although passive SETI has its detractors, whether or not to practice active SETI is extremely controversial among astronomers. Seth Shostak, senior astronomer at the SETI Institute in Mountain View, California, told Space.com, "It's a subject of discussion, I'll put it that way. There have been many workshops and symposia over the years to discuss Active SETI, and because it has a highly emotional component, it's like a third rail in a way,"
The pro-active SETI community argues that Earth is already very visible by sending signals to space via routine television broadcasts, military radar, etc. But, others argue that this is not really true, that, the signals dissipate as they travel, and that their presence is very different than targeting strong beams to specific planets and solar systems.
The anti-active SETI community argues that announcing our existence is dangerous, that any civilization that can receive and comprehend our messages is likely more technologically advanced than us. And, they point out that from our experiences here on Earth, good things don't often happen when more technically advanced civilizations meet less advanced ones (Europeans and Native Americans for example) or when "superior" species meet "inferior" species. (Think about the many now extinct plants and animals.)
Even a strong passive SETI supporter like Carl Sagan thought it was unwise to broadcast our existence to a universe that we really know little about. Rather, he thought we should take our time to listen and discuss what we learn before reaching out.
Weighing in on the argument is Steven Dick, the Baruch S. Blumberg NASA/Library of Congress Chair in Astrobiology at the Kluge Center in Washington, DC. He told Space.com, "I am for passive SETI programs, and in fact would advocate for renewed government funding after a 20-year lapse. That's because the existence of extraterrestrial intelligence is one of the great unsolved mysteries of science. On the other hand, I would not propose government funding for messaging extraterrestrial intelligence. I think we need to find ET first, and then have a period where a team consisting of scientists, social scientists and humanities people consider what the message should be."
Although there are no known active SETI projects being conducted by governments or major observatories, there are groups that have expressed interest in conducting active SETI. As technology advances, the active vs. passive argument may become moot as the capability to perform active SETI becomes more widespread. Even very wealthy individuals with an interest in space could launch an effort.
So what do you think? If you could initiate a "close encounter", would you?