Often when we think of aliens, we think of superintelligent beings far more advanced than ourselves. Since 1961 scientists have estimated the number of such advanced creatures using the famous Drake equation (see box at right). Frank Drake, who is now chairman of the Board of Trustees of the Search for Extraterrestrial Intelligence (SETI) Institute, came up with the equation to answer his own question: "What do we need to know about to discover life in space?" He assumed these other life-forms would necessarily possess the intelligence and technology to "broadcast" their existence to the universe.

But what if most aliens didn't have such smarts? What then would we need to know to find them? In fact, many scientists are now suggesting that when we discover extraterrestrial lifeif we doit won't resemble the cunning eight-eyed rivals of Star Trek episodes. Instead, they say, it will most likely come in the form of tiny microbes. With that in mind, astrobiologists have for some time carefully examined microbial life in extreme environments here on Earth. And now scientists at NASA's Jet Propulsion Lab (JPL) are working on ways to search for other types of microbial life that are "non-Earthcentric."

"It's a simple-sounding thing, but it's hard as heck to do," says Kenneth Nealson, a microbiologist who heads up the astrobiology unit at JPL. "Could we detect life if you don't get to use RNA, DNA or protein, or all the things people use to find it?" The answer isn't clear.

Nealson and his team are trying to "figure out how to frame the search for life if you didn't know what it was." So-called biosignatures, they say, will be key to finding places that may hold life and deserve further study. Those signatures could include interesting chemical compositions, meaningful structures or movements, signs of replication, or notable patterns of energy.

Chemical composition is a tough signature to find, Nealson says. Researchers don't know precisely what chemicals might be needed to create and support life. Some assume that carbon and water are likely candidates elsewhere in the universe, as they are herea belief that has fueled the excitement over detecting water on Mars and Europa, Jupiters icy moon. But not everyone agrees. Scientists have at least ruled out some elements: they say that life will not be silica-based because the bonding patterns are too weak. Carbon atoms, in contrast, form sturdy double bonds with one another.

An easier signature to recognize may well be distinctive flows of energy. Any form of life would necessarily create some kind of energy imbalance in its use of chemicals for fuel and in the resulting metabolized products, Nealson notes. And many life-forms on Earth have developed catalysts that allow them to get a jump on the normal chemical breakdowns in their environment. In other worlds, this acceleration of chemical-element interactions should stand out against everyday chemical reactions, such as the oxidation of minerals.

Writing in the first volume of a new journal called Astrobiology, Nealson and co-author Pamela Conrad, also at JPL, mapped out the beginnings of the technical path for tracking such energy flows, as well as other possible signs of life. They note that on-site laboratory methodsincluding atom probes and portable x-ray fluorescence or mass spectrometersoffer one way to start looking for signals on planets and other satellites close to home, among them Mars and Europa.

"The insight they have that may make a whole lot of sense on Mars is that you want to be able to scan quickly a lot of surface area and pick out those areas with some signature that says, 'Here is something potentially interesting,'" SETI researcher Christopher Chyba says. "It allows you to ignore the rest of the sample. You can't scan a square meter of rock looking for a bug."

Nealson and his team suggest that, through testing on Earth, scientists must first build a biosignature database of sorts. "We want to show that you would not miss life here using those methods," Nealson says. Such testing would also require taking measurements in a "dead" environment, such as the moon, to ensure that their approach didn't detect life where it wasn't. In the end, Chyba says, "it's a question of suspicion, not of proof.