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If “E.T.” is out there, whether in the form of intelligent beings or much simpler organisms, we may soon be hot on its trail. For the first time in history, the dream of searching for signs of life in other solar systems belongs not only on the philosopher’s wish list, but on the list of doable and planned human endeavors.

Momentum is gaining rapidly. Only 6 years ago, the first planet around another Sun-like star was discovered by scientists using Doppler Detection — a method that reveals Saturn-sized (or larger) planets close to their parent suns. Today, we know of more than 80 candidates for such worlds, and more are being found all the time.

Scientists crossed a new frontier in “exo-planet” research just last year when, using the Hubble Space Telescope, they detected sodium in the atmosphere of a large alien world orbiting the star HD 209458. The Hubble data not only revealed that exo-planets have atmospheres, but also that we can analyze them from a distance. For the first time, humans are discovering and exploring worlds outside the solar system.

So far, all known extra-solar planets are gas giants — unlikely abodes for life as we know it. In fact, some are so large that they might not be planets at all, but a kind of failed star called a “brown dwarf.” Of greater interest are Earth-size planets, which are too small for even the Hubble Space Telescope to detect. Nevertheless, astronomers believe they exist.

Enter Kepler, a space telescope approved recently by NASA.

Beginning in 2006, Kepler will monitor about 100,000 nearby stars, searching for the slight dimming that occurs when an orbiting planet blocks some of the parent star’s light. Because Kepler will be sensitive enough to detect planets as small as Earth, this celestial survey will give scientists an idea of how common Earth-like planets are — and identify candidates for further study.

If all goes as planned, an important new tool for exploring such planets will be operating by the end of this decade. Called the Terrestrial Planet Finder (TPF), this space telescope will use a technique called “interferometry” to dramatically reduce the obscuring glare from the planet’s parent star, allowing scientists to see the planet itself. The Web site for the European Space Agency’s similar Darwin project notes that, “Looking for planets around nearby stars is like trying to discern, from a vantage point 1000 km away, the feeble light from a candle next to a lighthouse.”

It is indeed a daunting challenge, but worth the effort. The goal is nothing short of finding worlds that could support life — and perhaps some that do.

Seeing E.T.‘s “breath”

Planets circling other stars are many light years away. (A light-year is the distance that light travels in a year — about 9.5 trillion km.) Even with the TPF’s advanced optics, Earth-like worlds would appear as a single pixel of light. How, then, will it be possible to learn much about them?

Amazingly, that tiny speck of light can speak volumes about the planet from which it came. Embedded in that light are the “fingerprints” of the chemicals that have touched it, including the gases in the planet’s atmosphere. By splitting the light into its component frequencies — which for visible light creates the familiar rainbow of colors — scientists can reveal these “fingerprints” and learn about the chemistry of the planet’s atmosphere.

If life is widespread on a planet, its atmosphere should show signs of life’s presence. Just as the air you exhale has more carbon dioxide and less oxygen than the air you inhale, the combined “breathing” of all the life on a planet will change the chemistry of its atmosphere. If life is plentiful on the planet, these changes may be large enough to notice.

A simple premise — but what would E.T.‘s breath look like? Which gases should we search for? We know the answers for our own world, but predicting how an alien biology might interact with its atmosphere is no simple matter.

“As astrobiologists we’ve got to be sure that we’re not too Earth-centric,” says Michael Meyer, senior scientist for astrobiology at NASA Headquarters in Washington, D.C.

The possibility that life elsewhere has a biology that’s radically different from our own is perhaps the most exciting and challenging part of astrobiology (not to mention a ubiquitous theme of science fiction). If life evolves by random mutations and natural selection, why should we expect alien life forms to be even remotely similar to Earthly life?

“We have to be very careful about how foreign biology might be different from our own, especially when you get to the bigger molecules” such as DNA, says David Des Marais, principal investigator for the Ames Research Center team of NASA’s Astrobiology Institute.

For example, people have speculated that silicon, a primary component of sand and a close cousin to carbon, could form the basis of an extraterrestrial biology. Alien life might forgo sunlight and depend instead on the geothermal energy in hydrogen and sulfur compounds emitted from the planet’s interior, much like the deep-sea vent ecosystems here on Earth. Or maybe the chemistry of alien life will be utterly different and unimaginable.

Fortunately, the chemical constraints within which life must function make it likely that simple molecules such as oxygen and carbon dioxide will play the same roles in an extraterrestrial biology as they do on Earth.

“Suppose,” says Meyer, “that there is silicon-based life. [It might be] photosynthetic, and you would still end up with oxygen in the atmosphere. You could go there and the life could be completely different, but some of the chemistry could still be the same [as on Earth].”

“The small molecules are going to be more universal,” agrees Des Marais. “Large molecules like DNA and chlorophyll represent later, highly significant innovations of life on Earth, but also the ones that may have differed elsewhere.”

For this and other reasons, the exploration of distant Earth-like planets with TPF will focus on simple gases such as oxygen, ozone, carbon dioxide, methane, and, of course, water vapor.

Oxygen and its tag-along cousin ozone will top the list of target molecules.

Without life, oxygen should be rare on rocky worlds. A small amount of it can be created without life by ultraviolet radiation that splits water vapor into hydrogen and oxygen. But that oxygen would be readily consumed by rocks and minerals on the planet’s surface in the “oxidizing” reactions that produce, for example, rust. Volcanic gases also react with oxygen and remove it from the atmosphere. Geological processes alone usually work against the accumulation of oxygen.

An oxygen-rich atmosphere is, therefore, out of chemical equilibrium, suggesting that some active agent — namely photosynthetic life — is constantly replenishing the supply. As Carl Sagan noted in a 1997 Scientific American article, “the great concentration of oxygen (20 percent) in Earth’s dense atmosphere is very hard to explain by [any means other than life.]” The same would be true of planets around other stars.

Next on the list of target molecules is methane. Scientists suspect that for roughly the first billion years of its history, life on Earth had not yet evolved oxygen-producing photosynthesis. Instead, the microorganisms that then dominated the planet tapped the energy in gases leaking out of the Earth’s interior, with some microbes creating methane as a byproduct.

On a planet with a similar geology to Earth, methane levels greater than about 100 parts per million would suggest the presence of life. But methane would be a more ambiguous discovery than oxygen, because planets of a different geological make-up might produce abundant methane without life.

Other details about these planets — such as their size, their distance from the parent star, their carbon dioxide and water vapor levels, and their reflectivity — will help scientists accurately interpret a methane or oxygen discovery. These other details can also be measured (or at least estimated) using TPF and other telescopes.

Some of these ideas have already been tested on a planet known to support life — Earth.

In 1990, the Galileo spacecraft made a flyby of our planet on its circuitous journey to Jupiter. As it passed, Galileo’s sensors detected high levels of oxygen and methane in Earth’s atmosphere and signs of chlorophyll on the ground. Chlorophyll, which absorbs light at the far-red end of the visible spectrum, is a “red flag” for life. The TPF won’t be sensitive to chlorophyll on a planet’s surface because atmospheric water vapor, which is opaque in the mid-infrared frequency range that TPF will observe, will hide the surface below. Even without chlorophyll, signs of oxygen and methane — which TPF can detect — would make a persuasive case for life.

If the TPF finds a habitable planet with lots of oxygen and some methane in its atmosphere, it would be a momentous discovery. But would such data really prove life is there? “Proof” is always a tall order in science, especially when discussing extraterrestrial life. Nevertheless, say astrobiologists, such evidence would be “very compelling.”

One day it might happen … and after catching its first whiff of E.T.‘s breath, humanity won’t likely give up the chase. The next step would be an even larger space telescope that could see the planet as more than one pixel, allowing scientists to see surface features such as continents and to observe the changing of the planet’s seasons. And perhaps by the end of the next decade it would be possible to send a probe across interstellar space to take a close-up look, which could finally provide incontrovertible evidence.

Proof will be for the patient: Even using advanced propulsion technologies, a probe might take decades to reach a neighboring star. But to answer a profound question that’s been asked by humanity for centuries, perhaps that isn’t too long to wait.