"Are we alone in the universe?" Short of receiving a convenient radio transmission from another civilization, how can we find out if a distant world harbors some form of life?
The discovery of more than 70 planets outside our solar system within less than a decade has brought a new sense of immediacy to the search for life. Scientists believe our best bet might be to build instruments capable of detecting life's chemical signatures, called biosignatures, or biomarkers
Terrestrial Planet Finder, a mission managed by JPL for NASA's Origins program, will be among the first generation of instruments capable of searching for the atmospheric "life signs" of habitable, or even inhabited, planets.
Terrestrial Planet Finder, scheduled for launch in 2014, will deploy revolutionary technologies to block the blinding glare of a star. By doing this, scientists will be able to detect planets as small as Earth, which are considered better prospects for life than the large planets detectable with current technology.
The closest planetary systems are many light years away, but the faint light the planets emit, if separated into its component frequencies, can provide a wealth of information. By analyzing the colors of infrared radiation detected by Terrestrial Planet Finder, astronomers can search for atmospheric gases such as carbon dioxide, water vapor and ozone.
The best candidates for closer study would be located in the habitable zone, the region around the system's star where we can expect to find liquid water, which is considered a prerequisite for life. If the planet is too hot, the water evaporates. If the planet is too cold, the water freezes. Earth is inside the habitable zone for our star, the Sun; the zone starts beyond Venus and ends before Mars.
Among the most reliable biomarkers we might find is oxygen -- a byproduct of photosynthesis on Earth. Oxygen molecules don't linger in the atmosphere, but combine with other molecular types in a process known as oxidation. An even more valuable biomarker is ozone, a form of oxygen that's easier to detect by analyzing the wavelengths of light. So, a planet with an atmosphere rich in oxygen or ozone implies the presence of a source to keep it replenished -- in other words, life, right?
Not so fast, says James F. Kasting of Pennsylvania State University, a member of the Terrestrial Planet Finder science working group.
"We know of non-biological processes that can also result in an oxygen-rich atmosphere," Kasting said. "The runaway greenhouse effect on Venus is one example. A frozen, Mars-like planet big enough to hold its oxygen would be another."
Still, the presence of ozone would at least suggest we're "getting warm" in the search for life. What additional clues could we look for?
The most persuasive indicator of life, Kasting says, would be the simultaneous presence of oxygen or ozone, along with another chemical such as methane or nitrous oxide.
These gases are more abundant than we might expect in Earth's atmosphere. They are present because they, too, are produced by organisms. Methane comes from a type of bacteria that lives in soils without oxygen, such as rice paddies, and in the intestines of cows and sheep. Nitrous oxide comes from a type of bacteria in the ocean and in soils without oxygen.
James Lovelock, a British scientist who has written numerous books on the "Gaia Hypothesis" -- the theory that life controls atmospheric composition and climate -- suggested more than 30 years ago that the simultaneous presence of oxygen and a reduced gas like nitrous oxide or methane would be strong evidence for life. This advice is still considered good today, Kasting says.
In any case, the large-scale chemical clues won't tell us about the complexity of the discovered life; it could be either algae or a developed civilization.
It's possible that planets without oxygen could sustain life as well. Photosynthesis might conceivably occur with another element, such as sulfur, playing the role of oxygen. In the search for life, scientists acknowledge, we must control our assumptions of just what it means to be living.