The discovery of about 100 extrasolar planets over the past decade has placed a momentous task on the scientific agenda: finding planets that could harbor life. Most of the newly discovered planets are gas giants that orbit close to their stars. They're broiling hot, and probably dead. The job of Terrestrial Planet Finder (TPF) is to find "terrestrial" (Earthlike) planets, and then to scan them for biosignatures - chemical signs of life.

Reading biosignatures requires knowing how life would change a planet. But Earth is the only planet that we know for certain is a living world. Although Earth now glows with a distinctive biosignature of oxygen and methane, it has not always done so. To be comprehensive, TPF must be able to search for far more than just present-day terrestrial conditions. And thus its search algorithm and technology rest on understanding how life has affected our planet's atmosphere over the entire history of life on Earth, from the time (some 3 to 4 billion years ago) when life first emerged until the present.

When launched in 10 to 15 years, TPF will study many different aspects of planets beyond the solar system, including their formation, abundance, locations, and suitability for life. TPF will focus on stars that could have planetary systems, says James Kasting, a professor of geoscience at Penn State who has worked on the biosignature issue for the mission. "The baseline plan is look out 40 to 50 light years [about 250 to 300 trillion miles], where there are several thousand stars. Within that range are about 120 single solar-type stars that we think are most likely to harbor Earthlike planets."

To be of interest, a planet must have the fundamental requirements of familiar life - warmth, energy and liquid water - because nobody can figure out how to recognize the effects of unforeseeable life forms. "The best we can do is assume that life requires the kind of chemistry that we understand on Earth: carbon chemistry dependent on liquid water," says spectroscopist Wesley Traub of the Smithsonian Center for Astrophysics. "I have resigned myself to look for planets that look just like the past or present Earth."

TPF's telescope will block light from a planet's star, then perform a spectroscopic analysis on the faint light reflecting from the planet's surface. The instrument will use absorption spectroscopy to measure the identities and abundances of gas molecules in the planet's atmosphere that block specific frequencies of light reflected by the planet.

As mission planners move toward a decision on whether to build a spectroscope that will detect infrared or visible light, they have focused on four gases that are found in Earth's atmosphere and linked to life:

• Water vapor A baseline sign, indicating the presence of liquid water, a requirement of known life.
• Carbon dioxide Can be created by biological and non-biological processes. Because it is necessary for photosynthesis, it would indicate the possible presence of green plants.
• Methane Considered suggestive of life, it also can be made both by biological and non-biological processes.
• Molecular oxygen (O2) - or its proxy, ozone (O3). The most reliable indicator of the presence of life, but still not conclusive.

A Discussion of Gas

Unless molecular oxygen in the atmosphere is constantly replenished by photosynthesis, it is quickly consumed in chemical reactions, in the atmosphere, on land and in seawater. So the presence of a large amount of oxygen in an extrasolar planet's atmosphere would be a sign that it might host an ecosystem like present-day Earth's.

Measuring oxygen may also prove useful, indirectly, in detecting living planets that resemble Earth before the rise of photosynthesis.

"We can tell the ages of stars to moderate precision," says Neville Woolf, a telescope builder at the University of Arizona who has advised the TPF project, "and so if TPF were to find that younger stars had planets without oxygen, and older stars had planets with oxygen, we would think that life had probably started on the younger planets, but not yet taken sufficient control of the environment as to be visible."

An additional oxygen-related biosignature is the possibility of detecting green plants that make oxygen. Chlorophyll reflects near-infrared light very strongly, a phenomenon known as the "red edge" because the light is just beyond the range of colors human eyes can see. (If humans could see the red edge, plants would look red instead of green.) Near-infrared cameras would have no trouble picking up this distinctive signal.

Although methane is often biogenic, detecting it on a distant world would not automatically indicate the presence of life. Jupiter and Saturn, for example, have traces of it. Methane, Traub points out, is "produced easily in primitive solar nebula. There is a huge amount of methane floating around in the universe." Moreover, TPF probably could not see methane at Earth's present concentration, 1.6 parts per million (ppm), since its spectroscopic lines overlap those of water.

However, methane levels around 1,000 ppm may have occurred on Earth between 2.3 and 3 billion years ago, produced as a waste byproduct by primitive microorganisms called methanogens. This strong a methane signal would probably be visible to a TPF-style detector, Kasting says, who adds that methane would be a "suggestive but not convincing" biosignature.

Finding oxygen along with methane might constitute the most convincing biosignature. "On Earth, we have loads of oxygen, so you should never see any methane, it would all be oxidized by oxygen, to form water and carbon dioxide," says Traub, who has worked on the biosignature issue. "The fact that we do see methane in Earth's atmosphere means it's not in equilibrium. That methane has to be continually produced." And life, in the form of methanogenic bacteria and photosynthetic plants, is the most likely source of the two gases.

In the 1960s, Kasting adds, British biologist James Lovelock proposed that the simultaneous presence of oxygen and a reduced gas such as methane would be a convincing indication of life. "Nothing is totally unambiguous," says Kasting, "but this is the best test for life."

However, many living worlds may never reach the stage of having much oxygen in the atmosphere. On Earth, to take the only example we know of, cyanobacteria did not start starting cranking out oxygen until some time before 2.3 billion ago. Primitive life on terrestrial planets may not have had time to progress beyond this stage, or may be snuffed before photosynthesis arises. How to find such worlds?

What's Next

As TPF planners move toward a decision on whether to build a spectroscope that will measure visible or infrared light, their ability to distinguish a wide variety of biosignatures depends on improving our picture of how early life changed Earth billions of years ago.

One possibility now getting some attention is to search for methanethiol (CH3SH), which is produced during the decay of biological material. This compound, also called methyl mercaptan, is created through the degradation of the amino acid methionine," says Carl Pilcher of the Astronomy and Physics Division at NASA headquarters. Since all life today uses the same 20 amino acids, he says, "There is every reason to think methionine was used by early life, and every reason to think the same process of degradation was going on then."

While Pilcher acknowledges that methanethiol is simply an early-stage proposal that may not pan out, it would address a difficult problem facing TPF. "We certainly understand what to look for once there is oxygen in the atmosphere - you look for oxygen or ozone, and traces of reduced gases like methane that are way out of equilibrium with the oxygenated atmosphere. What you look for in the early atmosphere is really the challenge."

Astrobiologists advising the TPF project face a basic limitation - nobody can predict the nature of extraterrestrial life. Woolf points out that Earth's subterranean biosphere, which by some estimates contains half of the entire planet's biomass, was essentially unknown a decade or two ago. In designing TPF, he says, "We cannot look for generalized life on other worlds. We can only look for those particular forms of life that produce signatures that are unlikely to arise from any other process."

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