The search for life on planets orbiting distant stars is picking up speed. Last month, NASA gave the go-ahead to the Kepler space telescope, which will try to infer the presence of Earthlike planets orbiting other stars. Launch is set for 2006. A few years later, NASA’s Terrestrial Planet Finder (TPF) and the European Space Agency’s Darwin project will attempt to glimpse these planets directly. The pictures won’t be much to look at—just a single bright pixel. Yet the range of color present in that tiny speck of light will reveal the gases that make up the planet’s atmosphere.
(The Hubble Space Telescope recently made the first identification of gases in an extrasolar planet’s atmosphere, using slightly different means.)

Oxygen in a planet’s atmosphere would almost surely point to life. The gas is readily absorbed by minerals if there aren’t biological processes (primarily photosynthesis) pumping it back into the atmosphere. But even Earth’s atmosphere didn’t contain much oxygen until about 2.2 billion years ago, whereas life arose much earlier, around 3.9 billion years ago. As NASA astrobiologists have put it, if we saw the early Earth, would we recognize it? Astrobiologists will search primarily for oxygen, ozone, carbon dioxide, methane, and water vapor. But alien life might not discharge much gas into an atmosphere because it lives beneath the planet’s surface. (Many astrobiologists suspect that this is the case on Jupiter’s moon Europa.) Or perhaps the chemistry of life on distant planets will be so unfamiliar that we’ll overlook its traces. Finding out which gases point to the presence of life is one of astrobiology’s major objectives.

Eventually the proposed Planet Imager spacecraft may give us a very blurry image of a planet. But ultimately the only way to confirm the presence of life on a planet is to go there. While an interstellar probe is a long way off, concepts for propulsion systems that can make the trip are already on the drawing board.