Expectations are running high that some time next year astronomers using NASA's Kepler spacecraft will announce the discovery that planet hunters have been waiting for: the first Earth-size exoplanet found in a region around a sunlike star where life could flourish. That exoplanet will almost certainly lie too far from Earth to be scrutinized, but it will nonetheless throw into high gear a search for the fingerprints of life—the chemical compounds that could indicate whether an exoplanet in the habitable zone, the life-friendly region where liquid water can survive, actually harbors life.
But even as researchers are gaining a deeper understanding of the bio-signatures that may be present in exoplanetary atmospheres, scientists face a roadblock. A proposed NASA mission called the Terrestrial Planet Finder (TPF), designed to search for these compounds among planets orbiting nearby stars—those that lie about one hundredth the distance of the orbs Kepler can find—lost its funding in 2007 amid rising costs for the James Webb Space Telescope, Hubble's successor.
The TPF would block the blinding glare from nearby, sunlike stars in order to take portraits of the planets that orbit them. In one scheme, a single large telescope outfitted with a mask, or coronagraph, would blot out starlight and image planets as they appear in reflected visible light. In another strategy, several telescopes flying in information would act in concert to zero out infrared light from a parent star and record the heat radiated by the star's planets at infrared wavelengths.
Light collected by the TPF and separated into its component wavelengths, or spectra, could reveal the presence of bio-signatures. Water vapor, oxygen and methane in the atmosphere of an exoplanet would offer evidence of a life-friendly environment as well as biological processes akin to photosynthesis and respiration on Earth, notes Geoff Marcy of the University of California, Berkeley.
"The galaxy may be lousy with microbial life, but currently we have no clue," he adds. "It is a tragedy of modern science that the Terrestrial Planet Finder cannot be supported, either in the U.S. or Europe, due to budget pressures."
Astronomers still hope to revive some version of TPF, but it would take a decade for the mission to get back on track, Marcy estimates. In the meantime studies by exoplanet researchers including Sara Seager of the Massachusetts Institute of Technology and Victoria Meadows of the University of Washington in Seattle are honing—and expanding—the list of compounds that may serve as biomarkers for exoplanets orbiting stars of different sizes and ages.
With the chances of looking for chemical markers of life beyond the solar system initially few and far between, "we want to make sure we have the best possible understanding of bio-signatures," Meadows says. "We don't want to be fooled."
Much of the new work focuses on planets orbiting M dwarf stars, which are about one-half to one-tenth the sun's mass and account for about 75 percent of all the stars in the galaxy. Because M dwarfs are much cooler than the sun, their habitable zones are only about one tenth as far from them as Earth lies from the sun.
That proximity makes it impossible for the TPF to image those planets. However, the James Webb Space Telescope, now scheduled for launch in 2018, has a chance of examining the atmospheres of a handful of these bodies. So might a new generation of extremely large ground-based telescopes, with mirrors of 30 meters or more, that have recently been proposed.
Some of the exoplanets these telescopes will attempt to study have a rare alignment. Like the more distant exoplanets identified by Kepler, they regularly pass in front of, or transit, their parent stars as seen by the detectors. During a transit, starlight filters through an exoplanet's atmosphere, with each chemical constituent leaving its own imprint on the light. The signal is extremely faint but planets in the habitable zone of M stars make frequent transits, enabling astronomers to accumulate individual observations to make a stronger detection. "The habitable zone of M stars are the first places that we can look for bio-signatures," Seager says.