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.
Simulations of Earthlike planets by Meadows and her colleagues over the past several years have revealed that M dwarfs may better preserve some of the fragile biomarkers that are easily destroyed by the radiation of more massive stars. Consider the simultaneous presence of high abundances of methane and ozone, which researchers first proposed in 1965 as a strong indicator of life. Only biological activity is capable of continually maintaining high levels of the two compounds, which readily react with each other and deplete the original supply.
M dwarfs produce much less near-ultraviolet radiation—which breaks ozone molecules into atomic oxygen and OH and hastens the destruction of methane—than sunlike stars do. As a result, methane would last about 20 times longer (about 200 years) and would have a predicted concentration 200 times greater on an Earthlike planet in the habitable zone around an M dwarf than the same planet in the habitable zone around the sun, Meadows and her collaborators calculate.
Similarly, two other earthly bio-signatures—methyl chloride and nitrous oxide—may be more prevalent and easier to detect on terrestrial planets circling M dwarfs, Meadows says.
M stars, K dwarfs and beyond
No survey has yet identified an Earth-size planet in the habitable zone around an M star, and a space mission is needed to conduct a thorough search, says Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the Max Planck Institute for Astronomy in Heidelberg, Germany. One proposed mission, the Transiting Exoplanet Survey Satellite, would scan the entire sky for Earth-size and larger exoplanets around M stars as well as slightly more massive stars called K dwarfs. Last year the project, led by George Ricker of M.I.T., received a $1-million grant from NASA for further study.
Thinking beyond M stars—Seager and Meadows have also expanded the list of possible bio-signatures. In the January Astrobiology, Seager, Matthew Schrenk of East Carolina University in Greenville, N.C., and William Bains of Cambridge, England–based consultant firm Rufus Scientific note that most studies that examine possible bio-signature gases limit their scope to ozone or oxygen, methane and nitrous oxide. These compounds are not only the main signs of life on Earth but are the direct product of chemical reactions that generate the energy and structural components of life on the planet. Microorganisms on Earth, however, produce a much broader range of gases that Seager and her colleagues label secondary by-products and which are generated for unknown reasons and may be specific to particular species. One terrestrial example is dimethyl sulfide, produced by marine phytoplankton.
Although these secondary by-products only occur in small concentrations on Earth, they could be a dominant bio-signature on other types of habitable exoplanets. The ideas are still preliminary, but Seager and her collaborators suggest that high concentrations of unusual or complex molecules in the atmosphere of an exoplanet could be a new type of bio-signature.
In the June 2011 Astrobiology, Meadows and her collaborators also broaden the scope of possible bio-signatures. Motivated by evidence that single-cell bacteria thrived on the early Earth well before oxygen dominated the planet's atmosphere, the team simulated the search for signs of life on oxygen-poor exoplanets. Their work revealed that sulfur gases were produced by organisms in such environments, but that these gases did not build up in the atmospheres of exoplanets. Instead, the sulfur compounds were destroyed in a series of reactions that ultimately produced ethane. High ethane concentrations therefore should be added to the roster of compounds that indicate biological activity, Meadows says. In fact, it could be the dominant signature of life on exoplanets that lack oxygen.
Overall, Seager says, "I'm excited, because I feel like we're really on the verge of understanding the biosignatures on exoplanets.
"We're gathering all the tools we need to make predictions and guide design of the instruments that will actually do the job of finding signs of life."