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.