The search for habitable planets and life is founded upon the premise that the effects of even the most basic forms of life on a planet are global, and that evidence for life, or biosignatures, from the planet's atmosphere or surface will be recognizable in the spectrum of the planet's light. Observations across as broad a wavelength range as possible are needed to fully characterize a planet's habitability and to detect signs of life.
Direct imaging detection and spectroscopic characterization of nearby Earthlike planets will be undertaken by the Terrestrial Planet Finder missions. The TPF Coronagraph (TPF-C), planned for launch in 2014, will operate at visible wavelengths. It will suppress the light of the central star to unprecedented levels, allowing it to search for terrestrial planets in ~150 nearby planetary systems. TPF-C will be followed about five years later by the TPF Interferometer (TPF-I). TPF-I will operate in the mid-IR and will survey a larger volume of our solar neighborhood, searching for terrestrial planets around as many as 500 nearby stars.
Understanding biosignatures
The search for life elsewere in the universe begins with an understanding of the biosignatures of our own world. Earth has surface biosignatures due to vegetation, and several atmospheric biosignatures, including the characteristic spectra of life-related compounds like oxygen - produced by photosynthetic bacteria and plants - and its photochemical product, ozone. The most convincing spectroscopic evidence for life as we know it is the simultaneous detection of large amounts of oxygen as well as a reduced gas, such as methane or nitrous oxide, which can be produced by living organisms. Oxygen, methane, and nitrous oxide are produced in large amounts by plants, animals, and bacteria on Earth today, and they are orders of magnitude out of thermodynamic equilibrium with each other.
However, we should not expect other habitable worlds to be exactly like our own. We must furthermore be able to identify potential "false-positives," the nonbiological generation of planetary characteristics that mimic biosignatures. For example, while atmospheric methane may be a possible biomarker on a planet like Earth, especially when seen in the presence of oxygen, on a body like Titan it is simply a component of the atmosphere that is non-biologically-generated. Theoretical and experimental research and analysis are necessary to secure a detailed understanding of the biosignatures that might be found.
TPF-C and TPF-I will have sufficient spectroscopic capability to detect evidence for gases such as carbon dioxide or water vapor. The visible and infrared spectrum, in conjunction with theoretical and empirical models, can tell us about the amount of atmosphere, the gases present in the atmosphere, the presence of clouds, the degree and variability of cloud cover or airborne dust, and the presence of a greenhouse effect. The concentration of greenhouse gases can determine whether the surface is warm enough to maintain liquid water, even if (as for Earth) the equilibrium temperature without such gases would result in a frozen surface. Clouds and dust aerosols can determine the amount of light absorbed and reflected, and, thus, the surface temperature. Spectra can also tell us about the surface, whether it is rock-like with little or no overlying atmosphere, or covered with an ocean.
Beyond TPF
Beyond the TPF missions, the next-generation Life Finder mission would use a greater collecting area and spectral resolution to provide a more sensitive search for additional biosignatures and extend our search for Earthlike worlds to perhaps thousands of stars. The dual goals of extending our search to more planetary systems and providing greater time-resolved spectral information will challenge our imagination and technical prowess for decades to come. The Decadal Review (2000) strongly endorsed the search for life beyond our solar system, noting that "This goal is so challenging and of such importance that it could occupy astronomers for the foreseeable future."
