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Astronomical Detection of Biosignatures From Extrasolar Planets

Motivated by recent discoveries of a multitude of extrasolar planets, NASA has initiated a series of studies for space-based observatories that will be able to search for life on these worlds. To optimize designs of these NASA missions and to interpret the data returned, we need to be able to recognize habitable worlds and to discriminate between planets with and without life. Jet Propulsion Laboratory Team 2 (JPL 2) research supports these endeavors.

Principal Research Goals of the JPL Team 2

• Understand the plausible range of atmospheric and surface compositions for terrestrial planets
• Learn how to recognize the presence of life on extrasolar planets by identifying signatures of life in their spectra

To achieve these goals, we work to develop a suite of innovative modeling tools to simulate the environments and spectra of extrasolar planets. These modeling tools constitute a Virtual Planetary Laboratory, used to explore the plausible range of atmospheric compositions and globally-averaged spectra for early Earth, other planets in our solar system, and for extrasolar planets both with and without life. These tools will provide the first models to couple the radiative fluxes, climate, chemistry, geology and biology of a terrestrial planet, all to produce a self-consistent planetary state.

Research Tasks of the JPL Team 2

• Task 1: Use a radiative-transfer model to explore sensitivity to life and planetary characteristics in disk-averaged spectra of terrestrial planets.
• Task 2: Couple the radiative and climate models together and validate this combined model using products generated in Task 1.
• Task 3: Integrate an atmospheric chemistry model with the climate-radiative transfer model developed in Task 2, and use this combined model to explore plausible extrasolar environments, and the environments of Earth through the past 4.3 billion years.
• Task 4: Define and develop geological and exospheric modules for terrestrial planets, and integrate them with the combined climate-chemistry of Task 3. This model (The Abiotic Planet Model) will be used to further explore the plausible range of extrasolar planet environments, and search for “false positives” i.e. planets without life that may display some of the characteristics expected of an inhabited planet.
• Task 5: Define and develop biological modules and integrate them with the Abiotic Planet Model developed in Task 4. This combined model, the Inhabited Planet Model, will be used to explore the interactions of life with the environment of an inhabited planet, and to define the signatures of life on the surface and in the atmosphere of a planet.”

Components of the
Virtual Planetary Lab

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Results of this research will provide an improved understanding of the range of atmospheric compositions that are possible for planets with and without life. It will also help to quantify the effect of life on the atmospheric spectrum and composition of a planet. The models will provide a comprehensive spectral catalog, a “menu” of biosignatures, which will be used to determine the optimum wavelength range, spectral resolution, and sensitivity required to remotely sense the signs of life in the atmosphere or on the surface of another world. Having a spectral catalog of biosignatures will specifically address the question of false positive detections of life.

These studies will provide recommendations for the design and optimization of search strategies for future NASA planet detecting and characterizing missions such as TPF (Terrestrial Planet Finder, to be launched in 2011) and Life Finder (the next generation follow-on mission).