Task E: The Observer

In this Task, VPL Team members make observations of Solar System and extrasolar planets, develop astronomical and remote-sensing retrieval methods, and use telescope and instrument simulators to determine required measurements, observing strategies, and analysis techniques for exoplanet characterization. New simulation capabilities are validated against the Earth datasets provided in Task A. To assess the detectability of signs of habitability and life, input spectra for the simulators and detectability calculations are generated from the environments developed in Tasks A, B, C and D.

Atmospheric spectra of a handful of transiting Jovian and mini-Neptune planets currently exist, but spectroscopy of transiting super-Earths will likely await the launch of NASA’s JWST. Direct imaging of Earth-sized planets in the habitable zone awaits the development of TPF-class missions. In all these cases, the data returned will be far more challenging to interpret than conventional terrestrial or planetary remote-sensing observations. The spectral resolution, spectral range and S/N are likely to be minimal, and the disk of the planet will be unresolved (disk-integrated), so planetary spectra will include mixtures of surface types and cloudy and clear scenes, and the planetary environment may be unlike anything currently known.  

To prepare for the interpretation of this challenging data, VPL team members explore and quantify the types of signals we might expect to see from different metabolisms and planetary environments. Examples of this work are given below. 

Detectability of Life on An Anoxic World

For a third of life’s history on Earth, the planet’s atmosphere contained little or no oxygen. In such an "anoxic" environment, two of the main biosignature gases (O2 and O3) would not build up to detectable concentrations. In Domagal-Goldman et al. (2011) VPL resesarchers show how biogenic sulfur gases could be used as biosignatures for such a planet. We found that for extremely high production rates of these gases, or for planets around stars with low UV-fluxes, these gases would be directly detectable. We furthermore found that for lower (modern-day) fluxes on planets around cooler stars than our Sun, the effects of these gases on atmospheric chemistry could be detected via an increase in the planet’s ethane to methane ratio. This work lays out a strategy for finding life on an anoxic planet, a type of biosphere that could represent a significant portion of the inhabited planets in the universe.

Detectability of Planetary Characteristics and Biosignatures for TPF

We are currently running simulators for Terrestrial Planet Finder Coronograph (S. Heap and D. Lindler), Interferometer (T. Velusamy) and Occulter (W. Cash) mission concepts. We are using these simulators to explore the detectability of signs of habitability and biosignatures from oxygenic photosynthetic, as well as the other metabolisms described here (Evans et al., 2011). We find that it is in fact easier to detect signs of photosynthesis on cloud-covered Earth-like planets, due to the tendency for oxygen to be evenly mixed, and ozone to be concentrated above the cloud layer.  We are currently working to inter-compare these simulators, which will help us to describe the efficacy with which these various observatories can characterize potentially habitable exoplanets, and allow us to make suggestions regarding the ideal wavelength ranges and spectral resolutions for a TPF-class mission.