Characterizing Habitable Extrasolar Planets using Spectral Fingerprints

The detection and characterization of Earth-like planet is approaching rapidly thanks to radial velocity surveys (HARPS), transit searches (Corot, Kepler) and space observatories dedicated to their characterization are already in development phase (James Webb Space Telescope), large ground based telescopes (ELT, TNT, GMT), and dedicated space-based missions like Darwin, Terrestrial Planet Finder, New World Observer). In this paper we discuss how we can read a planets spectrum to assess its habitability and search for the signatures of a biosphere. Identifying signs of life implies understanding how the observed atmosphere physically and chemically works and thus to gather information on the planet in addition to the observing its spectral fingerprint.Comment: 14pg, 4 figures, Accepted CRAS (Proceedings of the National Academy of Science France), Palevol serie

Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f

Planets composed of large quantities of water that reside in the habitable zone are expected to have distinct geophysics and geochemistry of their surfaces and atmospheres. We explore these properties motivated by two key questions: whether such planets could provide habitable conditions and whether they exhibit discernable spectral features that distinguish a water-planet from a rocky Earth-like planet. We show that the recently discovered planets Kepler-62e and -62f are the first viable candidates for habitable zone water-planet. We use these planets as test cases for discussing those differences in detail. We generate atmospheric spectral models and find that potentially habitable water-planets show a distinctive spectral fingerprint in transit depending on their position in the habitable zone.Comment: 8 pages, 4 figures, ApJ, 775, L4

UV Surface Environment of Earth-like Planets Orbiting FGKM Stars Through Geological Evolution

The UV environment of a host star affects the photochemistry in the atmosphere, and ultimately the surface UV environment for terrestrial planets and therefore the conditions for the origin and evolution of life. We model the surface UV radiation environment for Earth-sized planets orbiting FGKM stars at the 1AU equivalent distance for Earth through its geological evolution. We explore four different types of atmospheres corresponding to an early Earth atmosphere at 3.9 Gyr ago and three atmospheres covering the rise of oxygen to present day levels at 2.0 Gyr ago, 0.8 Gyr ago and modern Earth (Following Kaltenegger et al. 2007). In addition to calculating the UV flux on the surface of the planet, we model the biologically effective irradiance, using DNA damage as a proxy for biological damage. We find that a pre-biotic Earth (3.9 Gyr ago) orbiting an F0V star receives 6 times the biologically effective radiation as around the early Sun and 3520 times the modern Earth-Sun levels. A pre-biotic Earth orbiting GJ 581 (M3.5V) receives 300 times less biologically effective radiation, about 2 times modern Earth-Sun levels. The UV fluxes calculated here provide a grid of model UV environments during the evolution of an Earth-like planet orbiting a range of stars. These models can be used as inputs into photo-biological experiments and for pre-biotic chemistry and early life evolution experiments.Comment: 10 pages, 5 figure

Effect of UV Radiation on the Spectral Fingerprints of Earth-like Planets Orbiting M dwarfs

We model the atmospheres and spectra of Earth-like planets orbiting the entire grid of M dwarfs for active and inactive stellar models with $T_{eff}$ = 2300K to $T_{eff}$ = 3800K and for six observed MUSCLES M dwarfs with UV radiation data. We set the Earth-like planets at the 1AU equivalent distance and show spectra from the VIS to IR (0.4$\mu$m - 20$\mu$m) to compare detectability of features in different wavelength ranges with JWST and other future ground- and spaced-based missions to characterize exo-Earths. We focus on the effect of UV activity levels on detectable atmospheric features that indicate habitability on Earth, namely: H$_2$O, O$_3$, CH$_4$, N$_2$O and CH$_3$Cl. To observe signatures of life - O$_2$/O$_3$ in combination with reducing species like CH$_4$, we find that early and active M dwarfs are the best targets of the M star grid for future telescopes. The O$_2$ spectral feature at 0.76$\mu$m is increasingly difficult to detect in reflected light of later M dwarfs due to low stellar flux in that wavelength region. N$_2$O, another biosignature detectable in the IR, builds up to observable concentrations in our planetary models around M dwarfs with low UV flux. CH$_3$Cl could become detectable, depending on the depth of the overlapping N$_2$O feature. We present a spectral database of Earth-like planets around cool stars for directly imaged planets as a framework for interpreting future lightcurves, direct imaging, and secondary eclipse measurements of the atmospheres of terrestrial planets in the HZ to design and assess future telescope capabilities.Comment: in press, ApJ (submitted August 18, 2014), 16 pages, 12 figure

Detecting planetary geochemical cycles on exoplanets: Atmospheric signatures and the case of SO2

We study the spectrum of a planetary atmosphere to derive detectable features in low resolution of different global geochemical cycles on exoplanets - using the sulphur cycle as our example. We derive low resolution detectable features for first generation space- and ground- based telescopes as a first step in comparative planetology. We assume that the surfaces and atmospheres of terrestrial exoplanets (Earth-like and super-Earths) will most often be dominated by a specific geochemical cycle. Here we concentrate on the sulphur cycle driven by outgassing of SO2 and H2S followed by their transformation to other sulphur-bearing species which is clearly distinguishable from the carbon cycle which is driven by outgassing of CO2. Due to increased volcanism, the sulphur cycle is potentially the dominant global geochemical cycle on dry super-Earths with active tectonics. We calculate planetary emission, reflection and transmission spectrum from 0.4 to 40 micrometer with high and low resolution to assess detectable features using current and Archean Earth models with varying SO2 and H2S concentrations to explore reducing and oxidizing habitable environments on rocky planets. We find specific spectral signatures that are observable with low resolution in a planetary atmosphere with high SO2 and H2S concentration. Therefore first generation space and ground based telescopes can test our understanding of geochemical cycles on rocky planets and potentially distinguish planetary environments dominated by the carbon and sulphur cycle.Comment: 9 pages, 6 figures, ApJ accepted - detailed discussion adde