Philip von Paris

Potential biosignatures in super-Earth atmospheres - I. Spectral appearance of super-Earths around M dwarfs

Atmospheric temperature and mixing ratio profiles of terres trial planets vary with the spectral energy flux distribution for di fferent types of M-dwarf stars and the planetary gravity. We investigate the resulting effects on the spectral appearance of molecular absorption bands, which are relevant as indicators for potential planetary habitability during primary and secondary eclipse for transiting terrestrial planets with Earth-like biomass emissi ons. Atmospheric profiles are computed using a plane-parallel, 1D climate model coupled with a chemistry model. We then calculate simulated spectra using a line-by-line radiative transfer model. We find that emission spectra during secondary eclipse show i ncreasing absorption of methane, water, and ozone for planets orbiting quiet M0-M3 dwarfs and the active M-type star AD Leo compared with solar-type central stars. However, for planets orbiting very cool and quiet M dwarfs (M4 to M7), increasing temperatures in the mid-atmosphere lead to reduced absorption signals, which impedes the detection of molecules in these scenarios. Transmission spectra during primary eclipse show strong absorption features of CH4, N2O and H2O for planets orbiting quiet M0-M7 stars and AD Leo. The N2O absorption of an Earth-sized planet orbiting a quiet M7 star can even be as strong as the CO2 signal. However, ozone absorption decreases for planets orbiting these cool central stars owing to chemical effects in the atmosphere. To investigate the effect on the spectroscopic detection of absorption bands with potential future satellite missions, we compute signal-to-noise-ratios (SNR) for a James Webb Space Telescope (JWST)-like aperture telescope.

A Two-Tiered Approach to Assessing the Habitability of Exoplanets

In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability. The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature. For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions. As such, the PHI requires more detailed knowledge than is available for any exoplanet at this time. However, future missions such as the Terrestrial Planet Finder will collect this information and advance the PHI. Both indices are formulated in a way that enables their values to be updated as technology and our knowledge about habitable planets, moons, and life advances. Applying the proposed metrics to bodies within our Solar System for comparison reveals two planets in the Gliese 581 system, GJ 581 c and d, with an ESI comparable to that of Mars and a PHI between that of Europa and Enceladus.

FORMATION, TIDAL EVOLUTION, AND HABITABILITY OF THE KEPLER-186 SYSTEM

The Kepler-186 system consists of ve planets orbiting an early M dwarf. The planets have physical radii of 1.0{1.50 R and orbital periods of 4{130 days. The 1:1R Kepler-186f with a period of 130 days is of particular interest. Its insolation of roughly 0:32 S places it within the surface liquid water habitable zone (HZ). We present a multifaceted study of the Kepler-186 system, using two sets of parameters which are consistent with the data and also self-consistent. First, we show that the distribution of planet masses can be roughly reproduced if the planets were accreted from a high surface density disk presumably sculpted by an earlier phase of migration. However, our simulations predict the existence of one to two undetected planets between planets e and f. Next, we present a dynamical analysis of the system including the eect of tides. The timescale for tidal evolution is short enough that the four inner planets must have small obliquities and near-synchronous rotation rates. The tidal evolution of Kepler-186f is slow enough that its current spin state depends on a combination of its initial spin state, its dissipation rate, and the stellar age. Finally, we study the habitability of Kepler-186f with a one-dimensional climate model. The planet’s surface temperature can be raised above 273 K with 0.5{5 bars of CO2, depending on the amount of N2 present. Kepler-186f represents a case study of an Earth-sized planet in the cooler regions of the HZ of a cool star. Subject headings: methods: numerical { planets and satellites: atmospheres { planets and satellites: dynamical evolution and stability { planets and satellites: formation { stars: individual (Kepler-186, KIC 8120608)

Response of Atmospheric Biomarkers to NOx-Induced Photochemistry Generated by Stellar Cosmic Rays for Earth-like Planets in the Habitable Zone of M Dwarf Stars

Understanding whether M dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M dwarfs ranging from quiet up to strong flaring conditions and investigate one particular effect upon biomarkers, namely, the ability of secondary electrons caused by stellar cosmic rays to break up atmospheric molecular nitrogen (N(2)), which leads to production of nitrogen oxides (NO(x)) in the planetary atmosphere, hence affecting biomarkers such as ozone (O(3)). We apply a stationary model, that is, without a time dependence; hence we are calculating the limiting case where the atmospheric chemistry response time of the biomarkers is assumed to be slow and remains constant compared with rapid forcing by the impinging stellar flares. This point should be further explored in future work with time-dependent models. We estimate the NO(x) production using an air shower approach and evaluate the implications using a climate-chemical model of the planetary atmosphere. O(3) formation proceeds via the reaction O+O(2)+M→O(3)+M. At high NO(x) abundances, the O atoms arise mainly from NO(2) photolysis, whereas on Earth this occurs via the photolysis of molecular oxygen (O(2)). For the flaring case, O(3) is mainly destroyed via direct titration, NO+O(3)→NO(2)+O(2), and not via the familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios with low O(3), Rayleigh scattering by the main atmospheric gases (O(2), N(2), and CO(2)) became more important for shielding the planetary surface from UV radiation. A major result of this work is that the biomarker O(3) survived all the stellar-activity scenarios considered except for the strong case, whereas the biomarker nitrous oxide (N(2)O) could survive in the planetary atmosphere under all conditions of stellar activity considered here, which clearly has important implications for missions that aim to detect spectroscopic biomarkers.

Pre-Discovery Observations of CoRoT-1b and CoRoT-2b with the BEST Survey

The Berlin Exoplanet Search Telescope (BEST) wide-angle telescope installed at the Observatoire de Haute-Provence and operated in remote control from Berlin by the Institut fur Planetenforschung, DLR, has observed the CoRoT target fields prior to the mission. The resulting archive of stellar photometric light curves is used to search for deep transit events announced during CoRoTs alarm mode to aid in fast photometric confirmation of these events. The initial run field of CoRoT (IRa01) was observed with BEST in 2006 November and December for 12 nights. The first long run field (LRc01) was observed from 2005 June to September for 35 nights. After standard CCD data reduction, aperture photometry has been performed using the ISIS image subtraction method. About 30,000 light curves were obtained in each field. Transits of the first detected planets by the CoRoT mission, CoRoT-1b and CoRoT-2b, were found in archived data of the BEST survey and their light curves are presented here. Such detections provide useful information at the early stage of the organization of follow-up observations of satellite alarm-mode planet candidates. In addition, no period change was found over ~4 years between the first BEST observation and last available transit observations.

A Dynamic Scheme to Assess Habitability of Exoplanets

In the next few years, the number of catalogued exoplanets will be counted in the thousands and with it the need will arise to prioritize them in regard to habitability and the potential presence of life. Here, we suggest a first attempt of a dynamic scheme for classification based on our current understanding of parameters that are consistent and beneficial for the presence of life. These parameters include the presence of (1) a terrestrial planet or moon, which (2) is endowed with a significant atmosphere; the presence of (3) a magnetic field enveloping the exoplanet; (4) internal differentiation and plate tectonics; (5) detectable surface liquids, preferably water, on the surface of the planetary body; and (6) the detection of geoindicators; and (7) bioindicators. We propose to calculate a habitability index (HI) based on these parameters, but realize the assumptions that go into this value and the very bias introduced by the limitations of the detection methods. However, in order to account for the assumptions and limitations of this method, the HI index is an open scheme that can be updated as technology and our knowledge about habitable planets advances.