P von Paris

Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones

The effects of multi-layered clouds in the atmospheres of Earth-like planets orbiting different types of stars are studied. The radiative effects of cloud particles are directly correlated with their wavelength-dependent optical properties. Therefore the incident stellar spectra may play an important role for the climatic effect of clouds. We discuss the influence of clouds with mean properties measured in the Earths atmosphere on the surface temperatures and Bond albedos of Earth-like planets orbiting different types of main sequence dwarf stars.

Spectral features of Earth-like planets and their detectability at different orbital distances around F, G, and K-type stars

Context. In recent years, more and more transiting terrestrial extrasolar planets have been found. Spectroscopy already yielded the detection of molecular absorption bands in the atmospheres of Jupiter and Neptune-sized exoplanets. Detecting spectral features in the atmosphere of terrestrial planets is the next great challenge for exoplanet characterization. Aims. We investigate the spectral appearance of Earth-like exoplanets in the habitable zone (HZ) of different main sequence (F, G, and K-type) stars at different orbital distances. We furthermore discuss for which of these scenarios biomarker absorption bands and related compounds may be detected during primary or secondary transit with near-future telescopes and instruments. Methods. Atmospheric profiles from a 1D cloud-free atmospheric climate-photochemistry model were used to compute primary and secondary eclipse infrared spectra. The spectra were analyzed taking into account different filter bandpasses of two photometric instruments planned to be mounted to the James Webb Space Telescope (JWST). We analyzed in which filters and for which scenarios molecular absorption bands are detectable when using the space-borne JWST or the ground-based European Extremely Large Telescope (E-ELT). Results. Absorption bands of carbon dioxide (CO2), water (H2O), methane (CH4) and ozone (O3) are clearly visible in both highresolution spectra as well as in the filters of photometric instruments. However, only during primary eclipse absorption bands of CO2, H2 Oa nd O 3 are detectable for all scenarios when using photometric instruments and an E-ELT-like telescope setup. CH4 is only detectable at the outer HZ of the K-type star since here the atmospheric modeling results in very high abundances. Since the detectable CO2 and H2O absorption bands overlap, separate bands need to be observed to prove their existence in the planetary atmosphere. In order to detect H2O in a separate band, a ratio S /N > 7 needs to be achieved for E-ELT observations, e.g. by co-adding at least 10 transit observations. Using a space-borne telescope like the JWST enables the detection of CO2 at 4.3 μm, which is not possible for ground-based observations due to the Earth’s atmospheric absorption. Hence combining observations of space-borne and groundbased telescopes might allow to detect the presence of the biomarker molecule O3 and the related compounds H2 Oa nd CO 2 in a planetary atmosphere. Other absorption bands using the JWST can only be detected for much higher S/Ns, which is not achievable by just co-adding transit observations since this would be far beyond the planned mission time of JWST.

Transiting exoplanets from the CoRoT space mission XIII. CoRoT-13b: a dense hot Jupiter in transit around a star with solar metallicity and super-solar lithium content

We announce the discovery of the transiting planet CoRoT-13b. Ground-based follow-up in CFHT and IAC80 confirmed CoRoT’s observations. The mass of the planet was measured with the HARPS spectrograph and the properties of the host star were obtained analyzing HIRES spectra from the Keck telescope. It is a hot Jupiter-like planet with an orbital period of 4.04 days, 1.3 Jupiter masses, 0.9 Jupiter radii, and a density of 2.34 g cm −3 . It orbits a G0V star with Teff = 5 945 K, M∗ = 1.09 M� , R∗ = 1.01 R� , solar metallicity, a lithium content of +1.45 dex, and an estimated age of between 0.12 and 3.15 Gyr. The lithium abundance of the star is consistent with its effective temperature, activity level, and age range derived from the stellar analysis. The density of the planet is extreme for its mass, implies that heavy elements are present with a mass of between about 140 and 300 M⊕.

Clouds in the atmospheres of extrasolar planets - II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds

Aims. We study the impact of multi-layered clouds (low-level water and high-level ice clouds) on the thermal emission spectra of Earth-like planets orbiting different types of stars. Clouds have an important influence on such planetary emission spectra due to their wavelength dependent absorption and scattering properties. We also investigate the influence of clouds on the ability to derive information about planetary surface temperatures from low-resolution spectra. Methods. We use a previously developed parametric cloud model based on observations in the Earth’s atmosphere, coupled to a onedimensional radiative-convective steady state climate model. This model is applied here to study the effect of clouds on the thermal emission spectra of Earth-like extrasolar planets in dependence of the type of central star. Results. The presence of clouds lead in general to a decrease of the planetary IR spectrum associated with the dampening of spectral absorption features such as the 9.6 µm absorption band of O3 for example. This dampening is not limited to absorption features originating below the cloud layers but was also found for features forming above the clouds. When only single cloud layers are considered, both cloud types exhibit basically the same effects on the spectrum but the underlying physical processes are clearly different. For model scenarios where multi-layered clouds have been considered with coverages which yield mean Earth surface temperatures, the low-level clouds have only a small influence on the thermal emission spectra. In these cases the major differences are caused by highlevel ice clouds. The largest effect was found for a planet orbiting the F-type star, where no absorption features can be distinguished in the low-resolution emission spectrum for high cloud coverages. However, for most central stars, planetary atmospheric absorption bands are present even at high cloud coverages. Clouds also affect the derivation of surface temperatures from low-resolution spectra when fitting black-body radiation curves to the spectral shape of the IR emission spectra. With increasing amount of high-level clouds the derived temperatures increasingly under-estimate the real planetary surface temperatures. Consequently, clouds can alter significantly the measured apparent temperature of a planet as well as the detectability of the characteristic spectral signatures in the infrared. Therefore, planets with observationally derived somewhat lower surface temperatures should not be discarded too quickly from the list of potential habitable planets before further investigations on the presence of clouds have been made.

The extrasolar planet Gliese 581d: a potentially habitable planet?

Aims. The planetary system around the M star Gliese 581 contains at least three close-in potentially low-mass planets, Gl 581c, d, and e. In order to address the question of the habitability of Gl 581d, we performed detailed atmospheric modeling studies for several planetary scenarios. Methods. A 1D radiative-convective model was used to calculate temperature and pressure profiles of model atmospheres, which we assumed to be composed of molecular nitrogen, water, and carbon dioxide. The model allows for changing surface pressures caused by evaporation/condensation of water and carbon dioxide. Furthermore, the treatment of the energy transport has been improved in the model to account in particular for high CO2, high-pressure Super-Earth conditions. Results. For four high-pressure scenarios of our study, the resulting surface temperatures were above 273 K, indicating a potential habitability of the planet. These scenarios include three CO2-dominated atmospheres (95% CO2 concentration with 5, 10, and 20 bar surface pressure) and a high-pressure CO2-enriched atmosphere (5% CO2 concentration with 20 bar surface pressure). For all other considered scenarios, the calculated Gl 581d surface temperatures were below the freezing point of water, suggesting that Gl 581d would not be habitable then. The results for our CO2-dominated scenarios confirm very recent model results by Wordsworth et al. (2010). However, our model calculations imply that also atmospheres that are not CO2-dominated (i.e., 5% vmr instead of 95% vmr) could result in habitable conditions for Gl 581d.

Characterization of potentially habitable planets: Retrieval of atmospheric and planetary properties from emission spectra

Context. An increasing number of potentially habitable terrestrial planets and planet candidates are found by ongoing planet search programs. The search for atmospheric signatures to establish planetary habitability and the presence of life might be possible in the future. Aims. We want to quantify the accuracy of retrieved atmospheric parameters (composition, temperature, pressure) that might be obtained from infrared emission spectroscopy. Methods. We use synthetic observations of the atmospheres of hypothetical potentially habitable planets. These were constructed with a parametrized atmosphere model, a high-resolution radiative transfer model and a simplified noise model. The simulated observations were used to fit the model parameters. Furthermore, classic statistical tools such as χ 2 statistics and least-square fits were used to analyze the simulated observations. Results. When adopting the design of currently planned or proposed exoplanet characterization missions, we find that emission spectroscopy could provide weak limits on the surface conditions of terrestrial planets, hence their potential habitability. However, these mission designs are unlikely to allow the composition of the atmosphere of a habitable planet to be characterized, even though CO2 is detected. Upon increasing the signal-to-noise ratios by about a factor of 2−5 (depending on spectral resolution) compared to current mission designs, the CO2 content could be characterized to within two orders of magnitude. The detection of the O3 biosignature remains marginal. The atmospheric temperature structure could not be constrained. Therefore, a full atmospheric characterization seems to be beyond the capabilities of such missions when using only emission spectroscopy during secondary eclipse or target visits. Other methods such as transmission spectroscopy or orbital photometry are probably needed in order to give additional constraints and break degeneracies.

Potential biosignatures in super-Earth atmospheres II. Photochemical responses.

Spectral characterization of super-Earth atmospheres for planets orbiting in the habitable zone of M dwarf stars is a key focus in exoplanet science. A central challenge is to understand and predict the expected spectral signals of atmospheric biosignatures (species associated with life). Our work applies a global-mean radiative-convective-photochemical column model assuming a planet with an Earth-like biomass and planetary development. We investigated planets with gravities of 1g and 3g and a surface pressure of 1 bar around central stars with spectral classes from M0 to M7. The spectral signals of the calculated planetary scenarios have been presented by in an earlier work by Rauer and colleagues. The main motivation of the present work is to perform a deeper analysis of the chemical processes in the planetary atmospheres. We apply a diagnostic tool, the Pathway Analysis Program, to shed light on the photochemical pathways that form and destroy biosignature species. Ozone is a potential biosignature for complex life. An important result of our analysis is a shift in the ozone photochemistry from mainly Chapman production (which dominates in Earths stratosphere) to smog-dominated ozone production for planets in the habitable zone of cooler (M5-M7)-class dwarf stars. This result is associated with a lower energy flux in the UVB wavelength range from the central star, hence slower planetary atmospheric photolysis of molecular oxygen, which slows the Chapman ozone production. This is important for future atmospheric characterization missions because it provides an indication of different chemical environments that can lead to very different responses of ozone, for example, cosmic rays. Nitrous oxide, a biosignature for simple bacterial life, is favored for low stratospheric UV conditions, that is, on planets orbiting cooler stars. Transport of this species from its surface source to the stratosphere where it is destroyed can also be a key process. Comparing 1g with 3g scenarios, our analysis suggests it is important to include the effects of interactive chemistry.

Spectroscopic characterization of the atmospheres of potentially habitable planets: GL 581 d as a model case study

Context. Were a potentially habitable planet to be discovered, the next step would be the search for an atmosphere and its characterization. Eventually, surface conditions, hence habitability, and biomarkers as indicators for life would be assessed. Aims. The super-Earth candidate Gliese (GL) 581 d is the first potentially habitable extrasolar planet so far discovered. Therefore, GL 581 d is used to illustrate a hypothetical detailed spectroscopic characterization of such planets. Methods. Atmospheric profiles for a wide range of possible one-dimensional (1D) radiative-convective model scenarios of GL 581 d were used to calculate high-resolution synthetic emission and transmission spectra. Atmospheres were assumed to be composed of N2 ,C O 2 ,a nd H 2O. From the spectra, signal-to-noise ratios (SNRs) were calculated for a telescope such as the planned James Webb Space Telescope (JWST). Exposure times were set to be equal to the duration of one transit. Results. The presence of the model atmospheres can be clearly inferred from the calculated synthetic spectra thanks to strong water and carbon-dioxide absorption bands. Surface temperatures can be inferred for model scenarios with optically thin spectral windows. Dense, CO2-rich (potentially habitable) scenarios do not enable us to determine the surface temperatures nor assess habitability. Degeneracies between CO2 concentration and surface pressure complicate the interpretation of the calculated spectra, hence the determination of atmospheric conditions. Still, inferring approximative CO2 concentrations and surface pressures is possible. In practice, detecting atmospheric signals is challenging because the calculated SNR values are well below unity in most of the cases. The SNR for a single transit was only barely larger than unity in some near-IR bands for transmission spectroscopy. Most interestingly, the false-positive detection of biomarker candidates such as methane and ozone might be possible in low resolution spectra because CO2 absorption bands overlap biomarker spectral bands. This can be avoided, however, by observing all main CO2 IR bands instead of concentrating on, e.g., the 4.3 or 15 μm bands only. Furthermore, a masking of ozone signatures by CO2 absorption bands is shown to be possible. Simulations imply that such a false-negative detection of ozone would be possible even for rather high ozone concentrations of up to 10 −5 .

3D climate modeling of Earth-like extrasolar planets orbiting different types of host stars

Abstract The potential habitability of a terrestrial planet is usually defined by the possible existence of liquid water on its surface, since life as we know it needs liquid water at least during a part of its life cycle. The potential presence of liquid water on a planetary surface depends on many factors such as, most importantly, surface temperatures. The properties of the planetary atmosphere and its interaction with the radiative energy provided by the planets host star are thereby of decisive importance. In this study we investigate the influence of different main-sequence stars (F, G, and K-type stars) upon the climate of Earth-like extrasolar planets and their potential habitability by applying a state-of-the-art three-dimensional (3D) Earth climate model accounting for local and dynamical processes. The calculations have been performed for planets with Earth-like atmospheres at orbital distances (and corresponding orbital periods) where the total amount of energy received from the various host stars equals the solar constant. In contrast to previous 3D modeling studies, we include the effect of ozone radiative heating upon the vertical temperature structure of the atmospheres. The global orbital mean results obtained have been compared to those of a one-dimensional (1D) radiative convective climate model to investigate the approximation of global mean 3D results by those of 1D models. The different stellar spectral energy distributions lead to different surface temperatures and due to ozone heating to very different vertical temperature structures. As previous 1D studies we find higher surface temperatures for the Earth-like planet around the K-type star, and lower temperatures for the planet around the F-type star compared to an Earth-like planet around the Sun. However, this effect is more pronounced in the 3D model results than in the 1D model because the 3D model accounts for feedback processes such as the ice-albedo and the water vapor feedback. Whether the 1D model may approximate the global mean of the 3D model results strongly depends on the choice of the relative humidity profile in the 1D model, which is used to determine the water vapor profile. Hence, possible changes in the hydrological cycle need to be accounted for when estimating the potential habitability of an extrasolar planet.

Clouds in the atmospheres of extrasolar planets - III. Impact of low and high-level clouds on the reflection spectra of Earth-like planets

Context. Owing to their wavelength dependent absorption and scattering properties, clouds have an important influence on spectral albedos and planetary reflection spectra. In addition, the spectral energy distribution of the incident stellar light determines the detectable absorption bands of atmospheric molecules in these reflection spectra. Aims. We study the influence of low-level water and high-level ice clouds on low-resolution reflection spectra and planetary albedos of Earth-like planets orbiting different types of stars in both the visible and near infrared wavelength range. Methods. We use a one-dimensional radiative-convective steady-state atmospheric model coupled with a parametric cloud model, based on observations in the Earth’s atmosphere to study the effect of both cloud types on the reflection spectra and albedos of Earthlike extrasolar planets at low resolution for various types of central stars. Results. We find that the high scattering efficiency of clouds causes both the amount of reflected light and the related depths of the absorption bands to be substantially larger than in comparison to the respective clear sky conditions. Low-level clouds have a stronger impact on the spectra than the high-level clouds because of their much larger scattering optical depth. The detectability of molecular features in near the UV – near IR wavelength range is strongly enhanced by the presence of clouds. However, the detectability of various chemical species in low-resolution reflection spectra depends strongly on the spectral energy distribution of the incident stellar radiation. In contrast to the reflection spectra the spectral planetary albedos enable molecular features to be detected without a direct influence of the spectral energy distribution of the stellar radiation. Here, clouds increase the contrast between the radiation fluxes of the planets and the respective central star by about one order of magnitude, but the resulting contrast values are still too low to be observable with the current generation of telescopes.