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Context. Oxygen (O) and carbon (C) have been inferred recently to be subsolar in abundance from spectra of the atmosphere of the transiting hot Jupiter HD 189733b. Yet, the mass and radius of the planet coupled with structure models indicate a strongly supersolar abundance of heavy elements in the interior of this object.

Aims.The recent Cassini discovery of water vapor plumes ejected from the south pole of the Saturnian satellite, Enceladus, presents a unique window of opportunity for the detection of extant life in our solar system.

A variety of terrestrial planets with different physical parameters and exotic atmospheres might plausibly exist outside our Solar System, waiting to be detected by the next generation of space-exploration missions. Some of these planets might be transiting their parent star. We present here a detailed study of the atmospheric signatures of transiting Earth-size exoplanets. We focus on a limited number of significant examples, for which we discuss the detectability of some of the possible molecules present in their atmospheres, such as water (H2O), carbon dioxide (CO2), ozone (O3), or molecular oxygen (O2). To this purpose, we developed a model to simulate transmission spectra of Earth-size exoplanets from the ultraviolet (UV) to the near infrared (NIR). According to our calculations, the signatures of planetary atmospheres represent an absorption of a few parts-per-million (ppm) in the stellar flux. The atmospheres of a few Earth-like planets can be detected with a 30–40 m telescope. The detection of the extensive atmospheres of tens of small satellites of giant exoplanets and hundreds of hypothetical ocean-planets can be achieved with 20–30 m and 10–20 m instruments, respectively, provided all these planets are frequent and they are efficiently surveyed. We also found that planets around K stars are favored, mainly because these stars are more numerous and smaller compared to G or F stars. While not addressed in this study, limitations might come from stellar photometric micro-variability.

The tau code is a line-by-line radiative transfer code, which is generally applicable for modelling transmission spectra of close-in extrasolar planets. The inputs are the assumed pressure–temperature profile of the planetary atmosphere, the continuum absorption coefficients and the absorption cross-sections for the trace molecular absorbers present in the model, as well as the fundamental system parameters taken from the published literature. The program then calculates the optical path through the planetary atmosphere of the radiation from the host star, and quantifies the absorption due to the modelled composition in a transmission spectrum of transit depth as a function of wavelength. The code is written in C++, parallelised using OpenMP, and is available for public download and use from http://www.ucl.ac.uk/exoplanets/. Program summary: Program title: tau Catalogue identifier: AEPN_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEPN_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 56728 No. of bytes in distributed program, including test data, etc.: 1907711 Distribution format: tar.gz Programming language: C++. Computer: non-specific. Operating system: Linux, Mac. Has the code been vectorised or parallelized?: Yes RAM: 300 MB Classification: 1.3. External routines: OpenMP (http://openmp.org/wp/) Nature of problem: Calculation of molecular absorption, and hence transit depths, as a function of wavelength for stellar radiation passing through planetary atmospheres. Solution method: Line-by-line calculation of wavelength-dependent optical depths using absorption cross-sections for various trace molecular absorbers. Additional comments: The distribution file contains, [•] ./doc/: readme and User Guide. [•] ./run/: sample input data files [•] ./out/sample_tau_output.dat: sample output file for Run Mode 9 (H2O at for HD189733b at ). Running time: From 0:5 to 500 s, depending on run parameters [ABSTRACT FROM AUTHOR]

The tau code is a 1D line-by-line radiative transfer code, which is generally applicable for modeling transmission spectra of close-in extrasolar planets. The inputs are the assumed temperature–pressure profile of the planetary atmosphere, the continuum absorption coefficients and the absorption cross-sections for the trace molecular absorbers present in the model, as well as the fundamental system parameters taken from the published literature. The program then calculates the optical path through the planetary atmosphere of the radiation from the host star, and quantifies the absorption due to the modeled composition in a transmission spectrum of transit depth as a function of wavelength. The code is written in C++, parallelized using OpenMP, and is available for public download and use from http://www.ucl.ac.uk/exoplanets/. New version program summary: Program title: tau Catalogue identifier: AEPN_v1_1 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEPN_v1_1.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 56203 No. of bytes in distributed program, including test data, etc.: 1679104 Distribution format: tar.gz Programming language: C++. Computer: Non-specific. Operating system: Linux, Mac. Has the code been vectorized or parallelized?: Yes RAM: 300 MB Classification: 1.3. External routines: OpenMP (http://openmp.org/wp) Does the new version supersede the previous version?: Yes Nature of problem: Calculation of molecular absorption, and hence transit depths, as a function of wavelength for stellar radiation passing through planetary atmospheres. Solution method: Line-by-line calculation of wavelength-dependent optical depths using absorption cross-sections for various trace molecular absorbers. Reasons for new version: Bug found in previous version whereby some values, defined as floats, took values too small to be represented by this datatype, and so were set to zero. Summary of revisions: All ‘float’ datatypes changed to ‘double’. Additional comments: The distribution file contains, [•] ./doc/: readme and User Guide. [•] ./run/: sample input data files [•] ./out/sample_tau_output.dat: sample output file for run mode ‘9’ (H2O at for HD189733b at ). Running time: From 0.5 to 500 s, depending on run parameters. [ABSTRACT FROM AUTHOR]