Infrared radiative transfer in atmospheres of Earth-like planets around F, G, K, and M stars

Abstract

Context. The atmosphere of Earth-like extrasolar planets orbiting different types of stars is influenced by the spectral dependence of the incoming stellar radiation. The changes in structure and composition affect atmospheric radiation, hence the spectral appearance of these exoplanets. Aims. We provide a thorough investigation of infrared radiative transfer in cloud-free exoplanets atmospheres by not only analyzing the planetary spectral appearance but also discussing the radiative processes behind the spectral features in detail and identifying the regions in the atmosphere that contribute most at a given wavelength. Methods. Using cloud-free scenarios provided by a one-dimensional radiative-convective steady-state atmospheric model, we computed high-resolution infrared transmission and emission spectra, as well as weighting functions for exoplanets located within the habitable zone of F, G, K, and M stars by means of a line-by-line molecular absorption model and a Schwarzschild solver for the radiative transfer. The monochromatic spectra were convolved with appropriate spectral response functions to study the effects of finite instrument resolution. Results. Spectra of the exoplanets of F, G, K, and M stars were analyzed in the 4.5 µm N 2 O band, the 4.3 µm and 15 µm CO 2 bands, the 7.7 µm CH 4 band, the 6.3 µm H 2 O band, and the 9.6 µm O 3 band. Differences in the state of the atmosphere of the exoplanets clearly show up in the thermal infrared spectra; absorption signatures known from Earth can be transformed to emission features (and vice versa). Weighting functions show that radiation in the absorption bands of the uniformly mixed gases (CO 2 , CH 4 , N 2 O) and (to some extent) ozone comes from the stratosphere and upper troposphere, and also indicate that changes in the atmospheres can shift sources of thermal radiation to lower or higher altitudes. Molecular absorption and/or emission features can be identified in the high-resolution spectra of all planets and in most reduced resolution spectra. Conclusions. Insight into radiative transfer processes is essential for analyzing exoplanet spectral observations; for instance, understanding the impact of the temperature profile (nb. non-existence of an inversion) on the CO 2 bands facilitates their interpretation and can help avoid false positive or negative estimates of O 3 . The detailed analysis of the radiation source and sink regions could even help give an indication about the feasibility of identifying molecular signatures in cloud-covered planets, i.e. radiation mainly coming from the upper atmosphere is less likely to be hidden by clouds. Infrared radiative transfer and biomarker detectability in cloud-covered exoplanets will be presented in a companion paper.

Spectra of Earth-like Planets through Geological Evolution around FGKM Stars

“…However features originating from below the cloud layers in the visible will be suppressed, particularly surface features like the VRE (Seager et al 2005;Tinetti et al 2006c;Rugheimer et al 2013). In the IR, clouds can dampen features (as seen in Kitzmann et al 2011a;Vasquez et al 2013), though they can also enhance features (e.g. Kaltenegger et al 2007;Rugheimer et al 2013).…”

Transmission spectroscopy with the ACE-FTS infrared spectral atlas of Earth: A model validation and feasibility study

“…More recently, GAR-LIC (or its Fortran 77 predecessor) has also been used for a variety of exoplanet studies, e.g. [14][15][16][17].…”

“…Segura et al [11] generated synthetic Vis-NIR and TIR spectra of an Earth-like planet orbiting around M dwarfs, and transit spectra of Earth and an Earth-like exoplanet orbiting an M star have been modeled by Kaltenegger and Traub [12]. The impact of the host star's type or distance on the spectral appearance of Earth-like planets has been considered by, e.g., Segura et al [13], Rauer et al [14], Hedelt et al [15], Vasquez et al [16,17], Rugheimer et al [18] and [19].…”

Utilitarian Opacity Model for Aggregate Particles in Protoplanetary Nebulae and Exoplanet Atmospheres

“…In protoplanetary nebulae, the relatively small fraction (by mass) of solid particles provides the bulk of the opacity (D'Alessio et al 1999(D'Alessio et al , 2001) unless the temperature is so large (above 1500K) that common geological solids evaporate and molecular species dominate (Nakamoto andNakagawa 1994, Ferguson et al 2005). In exoplanet atmospheres, which are generally much denser, the particle contributions are more situation-dependent (Marley et al 1999, Sudarsky et al 2000, 2003Ackerman and Marley 2001, Tsuji 2002, de Kok et al 2011, Madhusudhan et al 2011, Morley et al 2012Vasquez et al 2013).…”