Spectral variability of Earth-like planets

A further step is to use narrow band data to build the spectral signal of Earthlike planets.By the comparison with spectra of terrestrial planets, we can infer the physical conditions of the atmosphere, the state and abundances of the main atmospheric species, as well as the global parameters of the planetary climate. The study of narrow-band light curves will also give an important information about the dynamics and the evolution of the most relevant compounds. By the average of the signal, we can also build longitudinal light curves and study their distribution on the planet. We have used a new generic version of the LMD Global Climate Model to build the spectral light curves of the Earth-like planets studied in Chapter 5: the Earth, the Earth with a rotation rate of 10 days (Ete–10), the aquaplanet Ote–1 and the snowball aquaplanet SOte–1.

Detection of Earth-like planets 

The Earth has a brightness temperature of ∼255 K, and the correspondent blackbody emission has a peak at ∼11 µm. Then, the thermal infrared (5-20 µm) is the most favorable spectral range to detect Earth-like planets. The orbital parameters can be detected by radial velocity using high-resolution spectrometers. For an Earth analogue at a distance of 1 UA from a solar-type star, the flux contrast factor between the star and the planet F∗/Fpl is ≃106 , 1000 times larger than in the visible range, however in order to obtain an angular distance above the difraction limit λ/D ∼ 100 mas, a diameter D of 40 m for ground-telescopesis required (?). The spectral features of H2O,CO2, and O3 are detectable, however CH4 is just detectable in EarlyEarth atmospheres where CH4 is predominant (Table 7.1). In the case of rocky 143 Spectral variability of Earth-like planets  7. Spectral variability of Earth-like planets Figure 7.1 NOAA newest Geostationary Operational Environmental Satellite (GOES15) took its first imager full-disk infrared image of the Earth on April 26 starting at 17.30 UT. Each of the five imager spectral bands are shown. There is one visible band and four infrared bands (shortwave window, water vapor, longwave window and a CO2 sensitive band). Credit: NASA-NOAA-SSEC. planets around cold dwarf stars of masses between 0.1-0.2 M J the detection from the ground is possible by 4-8 m class telescopes for planets at a maximum distance of ≃50 pc, with the use of a telescope of 40 m as E-ELT (Mart´ın & Guenther, 2006) allows to acheive distances up to ≃200 pc (e.g., Oliva & Origlia, 2008; Pall´e et al., 2011). Furthermore, we can increase the S/N ratio, using instruments like HIRES and METIS on grounds-based telescopes like E-ELT, which will open the door to a new range of measurements:

1. i) High-resolution spectra.– Planetary spectra can be used to probe into the atmosphere of the planet by identifying the optical depths as they depend directly on wavelength. Features on the atmosphere can be then separated with a resolving power of R∼300, and used to recover the abundances, state, distribution and dynamics of atmospheric species. 
2. ii) Rotation rate and winds.– Orbital velocity maps (time vs. orbital velocity) can be used to measure the zonal winds and the rotation rate of the planet. The velocity deviation is clearly shown as a line broadening (Snellen et al., 2010). 

iii) Longitudinal spectral variations.– We can build averaged rotational light curves as a function of the planetary longitude (Section 2.3.2, Figure 5.18 and Figure 6.12) to illustrate the distribution and the dynamics of atmospheric compounds. Future space telescope missions will use the transit method on bright stars to detect Earth-like planets: PLATO (Catala & PLATO Consortium, 2008; Rauer & Catala, 2011), an ESA mission that is previewed to be launched in 2017-2018, will search transits around stars of 8-11 magnitudes; TESS (e.g., Ricker et al., 2009) is a NASA mission for stars fainter than 12 magnitudes and it will be operative in 2017. EChO (e.g., Tinetti et al., 2012) is the first space mission designed to study exoplanet atmospheres and it will provide high-resolution multiwavelength spectroscopic observations in the infrared range (0.4-11) µm with a contrast ratio F∗/Fpl∼ 105 . A solution to improve the observations without building large telescopes is to use the nulling interferometry technique (Bracewell, 1978), either by a structurally connected interferometer or by a flagship mission, with a 2 m diameter collectors will be capable of detecting more than 100 Earth size planets around nearby stars. However space projects as SIM (e.g., Tanner et al., 2006; Catanzarite et al., 2006; Tanner et al., 2010), DARWIN (e.g., Ollivier et al., 1999, 2001; Cockell et al., 2009) and TPF-I (e.g., Kaltenegger & Fridlund, 2006; Gappinger et al., 2007) have been cancelled. 

 Climate Model

 The generic model was developed at the Laboratoire de M´et´eorlogie Dynamique (LMD) to explore a broad range of conditions in terms of temperature, pressure, atmospheric composition. Its aim is the study early climates (Forget et al., 2013; Wordsworth et al., 2013) and exoplanet atmospheres (Wordsworth et al., 2011; Selsis et al., 2011; Leconte et al., 2013) and to produce time-dependent maps of narrow band fluxes to generate synthetic observables. To do that, most of the empirical parameterizations (for instance to model the condensation and precipitation) or the detailed properties (like the precise albedos and emissivities associated with the different types of soils, vegetations, ice/snow layers) are no longer used. They are replaced by simplified, but more generic physical laws controlled by a few global parameters (like the radius of the ice/water particles, the soil thermal inertia, 148 7. Spectral variability of Earth-like planets emissivity). This model was applied to the case of present Earth to test its ability to catch the main characteristics of its climate and the associated observables. Although the general properties of Earth climate are reproduced, the simulations produced with this model slightly depart from those obtained with the Earth LMDZ code. In particular the equilibrium and mean surface temperature are lower than that of Earth (or simulated with the Earth LMDZ GCM). This is mainly due to the fact that the radiative effect of ozone is not included (it would require to compute grids of highresolution absorption coefficients with an additional dimension), which results in a lower forcing. The simplified properties of clouds also yield a lower forcing due to a slightly higher albedo. We are currently working with recent generic model simulations for the Earth, the aquaplanet Ote–1, and an Earth with a 10 days rotation period (Chapter 5). In this section we present our preliminary results for the mid-infrared spectra and time series obtained for these cases. The analysis of these synthetic observables (from Figure 7.4 to Figure 7.7) will be done after this thesis. However, we can study some main features from the spectral light curves of Figure 7.4 and Figure 7.5. Relating the global brightness temperature of each band to the atmospheric profile, we can identify some atmospheric compounds correspondent to certain bands (named by their center wavelength): water vapor absorption in the low-medium-level of the troposphere (7.14 µm); “atmospheric window”, temperature of the low-cloud tops and the surface (11.02 µm), water vapor at the upper levels of the troposphere (40.00 µm), and carbon dioxide at the stratosphere (15.58 µm), the height levels vary in the snowball planet because of the lower lapse rate (Figure 5.11). The curves start at the spring equinox, then for an observer at the northern hemisphere (blue, green) the temperature rises at the surface (11.02 µm) during spring and decreases during autumn. Water vapor bands (7.14 µm, 40.00 µm) have a small seasonal variation because of the accumulation of latent heat associated with evaporation of water at the surface and condensation of water vapor in the troposphere.