Spectra of Earth-like Exoplanets with Different Rotation Periods

Abstract

At present, planets like the Earth have been found near other stars. We investigate the spectra of Earth-like planets but with different axial rotation periods. Using the general circulation model of the atmosphere called the Community Climate Model (CCM3) and considering the atmospheric circulation lasting for two years, we calculated the radiation spectra of the Earth and the exo-Earth rotating with periods of 1 and 100 days, respectively. The radiation spectra of the atmospheres were calculated with the SBDART code. We analyzed the spectrum of upward radiation at altitudes of 1 and 11 km in wavelength ranges of 1 to 18 μm and 0.3 to 1 μm. The following common features were obtained for the Earth and the exo-Earth: (1) the planets exhibit a wide absorption band of CO₂ around 14 μm; (2) the radiation spectra at different locations near the equator show no significant differences (however, for some regions, e.g., near the poles, there can be considerable differences in the spectra); and (3) if the spectrum is integrated over the entire disk of the Earth/exo-Earth, the difference in the spectral signal obtained in observations from different directions becomes substantially lower than the difference between the results of observations of individual regions of the planets; however, the difference in the integrated signal of the spectrum for the Earth and the exo-Earth is noticeable (for example, this difference is noticeable for the spectrum obtained at an altitude of 11 km, when observing the South and North Poles; though, the difference is small, if one observes the whole disk from different equatorial directions). The differences in the spectra of exoplanets, which differ from the Earth only in axial rotation period, are comparable to the differences associated with changes in the angle of viewing the planet. Consequently, if the observation angle is not known, the analysis of the spectrum of the planet cannot be used to determine its axial rotation period. The maximal differences in the spectra of Earth-like exoplanets were obtained for wavelengths of about 5–10 and 13–16 μm. By analyzing the spectrum at wavelengths around 9.4–10 µm, we can determine whether the atmosphere of the exoplanet contains ozone or not. In the diagrams for the upward radiation at an altitude of 11 km, there is no local minimum at wavelengths of 9.4–10 µm if ozone is absent; and, when the models contain ozone, this minimum is present. Since ozone is essential for life, the 9.4–10 µm band may be important for future observations of Earth-like exoplanets.