Caracterização de exoplanetas rochosos na zona de habitabilidade das estrelas hospedeiras

Abstract

The main objective of this work is the search of astrobiologically interesting planets through the physical characterization of rocky exoplanets within the habitable zone (HZ) of their host stars. To do this, we calculated the HZ for a sample of 9 potentially rocky exoplanets, identified by the Kepler mission, orbiting single stars with known physical and orbital parameters. We used a radiative-convective climate model (Kopparapu et al. 2014) and found that 2 of them, Kepler 22-b and Kepler 298-b, are very close to the inner edge of the HZ; 3 of them, Kepler 174-d, Kepler 186-f and Kepler 441-b, are near to the outer limit; and the remaining planets of our sample, Kepler 283-c, Kepler 440-b, Kepler 442-b and Kepler 443-b, are very well settled within the HZ of their host stars. Considering the planet-star distance and the luminosities of the host stars, and using the climate model, we were able to identify the possible type of atmosphere (Earth or Mars type) that these planets may have, and also the limits for surface temperature and pressure. Up to now no rocky planet has been detected in binary systems, however, we also present an alternative method to that one proposed by Kaltenegger et al. (2013) e Haghighipour et al. (2013) for the calculation of HZ in this kind of systems. We verified that the HZ can be determined by the calculation of the roots of a fourth-order polynomial with its coefficients depending on the true anomalies of the planet and the secondary star, the distance between the stars, the eccentricity of the binary system and the boundaries of the HZ in solar system. This method was applied to 3 circumbinary systems discovered by the Kepler mission: Kepler 16, Kepler 47 and Kepler 453, and our results are in a good agreement with the literature. As we know from Earth, a planet to be astrobiologically interesting has to be able to keep liquid water on its surface for a long period of time, in the order of billions of years, to allow the origin and development of life. Therefore, we have carried out a dynamical stability study of the planets of our sample, which belong to multiplanetary systems (Kepler 174, Kepler 186, Kepler 283 and Kepler 298), using the SWIFT symplectic integrator. As a result, we found some orbital configurations where Kepler 174-d, Kepler 186-f and Kepler 283-c, remain within the HZ along the complete integration time of the simulation. We conclude that Kepler 442-b, Kepler 443-b and Kepler 186-f are the most astrobiologically interesting planets in or sample, because they lie within the HZ of their host stars and according to their physical characteristics, they are rocky planets. The cases of Kepler 442-b and Kepler 443-b, are most likely to have an Earth-like atmosphere with surface temperatures between 240-320 K and 240-280 K, respectively, and surface pressures of ~1,4 e ~5,6 bar respectively. On the other hand, Kepler 186-f is more likely to have a Mars-like atmosphere with a surface temperature greater than 273 K and a surface pressure of 7.23 bar. Due to the temperature range and the higher pressures, comparing to Earth, all of these planets may support liquid water on their surfaces. However, it is extremely necessary to study the water phase diagram to confimr this scenario and the possible existence of liquid water. As a future perspective, it would be interesting to explore whether the planets could have atmospheres with other chemical compositions. One possibility is using the model atmosphere PHOENIX (Allard et al. 2012), trying to obtain theoretical spectra that will predict or not the existence of biosignatures features in the atmospheres of these planets, indicating an astrobiologically interesting scenario.