The evolution of giant planets

Abstract

As a whole this dissertation aims to understand giant planets as an entire class of astronomical objects. Initially we investigate the mechanics and evolutionary effects of phase separation in the deep interiors of giant planets. We present the first models of Saturn and Jupiter to couple their evolution to both a radiative-atmosphere grid and to high-pressure phase diagrams of hydrogen with helium and other admixtures. We find that previously calculated hydrogen-helium phase diagrams in which Saturn's interior reaches a region of predicted helium immiscibility do not allow enough energy release to prolong Saturn's cooling to its known age and effective temperature. We explore modifications to published phase diagrams that would lead to greater energy release. Alternatively, we also explore the evolutionary effects of the phase separation of an icy component. We then expand our inhomogeneous evolutionary models to the evolution of hypothetical extrasolar giant planets (EGPs) in the 0.15 to 3.0 Jupiter mass range, incorporating helium phase separation using the hydrogen-helium phase diagram we have calibrated to Jupiter and Saturn. We show how phase separation increases the luminosity, effective temperature, and radii, and decreases the atmospheric helium mass fraction, for various giant planets as a function of age. We also show the effects of irradiation and dense cores. Next we turn to the atmosphere of the transiting EGP, HD209458b. Using a self-consistent atmosphere code, we construct a new model of the planet's atmosphere to investigate the disparity between the observed strength of the sodium absorption feature at 589 nm and the predictions of previous models. For the atmospheric temperature-pressure profile we derive, silicate and iron clouds reside at a pressure of several mbar in the planet's atmosphere. These clouds lead to increased absorption in bands directly adjacent to the sodium line core. Using a non-LTE sodium ionization model, we show that ionization leads to a slight weakening of the sodium feature. The sensitivity of our conclusions to the derived atmospheric temperature-pressure profile is discussed. We show how our investigation leads to a better understanding of how the planetary radius measurements should be compared to model radii.