Gas Giants from a Protoplanetary Disk

Abstract

The Earth is not the only planet in the universe. It has been known for millennia that there are other planets and other bodies in the solar system. Moreover, as recent as, 1995 it has been confirmed that other stars also host planets. But how did they form? This is the main question that planet formation research has been trying to answer for at least a century. Since planet formation can not be observed directly due to instrumental limitations and limitations on human life times, this takes many generations. Each project contributing a small part to the puzzle that is planet formation. This thesis aims to address a small part of this puzzle. Planets are formed from same material as their host star. Interstellar space houses vast clouds of gas and dust. Under the influence of interstellar radiation, turbulence, magnetic fields, gravity and gas dynamics, pockets of higher density are formed in these clouds, called molecular cloud cores. Upon the collapse of a molecular cloud core (MCC), the material in the MCC will divide itself between the central body, the protostar, and a disk of material orbiting the protostar. This disk is mostly made up of gas and ∼1% of dust. Because planets are formed from these disks, these are referred to as a protoplanetary disks (PPDs). Observations of both PPDs and exoplanets are improving. However, current instruments are not able to observe planet formation in action. Therefore, models are used to predict the conditions and activities inside PPDs. This is where the availability of the thermo-chemical protoplanetary disk model ProDiMo presents the opportunity to link the observed composition of exoplanets to their formation path. ProDiMo is a 2D steady-state simulation code that solves the radiative and chemical equilibrium selfconsistently. Moreover, it generates a great deal of information about the PPD model. The temperature, pressure, density and chemical composition are just a few of the properties that it provides. Firstly, to benchmark the code, ProDiMo models are compared to a set of simple disk models. The comparison shows that ProDiMo is a very advanced code that is able to predict the PPD environment much better than other models that try to link the formation path to the composition. In addition AA Tau is found to be the most suitable PPD to perform planet formation. Because it is has parameters that are found close to median values. The ProDiMo model is used in combination with a mass accretion scenario. The formation scenario that is applied is called pebble accretion. In this scenario it is proposed that pebbles formed from dust at large radii drift to the inner disk where the pebbles are accreted by a planetesimal. As a result, the planetesimal can grow in mass up to tens of Earth masses (푀⊕), and is now called a core. The moment the core has a high enough mass and the pebble accretion has terminated early in the disk life time, before ∼3 Myr, it becomes a gas giant. This is done by accreting a massive amount of gas from the disk, and thereby clearing an annulus of gas from the disk. In the case of the AA Tau model, gas giants only form at radii between 2 and 8 AU. The largest gas giant at 4 AU has a mass of 110 푀⊕. By combining the model of AA Tau with the pebble accretion scenario, the composition of the resulting gas giants is determined. Planetesimal accretion leads to a stellar C/O of the ices in all of the cores formed between 0.5 and 25 AU. The atmospheres of gas giants formed between 2 and 4.8 AU, with C/O between 1.3 and 13, are up to an order of magnitude away from other similar models and observations. From 4.8 to 8 AU the model predicts a slowly decreasing C/O from 2.2 to 1.3. Compared to observations the C/O ratios computed for the AA Tau model are similar for planets formed beyond 4.8 AU. On the other hand, below 4.8 AU very large differences are seen. However, to determine why there is a large discrepancy between the computed C/O and the observed C/O, further research is required.