The chemical composition of hot and cold gas giants

Abstract: Exoplanet surveys have identified a category of giant planets that orbit very close to their host stars - the so-called hot Jupiters. The origin of such hot Jupiters is a long standing puzzle and has been closely investigated since the first discovery of such planets in 1995. The two competing formation paths suggests that they either migrated inwards during their formation process, through gravitational interactions with the disc, or that they formed further out in the disc and was scattered inwards later due to disc-free interactions. The question is, how do we separate between these two cases? One approach is to study their chemical composition. In this thesis I will study growth-tracks of hot and cold Jupiter planets that has been produced in a detailed model of an evolving protoplanetary disc, where all growth is due to the accretion of pebbles, and use these to deduce the chemical evolution of the core and gas envelope of such planets. Two sets of volume mixing ratios were used, where one is adapted from observations of gas and ice in protoplanetary environments and the other from theoretical calculations of a solar composition disc. The aim of the work is to calculate the carbon to oxygen number ratio of hot and cold Jupiter planets, where cold Jupiter planets have a semi-major axis larger than 1AU and represent the planets that might be scattered inwards due to disc-free interactions. For both sets of volume mixing ratios, I find that the gas envelope of hot Jupiters are oxygen dominated with a number ratio of around 0.7, while cold Jupiter planets has an equal amount of carbon and oxygen in their envelopes. The cores of both types of planets are heavily oxygen dominated, and when theoretically calculated volume mixing ratios are used some planetary cores even turn out to lack carbon completely. These results can be used to separate between the two formation paths of hot Jupiters. If an astronomer were to find a gas giant in a tight orbit with a C/O ratio in the envelope around unity, then it is most likely a cold Jupiter planet that migrated by interacting gravitationally with another planet or a binary star after the dissipation of the gas disc. To conclude, the carbon to oxygen number ratio in exoplanet atmospheres has shown to be useful when separating between the two formation paths of hot Jupiter planets.