Origins & Orbital Evolution of Hot and Warm Jupiters

Kurzfassung

Hot Jupiters were the first planets to be discovered in orbit around Sun-like stars. These first discoveries were surprising since this class of planets was not known from the Solar System. Their existence raised many questions, like how they got into these close orbits and how long they can survive there. This is because of the intense stellar irradiation that they receive, but also due to the strong tidal forces that they experience from being so close to their stellar hosts. While these tides act to erase the imprints of formation and migration from these planets, they can also lead to shrinkage of the planetary orbits due to angular momentum transfer from the planet to the star. This is because if the star is rotating more slowly than the planet is orbiting, the tidal bulges created by the tidal forces on the star are dragged around the stellar surface, lagging behind the sub-planetary point due to the friction within the star. Effectively, the stellar rotation is being accelerated by the tides that the planet is causing, leading to the planet to lose orbital angular momentum and its orbit to shrink slowly. Warm Jupiters, on the other hand, have longer orbital periods and are thought to be tidally detached from their stars due to greater orbital separations, meaning that they should stay in their primordial orbital configurations. This makes it possible to infer their origins more easily, if the system can be well characterised. This thesis addresses these topics with investigations of multiple hot and warm Jupiter systems. The former are examined for signs of orbital decay, i.e. a change of the planetary orbital period, and other sources of transit-timing variations. These could hint at the presence of nearby planetary companions, of which only a few have been discovered in these extreme systems. For the warm Jupiter in the sample, the so-called spin-orbit angle, meaning the angle between the stellar spin axis and the planetary orbital axis, is measured via the Rossiter- McLaughlin effect and compared to existing measurements of similar planets. The orbital decay and transit-timing variation study of the hot Jupiters were enabled by the precise photometry of the CHEOPS and TESS space telescopes, while the Rossiter-McLaughlin effect for the warm Jupiter in this sample is observed with radial velocity measurements of its host star during the planetary transit. In total, four hot Jupiter systems, KELT-9, KELT-16, WASP-4 and TOI-2109, are examined in Harre et al. (2023a, Paper I), Harre and Smith (2023, Paper III) and Harre et al. (submitted, Paper IV), with one warm Jupiter system being investigated in Harre et al. (2023b, Paper II). In the study of the hot Jupiter systems, their orbital decay rates were measured, and while it was not possible to confirm the orbital decay of any of the systems here, lower limits for their stars’ modified stellar tidal quality factors, a measurement that describes the efficiency of tidal interactions, were calculated. These can, if a large enough sample is established, be used to constrain the stellar interiors. For WASP-4 b, it was found that a previously discovered apparent orbital decay trend in the measured transit timings can be well described by the introduction of light-time effects due to the presence of a distant massive planetary companion, which has been recently discovered. For TOI-2109 b, the closest hot Jupiter to its star discovered so far, a sinusoidal trend in the transit timings was identified, hinting at the existence of a close-by outer companion. Only five other hot Jupiters with close-by companions have been discovered so far, only one of which hosts a close outer companion. Due to the low amplitude of the timing variations, the planetary companion could not yet be fully confirmed or characterised, though. Nevertheless, when correcting for the most likely timing variation trend, a tentative orbital decay rate is measured at 3σ. Regarding the origin of these planets, the presence of a close-by companion has interesting implications for their formation and migration, being largely inconsistent with high-eccentricity migration, which is thought to be the main mechanism that delivers hot Jupiters into their tight orbits. The measurement of spin-orbit alignment in the warm Jupiter system WASP-106 is in line with a quiescent formation or migration scenario, as is thought to be the prevalent mechanism for warm Jupiter migration. Most of the 16 systems that have this measurement taken imply the same, with only two of these systems showing moderate spin-orbit misalignments, that are slightly inconsistent with this theory. In summary, this thesis addresses the fundamental questions of exoplanet formation and migration, as well as system evolution by examining transit-timing variations of hot Jupiters under the influence of tides and potentially gravitational interactions of planetary companions, and also by measuring the alignment of the stellar obliquity in a warm Jupiter system via the Rossiter-McLaughlin effect. This work presents a step forward in the ongoing process of increasing our knowledge of the origins of hot and warm Jupiters and system evolution, highlighting the value that observations of these kinds of planets carry.