Alqasim, Ahlam Abdulghafoor M. Rafi; (2025) Characterizing Eccentric Exoplanet Systems and Investigating their Formation Pathways. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The orbital eccentricities of exoplanets provide key insights into their formation and dynamic evolution, and allow us to explore their origins and potential migration pathways. To understand and trace back what the eccentricity tells us about a planet’s history, it is important to precisely characterize the properties of eccentric exoplanets and investigate their broader implications on a statistical level. In this thesis, studies of transiting, long-period eccentric exoplanets are examined to answer these questions. The thesis first presents the characterized properties and possible formation history of TOI-757 b, a mini-Neptune on a 17.5-day orbit, using high-precision radial velocity measurements and space-based photometry. The planet was found to be highly eccentric (e = 0.39), with its formation likely influenced by higheccentricity migration due to an outer companion or early star-star interactions given the system’s old age and low metallicity. The TOI-757 system is then re-examined, and various diagnostic tests are explored to address challenges arising from current radial velocity fitting methods, particularly in systems with moderately active stars where planetary periods are similar to stellar signals. In a complimentary study to this, a broader population-level analysis of 92 transiting, long-period giant (TLG) planets was performed to explore the correlation of eccentricity with planet and host star properties. The results revealed that the host star metallicity plays a significant role in the eccentricity evolution of TLGs, a dependence which was only previously proven for close-in giant planets (Hot Jupiters). Additionally, when probing high-eccentricity tidal migration scenarios, planet-planet scattering was identified as a more dominant mechanism than the Kozai-Lidov effect, supported by a lack of eccentricity dependence on the presence of stellar companions, a lack of strong tidal dissipation effects, and an anti-correlation trend between eccentricity and planet multiplicity. These findings provide insights into the diverse origins of planetary eccentricities.

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