Space Journey Beyond Earth: Exploring the properties of exoplanets and multi-planetary systems

Doktorsavhandling, 2026

The discovery of more than 6300 planets beyond the Solar System has revealed a broad range in the physical and orbital properties of exoplanets. In contrast to this observed diversity, planets within many multi-planetary systems exhibit pronounced intra-system similarities in their radii, masses, and orbital spacings. This thesis investigates the planetary properties and system architectures, focusing on the observed intra-system similarities, the inferred planetary interior compositions, and the differences between single- and multi-planetary systems.

Paper A shows that planets within the same system tend to have similar orbital spacings even when their sizes or masses differ substantially. The spacing similarity is more pronounced in orbital period ratios than in mutual Hill separations, while the period ratios are more strongly correlated with the masses than with the radii of adjacent planets. These results indicate that uniform orbital spacings represent a common architectural property and are not necessarily associated with similarities in planetary size or mass.

Paper B compares transiting planets in observed single- and multi-planetary systems (singles and multis, respectively). After excluding hot Jupiters, the distributions of planet types and radii of singles multis orbiting FGK stars show no significant differences, suggesting a common underlying planet population. Around FGK host stars, however, an unexpected overabundance of multis is identified within a specific planet radius range. In contrast, singles and multis hosted by M dwarfs exhibit differences in their planet-type and radius distributions, although the small sample sizes render these results less conclusive.

Paper C finds that non-gas-giant planets in systems hosting detected gas giants are, on average, larger, more massive, and more irregularly spaced than planets in systems without detected gas giants. Adjacent planets exhibit significant similarities in radius, mass, and bulk density, whereas their inferred core and water mass fractions show substantial scatter and no strong correlations. These results indicate that similarities in bulk properties do not necessarily translate into similarities in interior compositions.

Together, these studies place new observational constraints on the processes governing planet formation and dynamical evolution, while also demonstrating that the observed planet multiplicity and intra-system similarities in mass, radius, and orbital spacing provide only a partial representation of planetary systems.