Into the stellar glare

Winterhalder, Thomas (2025): Into the stellar glare: leveraging astrometry to directly detect and characterise substellar companions and their satellites. Dissertation, LMU München: Fakultät für Physik

Abstract

The direct detection and characterisation of gas giant exoplanets is a key method for investigating the formation, evolution and atmospheric properties of these distant worlds. Yet, at present, the intense glare of the host stars and instrumental intricacies of the technique limit its application to massive planets orbiting their host stars on wide, far-out orbits. To shed light on the formation history of our own Solar System and assess how representative it is among other stellar systems, we need to expand the sample size of well-characterised exoplanets at short orbital separations. In this thesis, I present a novel approach to extending our direct detection sensitivity towards lower companion masses and shorter orbital separations. This effort builds on the combination of the astrometric and direct detection methods and enables a thorough and efficient characterisation of individual planets. In this sense, the work presented in this thesis lays the groundwork for building a population-level sample of directly detected, short-separation exoplanets. The structure in which the different topics are presented is intended to convey a coherent narrative that illustrates an advance in our direct detection targeting capabilities. This underlying theme draws a line from mostly blind direct imaging studies to semi-targeted approaches based on an informed target selection of stars likely to host an exoplanet, to ultimately arrive at a fully-targeted technique that is capable of predicting the exact position of a planet candidate relative to its host. After a brief introduction of the topics upon which this thesis rests, I present an updated study of HIP 99770 b, the first exoplanet directly detected on the basis of long-term proper motion irregularities presented by its host star. Drawing on data obtained by the GRAVITY interferometer, I constrain the orbital solution of the companion, infer its age and determine a set of atmospheric parameters. The results obtained from this thorough analysis offer valuable insights into the potential and limitations of the proper motion technique and will eventually help clarify the formation history of HIP 99770 b. I next demonstrate how astrometric data collected by the Gaia space telescope can be used to reliably predict the position of substellar companions relative to their hosts. This new technique differs from the proper motion method in that it facilitates precise and efficient GRAVITY follow-up observations in a fully targeted manner. After the successful detection and confirmation of eight candidates, I show how the combination of the underlying Gaia data with a single GRAVITY astrometric epoch can enable tight constraints on the orbits and dynamical masses of the targeted companions and thereby reveal their true nature: five of the newly detected companions are shown to be substellar. I continue by applying this technique to companion candidates in the planetary mass regime. After describing the target selection process, I present an analysis of the observational data we obtained for a single companion candidate and show that the target system actually corresponds to a stellar binary. I address how these objects can be mistaken for planet-hosting systems and discuss the non-detection in the context of other studies investigating the false-positive contamination of the Gaia data set. Applying the same method Gaia employs to detect planets around stars, I next demonstrate how the unmatched astrometric precision enabled by optical and near-infrared interferometry can be exploited to detect moons around exoplanets. Simulating the gravitational perturbations they induce in planetary orbits, I compute the first exomoon sensitivity curves for different interferometric instruments. To date, no such object has been robustly detected. Yet, they are likely to exist in large numbers and -- once detected -- exomoons will impact current theories of planet formation and our search for habitable worlds. Finally, I provide an outlook at what lies ahead for the techniques developed in this thesis. From the likely implications of upcoming data releases and observational facilities on exoplanet science to the prospects for exomoon detection and characterisation in the more distant future, I discuss a set of flagship science cases that serve to further contextualise the progress made in this thesis. Overall, the work presented in this thesis demonstrates how to harness precision astrometry for informing follow-up observations and characterising gas giant exoplanets. It highlights the potential of leveraging synergies between different detection methods and the unique capabilities that optical and near-infrared interferometry can bring to exoplanet research. All things considered, it paves the way towards building a population-level sample of directly detected planets resembling those we see in our own Solar System and understanding its formation history.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 12. September 2025
• 1. Bericht­erstatter:in: Glindemann, Andreas