Search for habitable planets eclipsing ultra-cool stars

As the technology behind instrumentation in astronomy improves, so does our ability to detect and characterise planets outside our solar system. With missions such as the James Webb Space Telescope (JWST), the field of exoplanetology is about to enter a new era where the characterisation of earth-sized temperate planets will become feasible.
Expected to be launched later this year, JWST will be able to identify rocky planets' atmospheres and search for molecular signatures of habitability and life. While more and more potentially habitable exoplanets are being discovered, knowing which targets to prioritise is of paramount importance. In that respect, planets orbiting ultra-cool stars have been identified as the most amenable ones for the first atmospheric exploration of potentially habitable terrestrial worlds in the next decade.

In this dissertation, I focus on the detection and characterisation of planets in the habitable zones of ultra-cool stars.
In my first Chapter, I introduce the concepts and notions necessary for the reader to understand the context, content, and challenges of this thesis. Using astrobiology as the guiding theme, I briefly expose some basic notions on the definitions of life, discuss its detectability on exoplanets and justify the opportunity offered by planets orbiting ultra-cool stars in the search for life elsewhere in the Universe.

In the second Chapter, I present my contribution to the SPECULOOS project (Search for habitable Planets EClipsing ULtra-cOOl Stars) which aims to search for terrestrial planets orbiting the smallest stars and substellar objects (<0.15 $R_\odot$) of the solar neighbourhood, using the transit method. I explain my implication in various aspects of SPECULOOS: from technical monitoring to data management and analysis, to the identification of new planetary candidates. I also present a versatile tool that I have developed to manage and optimise the observations of SPECULOOS targets by the different telescopes. Finally, I expose the performances of the survey and discus some specific discoveries.
In the third chapter, I explain the methods used to turn astronomical images into light curves. Then, I introduce different algorithms that I relied on during my PhD to analyse those light curves and deduce meaningful information on exoplanets and their host star.
The first exoplanetary system revealed by SPECULOOS is the TRAPPIST-1 system, which is composed of a nearby ultra-cool star orbited by seven transiting planets, three of which lie within the star's habitable zone. This system represents a truly unique opportunity for a detailed study of potentially rocky, temperate, Earth-sized exoplanets, and has therefore galvanised the exoplanet community to study it in details, both observationally and theoretically. During my thesis, I had the chance to be strongly involved in these studies.
In Chapter 4, I present the outcomes from the analysis I led of an intensive follow-up campaign carried out with the Spitzer space telescope. Through those analyses and thanks to the transit method, I was able to significantly improve the precision on several key parameters of the planets and star, and even to set upper constraints on the dayside temperature of the two inner planets. Such results will reveal particularly interesting to prepare the follow-up of the system with the JWST. In parallel, I discuss the impact of flares on the habitability of TRAPPIST-1 habitable zone planets, and notably infer where the planets stand with regards to the abiogenesis zone. Finally, I derive precise timings for all transits of the seven planets and explain how those were used to refine the planet masses using in a subsequent dynamical analysis.
I also had the opportunity to be involved in the follow-up of the TRAPPIST-1 system from the ground. In Chapter 5, I explain how I used a multitude of observations in various bandpasses to construct the broadband transmission spectra of the seven planets. From these spectra, I managed to set the first empirical constraints on the impact of stellar contamination (resulting from the presence of heterogeneities of the stellar's photosphere), and compare it to predictions from existing models. I then propose alternative morphologies (sizes and temperatures) for those photospheric heterogeneties and discuss their relevance. Ultimately, I present some on-going work that I am carrying out to unveil the nature of the stellar phostosphere using the star's multi-bandpasses photometric variability.

Finally, in my last Chapter, I summarise the main results presented in this thesis and raise some perspectives for the study of the TRAPPIST-1 system and the future of the SPECULOOS project, as well as some prospects for the work I will be undertaking after my PhD.