My thesis in 400 words: Caroline Piaulet-Ghorayeb

Caroline Piaulet-Ghorayeb recently completed her PhD degree at the Université de Montréal. Here, she summarizes her research project.

Almost thirty years ago, the discovery of a planet orbiting a star other than the Sun (exoplanet) revolutionised our vision of our place in the Universe. With more than 5,000 exoplanets confirmed, we now know that most stars harbour planetary systems. However, only a few dozen exoplanet atmospheres have been studied to date, most of them belonging to gas giants, which are more accessible. In the context of my thesis, I explored the diversity of atmospheres of exoplanets of all sizes, from small terrestrial planets to gas giant planets, to establish the link between their composition and their formation and evolution history. 

I began by looking at WASP-107 b, a planet about the size of Jupiter but around 10 times lighter, sometimes referred to as a “cotton candy planet”. After four years of observations with the Keck telescope in Hawaii, we obtained a new measurement of its mass, which is too low to explain its formation so close to its star (only 5.5% of the distance from the Earth to the Sun). We concluded that this type of planet could have formed much further away from its star and come close to it much later. 

I then studied the small planets Kepler-13 c and d (about 1.5 times the size of Earth), using data from the Kepler, Hubble and Spitzer space telescopes. By studying the motion of these planets, I concluded that their density was compatible with a water-rich composition, extending from their gaseous atmosphere to a high-pressure supercritical ocean above the rocky core. This would be a new type of planet, a warmer version of the icy moons that are found around the giant planets in our solar system like Europa and Ganymede. This hypothesis needs to be confirmed by further observations. I then explored the atmospheric composition of another small planet, GJ 9827 d, for which our observations with the JWST space telescope detected the presence of large quantities of water, compatible with the scenario proposed for Kepler-138 c and d. 

Finally, I analysed JWST observations of TRAPPIST-1 d, which is about 80% the size of the Earth and could harbour an ocean of liquid water. Despite the high precision of our observations, we were unable to confirm the presence of an atmosphere: either it is very thin, or it has been lost over time due to irradiation by the host star.

Thanks to increasingly precise observations, we are entering a fascinating era of discoveries that reveal the diversity of exoplanet compositions and bring us closer to the study of potentially habitable planets. 

More information 

Caroline completed her PhD degree between 2019 and 2024, under the supervision of IREx professor Björn Benneke. Her thesis is available on Papyrus.