My thesis in 400 words: Pierre-Alexis Roy

Pierre-Alexis Roy, a student at IREx, completed his PhD at Université de Montréal in August 2025. Here, he summarizes his doctoral research project.

It was in 1995 that the discovery of a Jupiter-like planet orbiting a Sun-like star opened a new chapter in astronomy: the study of exoplanets. Since then, astronomers have discovered thousands of planets orbiting stars across our Galaxy. This allowed us to ask: Are all planetary systems like our own? The answer is an obvious no. One of the reasons is that the vast majority of exoplanets we find are between the sizes of Earth and Neptune. We often call them sub-Neptunes. Since there are no sub-Neptunes in our Solar System, they remain a true mystery. What are they made of? Are they all alike? How do they form? Why are they so common?

These are exactly the kinds of questions I tried to answer during my PhD. To do so, I used space telescopes, including Spitzer, Hubble, and James Webb, to observe sub-Neptunes as they passed in front of their star, causing a tiny dip in its brightness, like a miniature eclipse, or transit. By observing the same phenomenon in spectroscopy, that is, by splitting the light into its different colours, I was able to study the chemical composition of their atmospheres. The starlight that grazes the planet is filtered through its atmosphere, and the specific absorption signal of each molecule is encoded in that dip in brightness.

These observations first allowed me to discover that TOI 824 b, a very hot and very dense sub-Neptune, likely lost most of its atmosphere in a collision with another planet. They also revealed that another planet, GJ 9827 d, is likely a water world: a planet whose atmosphere is made up mostly of water vapour, without hydrogen, very different from our own Neptune. Finally, by comparing the cold sub-Neptune LP 791-18 c to its counterparts K2-18 b and TOI-270 d, I was able to show that even sub-Neptunes that are nearly twins in terms of density and temperature can have very different atmospheres, in both cloud cover and chemical composition.

After five years of studying sub-Neptunes, we realise how incredibly diverse they are. Every time we observe one, we never know what we’ll find. It’s important to keep finding and studying them, so that one day we can truly understand the nature of sub-Neptunes. After all, they are the most common type of planet in the Galaxy!

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Pierre-Alexis completed his PhD at Université de Montréal between 2021 and 2025, under the supervision of Professor Björn Benneke (Université de Montréal). His thesis, “ Sonder la nature et l’origine des exoplanètes sous-Neptuniennes” (Probing the Nature and Origin of Sub-Neptunian Exoplanets), is available on Papyrus.