My thesis in 400 words: Charles Cadieux

Charles Cadieux recently completed his Ph.D. at the Université de Montréal last fall. Here, he summarizes his doctoral research project.

One of the big surprises in the rapidly evolving field of exoplanets over the past 30 years is that the most common exoplanets are intermediate in size between Earth and Neptune. A marked separation around 1.7 times the size of Earth distinguishes classes of exoplanets that are rocky, the super-Earths, from gas-rich sub-Neptunes.

During my PhD, I focused on three temperate super-Earth systems around M-type stars (red dwarfs), smaller and less massive than the Sun. My aim was to constrain the internal and atmospheric composition of the super-Earths in order to, among other things, test theories of planetary formation. To do this, I used the transit method, which measures the size of an exoplanet and, in some cases, the chemical composition of its atmosphere. I also used Doppler spectroscopy to determine the mass of exoplanets by measuring the velocity variations they induce on their host star.

A first key result of my thesis was the discovery of TOI-1452 b, a temperate super-Earth orbiting every 11.06 days around a red dwarf in the solar neighborhood. Our estimate of TOI-1452 b’s mass and radius, coupled with internal structure models, suggest that 20% of this planet’s mass could be water. TOI-1452 b is therefore a potential water world, a type of exoplanet predicted in theory but not yet formally confirmed by observations. These water worlds have a low density, suggesting a much richer water composition than Earth or the other Solar System’s rocky planets.

Next, we revisited the LHS 1140 system. An analysis of archival observations enabled us to refine the radius and mass measurements, revealing that one of the two super-Earths, LHS 1140 b, is less dense than previously estimated. The hypothesis that this is a mini-Neptune has also been ruled out thanks to observations obtained with the James Webb Space Telescope in December 2023. These observations also suggest the possibility of this planet having a nitrogen-rich atmosphere. If confirmed, this would be the first detection of an Earth-like atmosphere on a potentially habitable exoplanet!

Finally, I worked on the L 98-59 system, which comprises four small exoplanets in compact orbits of less than 12 days. Due to the gravitational interaction between the planets, the precise moment when they pass in front of their star varies. An analysis of these slight variations has revealed the detailed architecture of the system and confirmed the presence of a fifth planet, a super-Earth located in the habitable zone. This system features exoplanets with varied compositions, offering a unique opportunity to study planetary formation mechanisms.

In short, these discoveries, along with several other recently published results in the literature, point to the existence of water worlds. Around M dwarfs, these planets are likely to have formed far from their host star, where water is more abundant, before migrating inwards, where they are found today.

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Charles completed his PhD at the Université de Montréal between 2020 and 2025, under the supervision of Professor René Doyon of IREx. His thesis, Caractérisation de super-Terres tempérées autour d’étoiles de faible masse pour contraindre leur composition interne et atmosphérique, is available (in French) online.