My thesis in 400 words: Raphaël Hardy

Raphaël Hardy completed his Ph.D. at the Université de Montréal in February. Here, he summarizes his doctoral research project.

Hot Jupiters are fascinating and unique laboratories for planetary study. These giant exoplanets orbit their host stars in less than 10 days and reach temperatures above 700°C. Because they are so close to their stars, they are tidally locked. This means that one hemisphere always faces the star (the “day side”) and the other is permanently in the dark (the “night side”). It is similar to how the Moon always shows the same face to Earth. This creates a significant temperature difference between the two sides. As a result, very strong winds emerge, that can blow at over 30,000 kilometres per hour. These winds help redistribute heat from one side of the planet to the other.

Repeated observations have confirmed the presence of alkali metals like potassium and sodium in the atmospheres of these hot Jupiters. These metals are partially ionized, meaning they have lost some electrons. Because of this, they can interact with the planet’s global magnetic field. This interaction is highly sensitive to temperature. When partially ionized winds meet the magnetic field, the atmosphere heats up. Under certain conditions, this heating process can spiral out of control. The temperature then rises very quickly, by several hundred degrees. This phenomenon is known as thermo-resistive instability. It stops when the atmosphere gets hot enough for the magnetic field to generate waves that slow down the winds.

During my PhD, I worked on simulations that help us understand what happens in the atmospheres of hot Jupiters. We use magnetohydrodynamic models. These models describe how a partially ionized fluid, such as a planet’s atmosphere, moves in the presence of a magnetic field.

We developed three different models to study thermo-resistive instability. Each one has a different level of complexity.

• The first is a local, zero-dimensional model (0D). It focuses on a single point in space. This model allowed us to identify the temperature, pressure, and general conditions in which the instability occurs. We also discovered that the instability tends to recur every few weeks to a few months.
• The second is a one-dimensional model (1D), where only the radial dimension, i.e. the distance from the centre of the planet, is considered. This helped us understand how the instability spreads through the atmosphere and how the different layers, each with different pressures, interact.
• The third is called a pseudo-2D model. It builds on the 1D radial model but approximates a second, longitudinal (east–west) dimension. It focuses on a narrow equatorial band of the planet. This model allowed us to simulate how the hot spot (the hottest region on the day side of the planet) and the overall planet’s brightness evolve during the instability. We realised that the hot spot can shift by as much as 60 degrees in longitude, and that sudden changes in brightness are possible. These effects could potentially be observed with future instruments.

We concluded our study of this instability with a final, more mathematical project. In it, we explored the nonlinear and chaotic behaviour of the local model, which appears under specific conditions.

To learn more
Raphaël completed his PhD at Université de Montréal from 2020 to 2025 under the supervision of Professors Andrew Cumming (McGill University) and Paul Charbonneau (Université de Montréal). His thesis is available on Papyrus.