Louis-Philippe Coulombe, a PhD student at IREx, recently completed his degree at the Université de Montréal. Here, he summarizes his doctoral research on exoplanet atmospheres and the James Webb Space Telescope.
From three decades of surveys, astronomers have revealed a veritable zoo of exoplanets, planets that orbit other stars. These planets range from sizes and temperatures like that of Earth, all the way to planets larger than Jupiter, and reaching temperatures close to that of stars. This diversity raises a fundamental question: what determines the chemistry, temperatures, clouds, and weather of these distant worlds?
Artist’s impression of LTT 9779 b, an ultra-hot Neptune-sized exoplanet. Observations with the James Webb Space Telescope revealed variations in the planet’s temperature and reflectivity, showing that its western hemisphere is colder and cloudier, likely due to winds transporting heat eastward. Credit: Benoît Gougeon (UdeM)
For planets that pass in front of their stars from our point of view, astronomers can use several techniques to study their atmospheres. By observing different wavelengths (colours) of light, we can measure light reflected by a planet, heat emitted by it, and starlight filtered through its atmosphere. Together, these measurements reveal the composition, temperatures, and clouds of these alien worlds.
Before the James Webb Space Telescope (JWST), our ability to study exoplanet atmospheres was limited. One year after the start of my PhD, JWST launched and rapidly revolutionized the field. My PhD research aimed to determine and explore the capacities of the JWST for exoplanet atmosphere characterization via reflected light, thermal emission, and transmission spectroscopy.
In my first study, I investigated WASP-18 b, an extremely hot, Jupiter-sized planet. When the planet passed behind its star, JWST allowed us to isolate the light coming from the planet itself. We found a planetary atmospheric composition similar to that of the Sun and produced a map of the planet’s scorching dayside, revealing powerful forces shaping its winds.
Next, I used JWST’s NIRISS/SOSS instrument to study LTT 9779 b, an ultra-hot Neptune planet throughout an entire orbit around its host star. We found that the planet appears bright at shorter wavelengths because its atmosphere reflects starlight, while at infrared wavelengths we instead see heat emitted by the planet. This allowed us to map its reflectivity and temperature, revealing a western hemisphere that is colder and cloudier, likely because winds carry heat eastward.
Image courtesy of Louis-Philippe Coulombe.
Finally, I analyzed the transit of the warm super-Earth TOI-270 b, observed with the NIRSpec instrument onboard JWST. By refining measurements of the planet’s density and modeling its interior structure, we found that the planet could contain a few percent water by mass. JWST observations also revealed hints of water vapour in its atmosphere, making TOI-270 b a promising candidate for a water-rich world.
Throughout my PhD, I demonstrated how JWST can reveal the chemistry, temperatures, clouds, and weather of planets spanning a remarkable range of sizes and temperatures. Together, these observations bring us closer to understanding the extraordinary diversity of planetary atmospheres beyond our Solar System.
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