Kim Morel, a MSc student at IREx, recently completed her degree at the Université de Montréal. Here she summarizes her research project
When an exoplanet passes behind its star during its orbit, the total flux we receive from the system decreases, as the star momentarily blocks the light coming from the orbiting planet. This is known as a secondary eclipse. Observing this slight brightness decrease makes it possible to study the planet’s intrinsic radiation (its “thermal emission”) and the starlight it reflects. By decomposing the observed light into its different wavelengths, i.e. its various colors, we can also gather valuable information about the exoplanet’s atmosphere.
During my master’s degree, I attempted to better understand the atmosphere of the hot jupiter WASP-80 b using secondary eclipse observations obtained with the NIRISS instrument aboard the James Webb Space Telescope. Hot jupiters are gas giants with high temperatures due to their proximity to their star. This proximity also means that their rotation period is synchronized with their orbital period, with the same side always facing the star. These synchronized planets have a “day side” and “night side”. It’s precisely this “day side” that we observe, just before the star hides the planet.
By observing the thermal emission of these exoplanets, we can study how temperature varies with altitude in their atmosphere (on the day side) and also determine its composition. By studying their reflected light, we can determine whether aerosols (clouds or haze, for example) are present in the atmosphere, as it is these that reflect light. Aerosols, by reflecting part of the light coming from the star, prevent the energy contained in this radiation from penetrating the planet’s atmosphere. They can therefore affect the amount of energy a planet receives, and in turn, its equilibrium temperature.
For the planet WASP-80 b, we’ve discovered that some of the star’s light is being reflected because we can see a glow at certain wavelengths where the planet itself doesn’t give off much heat. By comparing the amount of light reflected as a function of wavelength with models of reflection by clouds, we determined that clouds of chromium or sodium sulfide could be present in its atmosphere. Furthermore, our measurement of reflected flux combined with our temperature measurement of its “day side” suggests that this planet redistributes its heat to its “night side” quite efficiently.
Hot jupiters are interesting objects for astronomers, as they do not exist in the Solar System. By determining the composition of their atmosphere and the meteorological processes at their surface, we can obtain information about their formation and evolution.
Kim completed her MSc degree between 2022 and 2024, under the supervision of IREx professor David Lafrenière. Her thesis will be available soon on Papyrus.
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