A groundbreaking map of an ultra-hot giant exoplanet: interview with Louis-Philippe Coulombe

Ultra-hot Jupiters have long captured astronomers’ curiosity. These giant gas planets orbit so close to their stars that they complete a full orbit in less than a single Earth day, reaching temperatures above 2000 °C. Thanks to NIRISS, the Canadian-built instrument on the James Webb Space Telescope (JWST), an international team led by Ryan C. Challener (Cornell University) and Megan Weiner Mansfield (University of Maryland) obtained the first-ever spectroscopic map of the dayside of an exoplanet, WASP-18 b.

To better understand the significance of this discovery, recently published in Nature Astronomy, we spoke with Louis-Philippe Coulombe, researcher at the Université de Montréal and a member of the Trottier Institute for Research on Exoplanets (IREx), who played a key role in the data analysis.

IREx: What makes this discovery special?

Louis-Philippe: This is the first time we’ve been able to create a two-dimensional map of an exoplanet at multiple wavelengths with JWST. The map shows in detail the planet’s dayside, the side that always faces its star.

It’s a powerful tool to study how temperature and atmospheric composition vary with location and depth. In short, it gives us both a horizontal and vertical view of this giant planet.

IREx: What personally fascinates you about this discovery?

Louis-Philippe: WASP-18 b is a truly extreme world: temperatures exceed 2000 °C on the dayside, generating supersonic winds that redistribute heat toward the dark nightside. There’s nothing like this in our Solar System! Studying such a planet really pushes our understanding of atmospheric physics to new limits.

IREx: How do you make a map like this?

Louis-Philippe: We point the telescope at the star during what’s called a secondary eclipse, the moment when the planet passes behind it. This allows us to analyse the planet’s light slice by slice: the gradual dimming tells us which regions are brightest.

By repeating the observation at different wavelengths, or “colours,” we can see what’s happening at various depths in the atmosphere.

IREx: How did this project come about, and what was your role?

Louis-Philippe: This work was carried out as part of JWST’s Transiting Exoplanet Community Early Release Science (ERS) observing programme, designed to showcase the telescope’s capabilities for exoplanet studies during its first months in operation.

In 2023, I published a paper in Nature presenting a single-wavelength map of WASP-18 b, which revealed a strong temperature difference between the planet’s centre and its edges on the dayside.

This new study goes much further: by combining several wavelengths, we can now probe different altitudes in the atmosphere. My contribution focused on data reduction and comparing the results with 3D simulations of heat circulation. Björn Benneke, now an adjunct professor at Université de Montréal, also helped develop the observing programme that led to this study.

IREx: What have you learned about WASP-18 b itself?

Louis-Philippe: We identified two distinct regions: a very hot spot at the centre of the dayside, where the planet receives the most energy, and a cooler ring near the terminator, the boundary between day and night.

Interestingly, the temperature contrast is smaller than expected. That suggests there are physical processes influencing how heat is redistributed that aren’t yet captured in our models, for example, cloud forming on the nightside, or chemical reactions involving hydrogen.

IREx: Why should people care about this discovery?

Louis-Philippe: This exoplanet is an extreme and fascinating world that helps us understand the physics and chemistry shaping the most intense planetary climates in the Universe.

Even though it’s found hundreds of light-years away, this technique lets us reveal what’s happening in its atmosphere, even deep below the visible layers. That’s quite remarkable!

This interview was adapted from a written exchange with Louis-Philippe Coulombe and the scientific paper in Nature Astronomy. It has been edited for clarity.

Learn more

The paper “Horizontal and vertical exoplanet thermal structure from a JWST spectroscopic eclipse map” was published in Nature Astronomy on October 28, 2025. The study was led by Ryan C. Challener (Cornell University) and Megan Weiner Mansfield (University of Maryland) with an international team. Louis-Philippe Coulombe and Björn Benneke, from Université de Montréal and the Trottier Institute for Research on Exoplanets (IREx), are co-authors.

Media contact
Nathalie Ouellette
Deputy Director and JWST Outreach Scientist for Canada
Trottier Institute for Research on Exoplanets (IREx), Université de Montréal
613-531-1762 | nathalie@astro.umontreal.ca

Scientific contact
Louis-Philippe Coulombe
Researcher
Trottier Institute for Research on Exoplanets (IREx), Université de Montréal
louis-philippe.coulombe@umontreal.ca