Romain Allart, a Trottier postdoctoral researcher at the Trottier Institute for Research on Exoplanets (iREx) at the Université de Montréal, recently carried out a study that examined eleven gas giant exoplanets with the SPIRou instrument. SPIRou is a spectrograph designed in part in the laboratories of the Observatoire du Mont-Mégantic, and installed on the Canada-France-Hawaii telescope. He answers our questions about this study, which was published in the June 22, 2023 issue of Astronomy & Astrophysics.
iREx: Can you tell us more about what you did?
Romain: Thanks to SPIRou, our team has studied eleven giant exoplanets – ranging in mass from a little less than Neptune to a bit more than Jupiter.
We think that these exoplanets, which orbit really close their stars, might be losing some of their atmosphere because the stars are blowing it away. To determine whether this is the case, we have attempted to detect the presence of helium in the atmosphere of these exoplanets. This very light gas can be used to measure the size of a planet’s atmosphere, and the extent to which its atmosphere escapes into space.
iREx: Why study planets of this size in particular?
Romain: We know of thousands of exoplanets, and one of our objectives now is to better understand how these different worlds form and evolve.
For this project, of our main goals was to understand why we find very few Neptune-sized planets close to their stars, whereas larger and smaller planets are more commonly found at such close distances. Astronomers refer to this observation as the “Neptune desert”.
One possible explanation is that, due to the star’s radiation, exoplanets of this size could be swollen with puffed up atmospheres that partly evaporate. By losing a portion of their atmosphere, these planets would become smaller.
iREx: What did you discover?
Romain: For three exoplanets, we confirmed the presence of helium, which had previously been announced by other teams (including one I led during my thesis in Switzerland!). With our obervations, we tried to determine the extent to which these planets are losing their atmosphere, in what direction, etc. But these calculations remain difficult!
For the others, we’ve detected nothing, which allows us to assert that no more than a certain quantity of helium is escaping from the atmosphere. This doesn’t mean that these planets won’t lose their atmospheres, but it’s a start!
iREx: What distinguishes your study from what had been done before?
Romain: There have been a few studies looking for the presence of helium on a handful of exoplanets, but this is the first time that a large number of exoplanets have been studied in a uniform way. This is essential in order to rule out other reasons why helium may or may not be detected: for example, instrument-related causes, the way the results are analyzed, or the stellar activity of the stars.
iREx: What do you like about this subject?
Romain: First of all, I find it fascinating to think that exoplanets can lose their atmospheres. I imagine it would look like a tail trailing behind these planets – a bit like a comet’s!
In general, I’m enthusiastic about the idea of starting to make more statistical analyses of exoplanet atmospheres. I believe that by studying the atmospheres of several exoplanets simultaneously and consistently, we’ll be able to unravel the mysteries of how exoplanets form and evolve.
iREx: How important has this study been in your career as an astrophysicist?
Romain: A great deal! I started this study when I arrived as a researcher at iREx in 2021. To carry it out, I utilized data obtained by students in the group and other data obtained as part of the SPIRou large planet survey. This survey was conducted by an international team that includes several Canadians. This enabled me to better integrate with the iREx team in Montreal and develop new collaborations.
From a more technical point of view, familiarizing myself with the SPIRou instrument has enabled me to better prepare myself to lead an ambitious observation program on a very similar instrument, NIRPS, recently installed on the European Southern Observatory’s 3.6-m telescope.
iREx: What’s next?
Romain: As a matter of fact, we’re now starting to study the atmospheres of over 75 exoplanets with NIRPS. This study, which will last 5 years, will enable us to extend our program to a larger number of planets.
In doing so, we hope to unlock the secret of the origin of the hot Neptune desert, and better understand the evolution of planets in general!
Note: This interview has been edited for clarity and conciseness.
Animation showing exoplanet HAT-P-11b passing in front of its star. credit: Denis Bajram, Animation: G. Ottoni
The article “Homogeneous search for helium in the atmosphere of 11 gas giant exoplanets with SPIRou” was published on June 22, 2023 in the journal Astronomy & Astrophysics (free version on arXiv). In addition to Romain Allart, the team includes David Lafrenière, Étienne Artigau, Neil Cook, Antoine Darveau-Bernier, Lisa Dang, Charles Cadieux, Stefan Pelletier, Michael Radica, Björn Benneke, Nicolas Cowan and René Doyon from iREx, as well as iREx alumni Anne Boucher, Pierrot-Baptiste Lemée-Jolicoeur and Ryan Cloutier, and 18 other co-authors from Switzerland, Chile, France, the USA, Taiwan, Hungary, Germany, Brazil and the Netherlands.