My Thesis in 400 words: Pierrot Lamontagne

Pierrot Lamontagne, a student at IREx, recently completed his Master’s degree at Université de Montréal. Here, he summarizes his research project.

Thanks to the Kepler mission, we learned two things that changed our understanding of the Universe. First, planets are everywhere: most stars have not just one, but several planets, meaning our Galaxy is home to hundreds of billions of worlds. Second, the most common type of planet out there doesn’t even exist in our Solar System: these are sub-Neptunes, planets whose size is between that of Earth and Neptune.

With no counterpart among the Sun’s planets, these worlds remain very mysterious. Are they small rocky cores surrounded by a thin atmosphere of hydrogen and helium? Or are they giant “ocean worlds,” rich in water and volatile ices? Or can they be any of the above? Current knowledge leaves the door open to several possibilities, and uncovering their true nature is one of the great challenges in the field of exoplanets.

During my Master’s, I focused on the GJ 3090 system, a star located 72 light-years away from Earth, around which a planet was discovered in 2022 using the transit method. This study provided us with a measurement of its radius as well as an imprecise estimation of its mass. To refine this measurement, we used the radial velocity method, which involves detecting the tiny wobbles of the star caused by the planet’s gravity, with the HARPS and NIRPS spectrographs, two ultra-precise instruments installed in Chile. By combining the mass (from radial velocities) with the radius (from the transit), we can determine a planet’s density, which is a key clue to its internal composition. The new density we obtained, combined with recent observations from the James Webb Space Telescope (published in a separate study), allowed us to classify GJ 3090 b as a planet possibly covered by a vast water-rich envelope.

Beyond this discovery, we were able to confirm the existence of a second planet in the system: GJ 3090 c. Despite its mass being about ten times that of Earth, this sub-Neptune was not initially detected because it does not pass in front of its star from our point of view. Detecting it was a real challenge: its orbital period is almost identical to the star’s rotation period, which makes the data harder to interpret. Stellar spots and magnetic activity can sometimes mimic the signal of a planet. It was only by using advanced statistical models that we were able to separate the true planetary signal from the star’s “noise.”

The GJ 3090 system is thus becoming a unique laboratory for solving the mystery of sub-Neptunes. Each of these discoveries brings us a little closer to answering a fundamental question: what do the most common planets in our Galaxy really look like?

Learn More

Pierrot completed his Master’s between 2023 and 2025 under the supervision of Professor David Lafrenière at iREx. His thesis, “Analyse conjointe des vitesses radiales de HARPS et NIRPS : une application sur le système GJ 3090” (Joint Analysis of HARPS and NIRPS Radial Velocities: An Application to the GJ 3090 System), is available on Papyrus.