My Thesis in 400 Words: Katherine Thibault

Katherine Thibault, a master’s student at iREx, recently completed her master’s degree at the Université de Montréal. Here she summarizes her research project.

The majority of stars, including our Sun, will become white dwarfs. A white dwarf is a small, dense stellar object that remains after small to medium-sized stars exhaust their nuclear fuel and shed their outer layers. Although many exoplanets have been discovered to date, few have been found orbiting white dwarfs.

However, a popular theory suggests that giant planets can survive around these stellar remnants. Will the giant planets in our Solar System survive the Sun’s demise? My master’s project aimed to find exoplanets by observing four white dwarfs with the MIRI instrument on the James Webb Space Telescope. MIRI is designed to observe our Universe in the mid-infrared spectrum, allowing astronomers to study a wide range of phenomena, including searching for exoplanets orbiting white dwarfs.

Image: Artist’s rendering of an exoplanet and a debris disk orbiting a white dwarf. Credit: NASA/JPL-Caltech

The search for planets around white dwarfs is a challenging one – not least because of their small size and few spectral lines. So I used the technique of phase-nucleus imaging, since it enables exoplanets to be explored at very small angular separations down to half the diffraction limit. The aim is to detect an astrophysical signal by cancelling the instrumental phase errors produced by the telescope. This signal is calibrated with a high signal-to-noise point source, called a calibrator, to eliminate the artifacts produced by the telescope’s detector.

By analyzing four white dwarfs—WD 2149+021, WD 1202-232, WD 1620-391, and WD 2105-82—using phase-nucleus imaging, we discovered two exoplanet candidates: WD 1202-232 b and WD 2105-82 b. These candidates are located 1230±20 milliarcseconds and 2210±20 milliarcseconds from their stars, with flux ratios of 63±2 and 29±3, respectively. If confirmed, the masses of WD 1202-232 b and WD 2105-82 b would be between 1 and 7 times, and 1 and 2 times, the mass of Jupiter, respectively. This discovery opens the door for more observations of white dwarfs to confirm the survival of giant planets after their stars’ deaths. This discovery paves the way for more white dwarf observations to confirm the survival of giant planets after the death of their host star.

In addition, to test the performance of phase-nucleus imaging with MIRI observations at different signal-to-noise regimes, I also carried out analysis and simulations of exoplanet recoveries around white dwarfs. The simulations showed that having a high signal-to-noise ratio for the observed target is more crucial than for the calibrator. However, the success of detecting companions varies greatly depending on their position and contrast relative to the star. For high signal-to-noise observations, the phase kernel imaging technique is promising for finding exoplanets around white dwarfs, due to its ability to detect astrophysical signals at very small angular separations.

More information

Katherine completed her master’s degree between 2022 and 2023, under the supervision of iREx professor David Lafrenière. Her thesis is available online via Papyrus.