Announcement of JWST time requests selected for Cycle 3

The star-forming region NGC 604 as seen by the JWST's NIRCam instrument. (Credit: NASA/ESA/CSA/STScI)
The star-forming region NGC 604 as seen by the JWST's NIRCam instrument. (Credit: NASA/ESA/CSA/STScI)

A total of 1931 telescope time proposals were submitted for the James Webb Space Telescope‘s (JWST) Cycle 3 General Observers (GO) program, requesting a total of 48 800 observing hours… This is a record for any telescope or observatory, whether in space or on the ground! The 253 successful proposals were finally announced by the Space Telescope Science Institute (STScI) on February 29, 2024, covering about 5 500 hours of observing time from July 2024 to June 2025. Cycle 3’s oversubscription rate was 9 to 1, meaning that for every 9 proposals received, only 1 was granted time. This is a testament to the international community’s engagement with the JWST and its impressive performance in the first two years of its scientific operations.

Despite the steep competition, proposals from astronomers at many Canadian institutions were selected during Cycle 3 for a total of nearly 263 hours awarded to Canadian Principal Investigators (PIs). This is a 30% increase compared to the previous Cycle 2 results! True to form, iREx researchers obtained nearly half of this Canadian time for a total of over 120 hours led by iREx PIs. In addition to these 4 programs with iREx PIs, iREx’s Loïc Albert is also co-PI on a 6.3 hour-long proposal using the MIRI instrument.

Find out more about iREx researchers’ Cycle 3 JWST programs here:


Loïc Albert, iREx researcher and the NIRISS Instrument Scientist.

Program 4857: Confirmation of Planetary Companions to White Dwarf Stars
PI: Fergal Mullaly (Orbital Insight)
Co-PIs: Loïc Albert (iREx/UdeM), Susan Mullaly (STScI)
Time awarded: 6.33h using MIRI/Imaging
Summary: Several tantalising JWST observing programs have already attempted to study exoplanets around white dwarfs and other stellar remnants. This proposal aims to use MIRI imaging to confirm two potential Jupiter-like planets orbiting white dwarfs. The team hope to verify the exoplanets’ movement relative to the white dwarfs, indicating they are true companions. Initial observations from JWST’s Cycle 1 identified these companions, which appear to be 1 to 5 times the mass of Jupiter. Given their close proximity to the white dwarfs, there is less than a 2% chance they are background galaxies, but a second image is needed to confirm their nature. If confirmed, these would be among the first true Jupiter analogs found around other stars, similar in mass, distance to their star, and age to the largest planet in our own Solar System. Their brightness will allow for spectroscopic analysis of their atmospheres, allowing the team to test different atmospheric models. If the companions are background sources, it challenges the idea that planets drive metal accretion in white dwarfs. On the other hand, confirming the planets are true companions would support this theory and suggest such systems are common in our Galaxy.


Joost Wardenier, Trottier Postdoctoral Fellow at iREx.

Program 5268: Around the world in less than two days: observing the spectral phase curve of an ultra-hot Jupiter with JWST/NIRSpec
PI: Joost Wardenier (iREx/UdeM)
co-PI: Thea Hood (IRAP)
Time awarded: 59.44h using NIRSpec/BOTS
Summary: This program wishes to observe the full phase curve of the ultra-hot Jupiter (UHJ) WASP-76 b using the NIRSpec instrument aboard the JWST. WASP-76 b is one of the most studied exoplanets, observed with at least eight ground-based instruments. Many different atoms and molecules have already been detected in its atmosphere thanks to data from the visible spectrum of light. By combining these data with JWST’s infrared measurements of elements like carbon, oxygen, and hydrogen, this team can uncover this exoplanet’s formation history in unprecedented detail. UHJs have complex 3D structures, with significant temperature and chemical differences between their permanent day and night sides. Using 1D models can lead to inaccurate results, so it is necessary to measure the planet’s full 3D temperature and cloud cover. The proposed comprehensive approach will refine our understanding of previous ground-based observations. Observing WASP-76 b’s full phase curve with high spectral resolution will provide a complete picture of its atmospheric temperature and chemistry, covering all key volatile molecules.


Michael Radica, Ph.D. student at iREx.

Michael Radica, Ph.D. student at iREx.

Program 5844: Starspots, Hazes, and Disequilibrium Chemistry: A Deep Dive into the Atmosphere of HAT-P-18 b
PI: Michael Radica (iREx/UdeM)
Time awarded: 16.4h using MIRI/LRS and NIRSpec/BOTS
Summary: Many exoplanet atmosphere characterisation results using JWST up until now have encountered the challenge of tackling contamination from the host star’s activity. Previous analyses of JWST NIRISS transit data for HAT-P-18 b have produced varying results regarding how the star’s inhomogeneous photosphere affects the transmission spectrum, leading to very different conclusions about the planet’s atmospheric composition. This proposal will observe two additional transits of HAT-P-18 b with both the NIRSpec and MIRI instruments to create a comprehensive transmission spectrum covering a wavelength range from 0.6-12 microns. Using JWST’s full capabilities, the team aims to distinguish between atmospheres shaped by hazes versus heterogeneities on the host star’s surface. This study will enhance our interpretation of transmission spectra of other planets with inferred scattering hazes and complete the chemical inventory of HAT-P-18b’s atmosphere. It should also shed light on the planet’s formation history and the disequilibrium processes influencing its evolution.


Pierre-Alexis Roy, Ph.D. student at iREx.

Program 5967: Exploring the desert: Thermal characterization of an exposed planetary core
PI: Pierre-Alexis Roy (iREx/UdeM)
Co-PI: Björn Benneke (iREx/UdeM)
Time awarded: 21.36h using NIRSpec/BOTS
Summary: Scientists have recently discovered a handful of ultra-dense and hot sub-Neptune type exoplanets despite this type of planet being quite rare (and thus said to be in a regime called the “hot-Neptune desert”). These sub-Neptunes are believed to be the exposed cores of gas giant planets like Jupiter, offering a unique chance to study them directly. This program proposes to study the sub-Neptune TOI-849 b using the NIRSpec instrument on the JWST. By observing three of its secondary eclipses, the team aims to measure the planet’s dayside temperature, heat distribution, and atmospheric composition, including its water and carbon dioxide content. These observations will help us understand the metallic content and the carbon-to-oxygen ratio of TOI-849 b’s atmosphere. This will confirm if it is an exposed core and reveal details about its formation. This study will not only advance our knowledge of sub-Neptunes but also provide insights into how planets form, giving us a direct look at a primordial planetary core.


Björn Benneke, Professor at iREx.

Program 6457: Thermal emission of a cool, potentially volcanically active exo-Earth
PI: Björn Benneke (iREx/UdeM)
Time awarded: 23.15h using MIRI/Imaging
Summary: Rocky planets are common in the Universe, but it is still a mystery whether many of them have atmospheres. To understand this better, more real data on how these atmospheres form, change, and survive is required. This proposal plans to study the recently discovered cool exo-Earth, LP 791-18 d, to see if it has an atmosphere. LP 791-18 d orbits a nearby M6-type dwarf star and is thought to be volcanically active, similar to Jupiter’s moon Io. This volcanic activity could keep replenishing its atmosphere. The team will observe the planet’s mid-infrared thermal emission using JWST’s MIRI instrument to check for an atmosphere. The observations will differentiate between a bare-rock planet and one with a carbon dioxide-rich atmosphere, even if it has thick clouds like Venus. If the team finds an atmosphere, it would show that cool, rocky planets around M dwarf stars can have atmospheres, encouraging further studies on the atmospheres of such planets.


Many more JWST Cycle 3 GO programs will be supported by Canadian and iREx researchers as co-Investigators. To see all selected Cycle GO programs, consult the STScI website. Congratulations to all astronomers with awarded time, as well as all astronomers who submitted proposals!

The next Call for Proposals for JWST’s Cycle 4 GO Programs is expected on August 1, 2024 with a submission deadline on October 16, 2024.