A team of astronomers, including McGill Professor Nicholas Cowan, has revealed the mysterious atmosphere of exoplanet HAT-P-18 b; It shed light on the intriguing mixture of gases, clouds, and even the effects of stellar activity.

Exoplanets, planets located outside our solar system, have fascinated scientists and the public alike and promise to reveal diverse planetary systems and potentially habitable worlds. Although very similar to our Earth, large gas giant planets located very close to their stars have proven to be ideal test targets for telescopes such as the James Webb Space Telescope (JWST) to improve the way astronomers understand exoplanets.

One such planet, HAT-P-18 b, is a “hot Saturn” planet more than 500 light-years away; Its mass is similar to that of Saturn, but its size is closer to the larger Jupiter. This gives the exoplanet a bulging atmosphere that is particularly ideal for analysis.

A team of astronomers led by researchers from the Trottier Institute for Exoplanet Research at McGill University and the University of Montreal (UdeM) used the power of the revolutionary Webb Telescope to study HAT-P-18 b. Their findings are detailed in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) provides a comprehensive portrait of Saturn’s hot atmosphere by delving into the complexities of separating atmospheric signals from stellar activity.

“The James Webb Space Telescope provides observations of exoplanets that are so precise that our understanding of their stars is limited. Fortunately, the same data, especially with Canada’s NIRISS instrument, allows us to measure what the star is doing during our observations and correct for these, so that these planets’ atmospheres “We can determine exactly what happened,” he said. McGill University Department of Earth and Planetary Science.

Passing over a spotted star

The Webb observations were made as the exoplanet HAT-P-18 b passed in front of its sun-like star. This is called the main transit and is crucial for detecting with astonishing precision and further characterizing an exoplanet hundreds of light-years away. Astronomers do not observe the light directly emitted by a distant planet. Instead, they study how the light of the central star is blocked and affected by the planet orbiting it.

Therefore, exoplanet hunters must grapple with the problem of distinguishing between signals resulting from the presence of a planet and signals resulting from the properties of the star. Like our Sun, stars do not have a smooth surface.

They may have dark star spots and bright regions that create signals that mimic the atmospheric properties of a planet. Ph.D. on exoplanet TRAPPIST-1 b and its star TRAPPIST-1. A recent study by. UdeM. Student Olivia Lim witnessed an explosion or flash on the star’s surface that affected her observations.

In the case of planet HAT-P-18 b, Webb captured the exoplanet directly as it passed over the dark spot on its star HAT-P-18. This is called the point event, and its impact is clearly visible in the data collected for the study. The team also reported the existence of many other star points on the surface of HAT-P-18 b that were not blocked by the exoplanet.

To accurately determine the atmospheric composition of an exoplanet, researchers determined that it was necessary to simulate the planet’s atmosphere as well as the star’s properties. They note that this assessment will be critical in processing future JWST observations to exploit the full potential of exoplanets.

H₂Oh, C.O.₂ and clouds in a fiery atmosphere

After carefully modeling both the exoplanet and the star in the HAT-P-18 system, the team of astronomers performed a comprehensive analysis of the composition of HAT-P-18 b’s atmosphere.

Researchers discovered the presence of water vapor (H) by examining the light passing through the exoplanet’s atmosphere past its host star.₂O) and carbon dioxide (CO₂). The researchers also identified the possible presence of sodium.

Adding to the intrigue of the findings, the team noticed strong signatures of a cloud layer in HAT-P-18 b’s atmosphere, which appeared to weaken the signals of many of its molecules. They also concluded that the star’s surface is covered with many dark spots, which can significantly affect the interpretation of the data.

An earlier analysis of the same JWST data by a team at Johns Hopkins University also found clear detections of water and CO. ₂ , but also reported that tiny particles called smog have been detected at high altitudes and that hints of methane (CH4) have been found. .

Studies by astronomers at the University of Montreal, which for the first time took into account the properties of the stars’ surface and the planet’s atmosphere, revealed a different picture. The detection of CH4 was not confirmed, and the amount of water they detected was ten times less than previously found.

They also found that nebula detection in the previous study may have been due to starspots on the star’s surface; This underlined the importance of considering the star in the analysis.

While molecules such as water, carbon dioxide and methane in certain proportions or combined with other molecules can be interpreted as biosignals or signs of life, HAT-P-18 b’s scorching temperatures approaching 600 degrees Celsius do not bode well. habitability of the planet.

The data used from JWST in this study was collected by Canada’s NIRISS (Near Infrared Imager and Slitless Spectrograph) instrument, which offers astronomers an unprecedented opportunity to distinguish between HAT-P-18 b’s many atmospheric features. another.

The results show that observations made in the far visible and near infrared wavelength range of the NIRISS instrument are important in resolving signals from the planet’s atmosphere and the star. Future observations with another JWST instrument, the Near Infrared Spectrograph (NIRSpec), will help refine the team’s findings, such as the detection of CO.₂and will shed even more light on the intricacies of this hot outer planet of Saturn.