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Science & Technology, Canada, (Commonwealth Union) – A group of astronomers from iREx, led by researchers at the University of Montreal, Trottier Institute for Research on Exoplanets, has delved into the mysterious atmosphere of the exoplanet HAT-P-18 b, uncovering insights into its intriguing combination of gases, clouds, and the influence of its host star’s activity.
Utilizing the cutting-edge capabilities of the James Webb Space Telescope (JWST), the team focused their efforts on studying this “hot Saturn” exoplanet.
Their recent findings, detailed in the journal Monthly Notices of the Royal Astronomical Society, provide a comprehensive understanding of HAT-P-18 b’s atmospheric composition, all while navigating the intricate task of distinguishing its atmospheric features from the effects of stellar activity.
HAT-P-18 b, positioned more than 500 light-years away, boasts a mass akin to Saturn’s but a size more akin to Jupiter, resulting in an exoplanet with an expanded atmosphere ripe for detailed analysis.
During a transit event, wherein HAT-P-18 b passed in front of its Sun-like star, the James Webb Space Telescope (JWST) captured crucial observations. These transits are pivotal for discerning and characterizing exoplanets located hundreds of light-years distant with remarkable precision.
Astronomers do not directly observe the light emitted by distant planets. Instead, they scrutinize how the light from the central star is obstructed and altered by its orbiting planet, necessitating the separation of signals originating from the planet and those stemming from the star’s inherent characteristics.
Much like our own Sun, stars exhibit non-uniform surfaces marked by dark spots and bright regions, which can produce signals mimicking an exoplanet’s atmospheric traits. A recent investigation led by doctoral student Olivia Lim from University of Montreal, focusing on the exoplanet TRAPPIST-1 b and its host star TRAPPIST-1, documented a flare eruption on the star’s surface that impacted observations.
In the instance of the exoplanet HAT-P-18 b, the Webb telescope captured a unique moment as the planet traversed a dark region on its host star, HAT-P-18. This event, known as a spot-crossing event, left a distinct imprint in the data gathered for the recent study. Additionally, the iREx team identified numerous other star spots on the surface of HAT-P-18 that remained visible despite the passage of the exoplanet.
To precisely ascertain the atmospheric makeup of the exoplanet, researchers needed to simultaneously account for both its own atmosphere and the idiosyncrasies of its host star. Their findings underscore the importance of such considerations for future observations of exoplanets using the Webb telescope, highlighting the necessity to fully leverage their potential.
“We found that accounting for stellar contamination implies the existence of spots and clouds instead of haze and recovers a water vapour abundance of almost an order of magnitude lower,” added the lead author Marylou Fournier-Tondreau.
“So considering the system’s host star makes a big difference,” said Fournier-Tondreau, who carried out the work as a master’s student at iREx and is presently conducting a Ph.D. at the University of Oxford.
“It’s actually the first time that we clearly disentangle the signature of hazes versus starspots, thanks to Canada’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument, which provides wider wavelength coverage extending into the visible light domain.”
Following detailed modeling of both the exoplanet and its host star within the HAT-P-18 system, the iREx astronomers meticulously dissected the atmospheric composition of HAT-P-18 b. By scrutinizing the light filtering through the exoplanet’s atmosphere during its transit across its host star, the researchers identified the presence of water vapor (H2O) and carbon dioxide (CO2).
Additionally, they detected potential traces of sodium and observed robust indications of a cloud layer in the atmosphere of HAT-P-18 b, which seems to attenuate the signals of many molecules present within it. Moreover, they concluded that the star’s surface exhibited numerous dark spots, a factor with significant implications for data interpretation.
An earlier analysis of the same JWST data, conducted by a team at Johns Hopkins University, had also identified water and CO2. However, they additionally reported the detection of hazes—small particles at high altitudes—and suggested the presence of methane (CH4). The iREx astronomers present a divergent interpretation of the findings.
The confirmation of CH4 detection remained elusive, and the iREx astronomers discovered that the water abundance they determined was tenfold lower than previously reported. Moreover, they suggested that the earlier study’s identification of hazes might instead stem from star spots on the surface of the star, underscoring the critical significance of incorporating the star’s characteristics into the analysis.
