Astronomers analyzed HAT-P-18b with the James Webb Space Telescope and discovered water vapor and CO2 in its atmosphere. They highlighted the difficulties in distinguishing between atmospheric and stellar signals and suggested that starspots significantly influence the interpretation of the data. (Artist's concept.) Photo credit: Issues.fr.com
Astronomers used JWST to study the atmosphere of the exoplanet HAT-P-18 b, finding water vapor and CO2, while focusing on the influence of the parent star's properties on data analysis.
Led by researchers from the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, a team of astronomers harnessed the power of the revolutionary James Webb Space Webb Telescope (JWST) to study “hot planets.” Saturn” exoplanet HAT-P-18 b.
Their results, published last month in the journal Monthly Notices of the Royal Astronomical Society, paint a comprehensive picture of HAT-P-18 b's atmosphere while exploring the major challenge of distinguishing its atmospheric signals from the activity of its star.
HAT-P-18 b is more than 500 light-years away and has a similar mass to Saturn but a size closer to that of the larger planet. Jupiter. This means that the exoplanet has an “inflated” atmosphere that is particularly suitable for analysis.
An artist's impression of the “hot Saturn” exoplanet HAT-P-18 b. Image credit: NASA/Eyes on Exoplanets
Skip a spotted star
The JWST observations were made as HAT-P-18 b passed in front of its Sun-like star. This moment is called a transit and is crucial for the discovery and further characterization of an exoplanet hundreds of light-years away with surprising precision.
Astronomers do not observe the light that the distant planet emits directly. Rather, they study how the host star's light is blocked and influenced by the planet orbiting it, and therefore must try to separate the signals caused by the planet's presence from those caused by the properties of the planet itself.
The light curve shows the brightness or brightness of the star over time. When the exoplanet passes over the star, called a transit, some of the star's light is blocked by the exoplanet. This causes the star's brightness to decrease. When a star spot is obscured on the star's surface or when the exoplanet crosses the dark spot, astronomers can detect a signal in the light curve in the form of a small bump at the bottom of the transiting light curve. Check out the full animation of this infographic below. Photo credit: B. Gougeon/Université de Montréal
Just like our sun, stars do not have a uniform surface. They can have dark starspots and bright regions that can produce signals that mimic a planet's atmospheric properties. A recent study of the exoplanet TRAPPIST-1 b and its star TRAPPIST-1 led by Olivia Lim, a doctoral student at UdeM, observed an outburst or flare on the star's surface that interfered with observations.
In the case of planet HAT-P-18 b, Webb captured the exoplanet as it flew over a dark spot on its star HAT-P-18. This is called a one-time crossover event and its impact was clear in the data collected for the new study. The iREx team also reported the presence of many other starspots on HAT-P-18's surface that were not obscured by the exoplanet.
To accurately determine the exoplanet's atmospheric composition, researchers had to simultaneously model the planet's atmosphere and the features of its star. In their study, they emphasize that such consideration will be crucial for processing future observations of exoplanets over Webb in order to fully exploit their potential.
“We found that accounting for stellar contamination implies the presence of spots and clouds instead of haze and recovers an almost order of magnitude smaller amount of water vapor,” said lead author Marylou Fournier-Tondreau.
“So taking into account the host star of the system makes a big difference,” added Fournier-Tondreau, who did the work as a master’s student at iREx and is now pursuing a doctorate. at the University of Oxford.
“This is actually the first time we can clearly separate the signature of haze spots from starspots, thanks to the Canadian NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument, which provides greater wavelength coverage, extending into the visible light range .” . »
H2O, CO2 and clouds in a warm atmosphere
After modeling the exoplanet and star in the HAT-P-18 system, iREx astronomers conducted a careful analysis of the atmospheric composition of HAT-P-18 b. By studying the light that passes through the exoplanet's atmosphere as it passes its parent star, researchers determined the presence of water vapor (H2O) and carbon dioxide (CO2).
The researchers also discovered the possible presence of sodium and observed clear signs of cloud formation in the atmosphere of HAT-P-18 b, which appears to dampen the signals of many molecules there. They also concluded that the star's surface was covered with numerous dark spots, which could significantly affect the interpretation of the data.
A previous analysis of the same JWST data by a team at Johns Hopkins University also found clear evidence of water and CO2, but also reported detection of small particles at high altitudes, called haze, and found evidence of methane (CH4). The iREx astronomers paint a different picture.
The detection of CH4 was not confirmed and the determined water abundance was ten times lower than previously determined. They also found that the haze detection in the previous study could instead be caused by star spots on the star's surface, highlighting the importance of considering the star in the analysis.
Could the exoplanet support life? Unlikely. While molecules such as water, carbon dioxide and methane in certain ratios or in combination with other molecules can be interpreted as biosignatures or signs of life, HAT-P-18 b's scorching temperatures, near 600 degrees Celsius, do not bode well for the planet's habitability.
Future observations with another JWST instrument, the Near Infrared Spectrograph (NIRSpec), promise to help refine the team's results, such as detecting CO2, and shed even more light on the intricacies of this hot exoplanet of Saturn.