Astronomers used the JWST (Webb Telescope) to study the atmosphere of the exoplanet HAT-P-18b, where they discovered water vapor and carbon dioxide, while highlighting the impact of the host star's properties on the data analysis. Led by researchers at the Université de Montréal's Trotier Institute for Exoplanets (iREx), a team of astronomers harnessed the power of the revolutionary James Webb Space Telescope (JWST) to study the "hot Saturn" exoplanet HAT-P-18b.

Their results, published last month in the journal Monthly Notices of the Royal Astronomical Society, paint a complete picture of HAT-P-18b's atmosphere while exploring the grand challenge of distinguishing its atmospheric signature from its stellar activity.

HAT-P-18b is located more than 500 light-years away, has a mass similar to Saturn, but is closer in size to the large planet Jupiter. Therefore, this exoplanet has an "inflated" atmosphere and is particularly suitable for analysis.

Artistic representation of the "hot Saturn" exoplanet HAT-P-18b.

The JWST observations were taken as HAT-P-18b passed in front of its sun-like star. This moment, known as a transit, is critical for detecting exoplanets from hundreds of light-years away with astonishing accuracy and further determining their characteristics.

Astronomers do not observe the direct light from distant planets. Instead, they studied how the central star's light is blocked and affected by planets orbiting it, and therefore had to try to separate the signal caused by the presence of the planet from the signal caused by the properties of the star itself.

The light curve shows the luminosity, or brightness, of a star over time. When an exoplanet passes over a star, known as a transit, some of the star's light is blocked by the exoplanet. As a result, the star's brightness decreases. When star spots on a star's surface are obscured, or when an exoplanet passes over a dark spot, astronomers can see a signal in the light curve, a small bump at the bottom of the transit light curve. See the full animation of the infographic below. Source: B.Gougeon/University of Montreal

Just like our sun, the star's surface is not uniform. They will have dark spots and bright regions, producing signals that mimic the properties of planetary atmospheres. A recent study of the exoplanet TRAPPIST-1b and its star TRAPPIST-1, led by UdeM PhD student Olivia Lim, witnessed an outburst, or flare, on the star's surface, which affected the observations.

In the case of planet HAT-P-18b, Webb captured the exoplanet just as it passed a dark spot on its star, HAT-P-18. This is known as a spot crossing event, and its effects are evident in the data collected by the new study. The iREx team also reported that there are many other star spots on the surface of HAT-P-18 that are not obscured by the exoplanets.

To accurately determine the composition of an exoplanet's atmosphere, researchers must simultaneously model the peculiarities of the planet's atmosphere and its star. In their study, they note that this consideration is critical for future exoplanet observations with the Webb telescope to realize its full potential.

"We found that accounting for stellar contamination implies the presence of flares and clouds rather than haze, and the recovered water vapor abundance is almost an order of magnitude lower," said lead author Marylou Fournier-Tondreau. Therefore, taking into account the host star of the system makes a big difference. In fact, this is the first time we've clearly distinguished hazy and star-spotted features, thanks to Canada's Near Infrared Imager and Seamless Spectrograph (NIRISS) instrument, which has a wider range of wavelengths that extends into the visible light realm. "

H2O, CO2 and clouds in the hot atmosphere

After modeling the exoplanets and stars in the HAT-P-18 system, iREx astronomers conducted a detailed analysis of HAT-P-18b's atmospheric composition. By detecting light in the exoplanet's atmosphere as it passes through its host star, researchers discovered the presence of water vapor (H2O) and carbon dioxide (CO2).

The researchers also detected the possible presence of sodium and observed strong signs of clouds in HAT-P-18b's atmosphere, which appear to weaken the signal from many of these molecules. They also concluded that the star's surface is covered in many dark spots, which can seriously affect the interpretation of the data.

An earlier analysis of the same data from JWST by a Johns Hopkins team also showed clear detections of water and carbon dioxide, but also reported detecting small particles called "hazy" particles high in the sky and finding telltale signs of methane (CH4). iREx astronomers paint a different picture.

The CH4 detection was not confirmed, and the water abundance they measured was 10 times lower than previously found. They also found that the haziness seen in previous studies could be caused by star spots on the star's surface, highlighting the importance of considering stars in analyses.

Can exoplanets support life? Not likely. While molecules like water, carbon dioxide and methane could be interpreted as biosignatures or signs of life in certain proportions or in combination with other molecules, HAT-P-18b's scorching temperatures of nearly 600 degrees Celsius don't bode well for the planet's habitability.

Future observations from JWST's other instrument, the Near Infrared Spectrograph (NIRSpec), are expected to help refine the team's results, such as the detection of carbon dioxide, and provide more information about the complexities of this hot Saturnian exoplanet.

Compiled source: ScitechDaily