April 24, 2024

Uncovering the secrets of a “hot Saturn” and its tarnished star

Hot Saturn exoplanet concept art

Astronomers analyzed HAT-P-18 b using the James Webb Space Telescope, revealing water vapor and CO2 in its atmosphere. They highlighted the challenges of distinguishing between atmospheric and stellar signals, suggesting that starspots significantly influence data interpretation. (Artist’s concept.) Credit: SciTechDaily.com

Astronomers used JWST to study the Earth’s atmosphere. Exoplanet HAT-P-18 b

An artist’s rendering of the “hot” exoplanet Saturn, HAT-P-18 b. Credit: NASA/Eyes on Exoplanets

Passing by a tarnished 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 to detecting and further characterizing an exoplanet hundreds of light-years away with surprising precision.

Astronomers do not observe the light emitted directly from the distant planet. Instead, they study how the light from the central star is being blocked and affected by the planet orbiting it, and so they must try to separate the signals caused by the planet’s presence from those caused by the star’s own properties.


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

Just like our Sun, stars do not have uniform surfaces. They can have dark star spots and bright regions, which can create signals that mimic a planet’s atmospheric attributes. A recent study of the exoplanet TRAPPIST-1 b and its star TRAPPIST-1, led by UdeM PhD student Olivia Lim, witnessed an eruption, or eruption, on the star’s surface, which affected observations.

In the case of the planet HAT-P-18 b, Webb captured the exoplanet just as it passed through a dark spot on its star, HAT-P-18. This is called a point crossover event and its effect was evident in the data collected for the new study. The iREx team also reported the presence of numerous other star spots on the surface of HAT-P-18 that were not blocked by the exoplanet.

To accurately determine the exoplanet’s atmospheric composition, researchers had to simultaneously model the planet’s atmosphere as well as the peculiarities of its star. In their study, they emphasize that such consideration will be crucial in handling future observations of exoplanets through Webb to fully take advantage of its potential.

“We found that accounting for stellar contamination implies the existence of patches and clouds rather than haze and recovers almost an order of magnitude lower water vapor abundance,” said lead author Marylou Fournier-Tondreau.

“So considering 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 his PhD. at the

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