Unraveling the secrets of “hot Saturn” and its speckled star

Astronomers used the James Webb Space Telescope to study the galactic atmosphere Exoplanet HAT-P-18 b, finds water vapor and carbon dioxide with an emphasis on the influence of the properties of the host star on data analysis.

Led by researchers from the Université de Montréal's Trottier Institute for Exoplanet Research (iREx), a team of astronomers has harnessed the power of the revolutionary James Webb Space Telescope (JWST) to study “hot planets.” Saturn“Exoplanet HAT-P-18 b.

Their findings were published last month in the journal Monthly Notices of the Royal Astronomical Societydraw a complete picture of HAT-P-18 b's atmosphere while exploring the major challenge of distinguishing atmospheric signals from the activity of its star.

HAT-P-18 b is located more than 500 light-years away, with a mass similar to that of Saturn but a size closer to that of the larger planet. Jupiter. As a result, the exoplanet has a “puffy” atmosphere that is particularly ideal for analysis.

Artistic representation of “hot Saturn” exoplanet, HAT-P-18 b. Credit: NASA/Eyes on Exoplanets

Passing over a speckled star

The observations were taken from the James Webb Space Telescope as HAT-P-18 b passed in front of its sun-like star. This moment is called a transit and is essential for detecting and characterizing an exoplanet hundreds of light-years away with amazing precision.

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

The light curve shows the brightness or brightness of a star over time. When an exoplanet passes over the star, known as a transit, part of the star's light is blocked by the exoplanet. As a result, the star's luminosity decreases. When a stellar spot occults the star's surface, or when an exoplanet passes over the dark spot, astronomers can see a signal in the light curve in the form of a small bump at the bottom of the passing light curve. Watch the full animation of this chart below. Source: B. Gougeon/Université de Montréal

Just like our Sun, stars do not have uniform surfaces. It can contain dark star spots and bright regions, which can generate signals that mimic features of the planet's atmosphere. A recent study of the exoplanet TRAPPIST-1 b and its star TRAPPIST-1, led by UdeM doctoral student Olivia Lim, witnessed an explosion, or flare, on the star's surface, affecting observations.

In the case of HAT-P-18 b, Webb was able to capture the exoplanet as it passed over a dark spot on its star HAT-P-18. This is called a localized crossover event, and its effect was evident in the data collected for the new study. The iREx team also reported the presence of several other star spots on the surface of HAT-P-18 that were not obscured by the exoplanet.

To accurately determine the exoplanet's atmospheric composition, researchers had to simultaneously model the planet's atmosphere as well as the properties of its star. They point out in their study that such a consideration will be crucial in addressing future Webb exoplanet observations to fully harness its potential.

“We found that accounting for stellar contamination means spots and clouds instead of haze and recovers water vapor abundance at approximately a lower rate,” said lead author Marilou Fournier-Tondreau.

“So looking at the host star of the system makes a big difference,” added Fournier Tondreau, who did this work as a master's student at iREx and is now pursuing a PhD. In the Oxford university.

“This is actually the first time we have clearly separated haze from star spots, thanks to the NIRISS (Near-Infrared Imager and Non-Slit Spectrograph) instrument in Canada, which provides broader wavelength coverage extending into the visible light range.”

Water, carbon dioxide and clouds in a burning atmosphere

After modeling the exoplanet and star in the HAT-P-18 system, iREx astronomers performed a microdissection of the composition of HAT-P-18 b's atmosphere. By examining the light filtering through the exoplanet's atmosphere as it passes its host star, the researchers detected the presence of water vapor (H2O) and carbon dioxide (CO2).

The researchers also explored the possible presence of sodium, and observed strong signs of a cloud surface in HAT-P-18 b's atmosphere, which appears to mute the signals of many molecules within it. They also concluded that the star's surface was covered in many dark spots that could significantly affect the interpretation of the data.

A previous analysis of the same James Webb Space Telescope data led by a team from Johns Hopkins University also revealed the clear detection of water and carbon dioxide, but also reported the detection of small high-altitude particles called aerosols and found hints of methane (CH4). iREx astronomers paint a different picture.

The discovery of CH4 has not been confirmed, and the abundance of the water they identified was ten times lower than previously detected. They also found that the previous study's discovery of the haze could be caused by star spots on the star's surface, highlighting the importance of taking the star into account in the analysis.

Can an exoplanet support life? Improbable. While molecules such as water, carbon dioxide and methane can be interpreted as biosignatures, or signs of life, in certain proportions or in combination with other molecules, HAT-P-18 b's scorching temperatures are close to 600 degrees. Celsius It does not bode well for the planet's habitability.

Future observations from another of the James Webb Space Telescope's instruments, the Near-Infrared Spectrometer (NIRSpec), promise to help improve the team's results, such as the discovery of carbon dioxide, and shed more light on the complexities of this hot exoplanet of Saturn.

Reference: “Near-infrared transmission spectroscopy of HAT-P-18 b with NIRISS: deconstructing planetary and stellar features in the JWST era” by Marilou Fournier Tondreau, Ryan J. MacDonald, Michael Radica, David Lafrenière, Louis Wilbanks, Carolyn Piolette, Louis Philippe Coulombe, Romain Allart, Kim Morel, Etienne Artigao, Loic Albert, Olivia Lim, Rene Doyon, Björn Beneke, Jason F. Roux, Antoine Darvaux-Bernier, Nicholas B. Cowan, Nicole K. Lewis, Neil James Cook, Laura Flagg, Frédéric Genest, Stephane Pelletier, Doug Johnston, Lisa Dang, Lisa Kaltenegger, Jake Taylor and Jake D. Turner, December 9, 2023, Monthly Notices of the Royal Astronomical Society.
doi: 10.1093/manras/stad3813