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How stellar magnetism is reshaping our view of distant worlds

Vaseline 4 weeks ago

Exoplanet magnetic field Art

Research on the exoplanet WASP-39b has revealed the need to incorporate stellar magnetic fields into models to match observations with theoretical predictions, greatly improving the accuracy of exoplanet studies. (Artist’s concept.) Credit: SciTechDaily.com

The brightness variations of its parent star show that an exoplanet‘s size and other properties can be determined. To avoid errors, the star’s magnetic field is decisive.

Located 700 light-years away from Earth in the constellation Virgo, the planet WASP-39b orbits the star WASP-39. The gas giant, which takes just over four days to complete one orbit, is one of the best-studied exoplanets.

Shortly after launch in July 2022, NASA’s James Webb Space Telescope turned its high-precision gaze on the distant planet. The data showed that there were large amounts of water vapor, methane and even, for the first time, carbon dioxide in WASP-39b’s atmosphere. A small sensation!

But there is still a fly in the ointment: researchers have not yet managed to reproduce all crucial details of the observations in model calculations. This prevents an even more accurate analysis of the data. In the new study led by the MPS, the authors, including researchers from the Massachusetts Institute of Technology (US), the Space Telescope Science Institute (US), Keele University (UK) and the University of Heidelberg (Germany), show a way to overcome this obstacle.

Challenges in interpreting exoplanet data

“The problems encountered in interpreting the data from WASP-39b are known from many other exoplanets – regardless of whether they are observed with Kepler, TESS, James Webb, or the future PLATO spacecraft,” explains MPS scientist Dr. Nadiia Kostogryz, first author of the new study, explains. “As with other stars orbited by exoplanets, WASP-39’s observed light curve is flatter than previous models can explain,” she adds.

Influence of stellar limb darkening on exoplanet light curves

Stars with a low magnetic field strength show more pronounced dimming of their limbs than stars with a strong magnetic field. This affects the shape of the light curve. Credit: MPS/hormesdesign.de

Researchers define a light curve as a measurement of a star’s brightness over time. The brightness of a star fluctuates constantly, for example because its brightness is subject to natural fluctuations. Exoplanets can also leave traces in the light curve. As an exoplanet passes in front of its star, as seen by an observer, it dims the starlight. This is reflected in the light curve as a regular decrease in brightness. Accurate evaluations of such curves provide information about the size and orbital period of the planet. Researchers can also gain information about the composition of the planet’s atmosphere if the star’s light is split into different wavelengths, or colors.

A closer look at a star’s brightness distribution

A star’s limb, the edge of the stellar disk, plays a decisive role in the interpretation of its light curve. As with the sun, the limb appears darker to the observer than the inner area. Further on, however, the star shines no less brightly. “Because the star is a sphere and the surface is curved, we look at the edge in higher and therefore cooler layers than in the center,” explains co-author and MPS director Prof. Dr. Laurent Gizon. “This area therefore appears darker to us,” he adds.

Limb dimming is known to affect the exact shape of the exoplanet signal in the light curve: the dimming determines how much a star’s brightness drops during a planetary transit and then rises again. However, it has not been possible to accurately reproduce observational data using conventional models of the stellar atmosphere. The decrease in brightness was always less abrupt than the model calculations suggested. “It was clear that we were missing a crucial piece of the puzzle to precisely understand the signal from the exoplanets,” says MPS director Prof. Dr. Sami Solanki, co-author of the current study.

Magnetic field is the missing piece of the puzzle

As the calculations published today show, the missing piece of the puzzle is the stellar magnetic field. Like the sun, many stars generate a magnetic field deep within their interior through enormous heat flows plasma. For the first time, the researchers were now able to include the magnetic field in their models of limb darkening. They could show that the strength of the magnetic field has an important effect: the limb darkening is pronounced in stars with a weak magnetic field, while it is weaker in stars with a strong magnetic field.

The researchers were also able to prove that the discrepancy between observational data and model calculations disappears when the star’s magnetic field is included in the calculations. To this end, the team used selected data from NASA‘s Kepler Space Telescope, which captured the light from thousands upon thousands of stars from 2009 to 2018. In a first step, the scientists modeled the atmosphere of typical Kepler stars in the presence of a magnetic field. In a second step, they then generated ‘artificial’ observation data from these calculations. As a comparison with the real data shows, the Kepler data is successfully reproduced by including the magnetic field.

The team also expanded its considerations to data from the James Webb Space Telescope. The telescope is able to split the light from distant stars into different wavelengths and thus look for the characteristic features of certain molecules in the atmospheres of the discovered planets. It turns out that the magnetic field of the parent star affects the darkening of the stellar limbs differently at different wavelengths – and therefore should be taken into account in future evaluations to achieve even more precise results.

From telescopes to models

“In recent decades and years, the way to make progress in exoplanet research has been to improve the hardware, the space telescopes designed to search for and characterize new worlds. The James Webb Space Telescope has taken this development to new limits,” said Dr. Alexander Shapiro, co-author of the current study and head of an ERC-funded research group at the MPS. “The next step now is to improve and refine the models to interpret this excellent data,” he adds.

To further this development, the researchers now want to extend their analyzes to stars that are clearly different from the Sun. Furthermore, their findings offer the opportunity to use the light curves of stars with exoplanets to infer the strength of the stellar magnetic field, which is often difficult to measure otherwise.

Reference: “Magnetic origins of the discrepancy between models and observations for stellar limb dimming” by Nadiia M. Kostogryz, Alexander I. Shapiro, Veronika Witzke, Robert H. Cameron, Laurent Gizon, Natalie A. Krivova, Hans-G. Ludwig, Pierre FL Maxted, Sara Seager, Sami K. Solanki and Jeff Valenti, April 12, 2024, Nature Astronomy.
DOI: 10.1038/s41550-024-02252-5