Dr. Nadiia Kostogryz of the Max Planck Institute for Solar System Research (MPS) and first author of the study explains, "The challenges in interpreting data from WASP-39b are not unique. Similar issues have arisen with data from exoplanets observed by other telescopes such as Kepler, TESS, and even the upcoming PLATO mission. The observed light curve from WASP-39 is flatter than our models predicted."
A light curve, which measures a star's brightness over time, reveals fluctuations due to factors like natural luminosity variations and the transit of exoplanets, which can obscure starlight. Such measurements are crucial as they provide insights into the planet's size, orbit, and atmospheric composition when the star's light is dissected into various wavelengths.
The MPS study highlights the significant role of the star's limb, the outer edge of the stellar disc, in these observations. "The limb darkening effect, where the edge of a star appears dimmer than its center, significantly influences the shape of the exoplanet signal in the light curve," says Prof. Dr. Laurent Gizon, coauthor and director of MPS. "Since the star is a sphere, its curvature means we see cooler, hence darker, layers at the limb than in the center."
The recent breakthrough, detailed in the study, is the incorporation of the star's magnetic field into limb darkening models. Like our Sun, many stars generate magnetic fields that affect their luminosity. The study demonstrates that the limb darkening effect varies with the magnetic field's strength, which has been a missing component in previous models.
"The inclusion of the magnetic field in the models aligns the theoretical predictions with the observed data," asserts Prof. Dr. Sami Solanki, coauthor and MPS director. This adaptation has allowed for a more accurate representation of the stellar limb darkening, resolving long-standing discrepancies between observed data and model predictions.
Further validation came from examining data from NASA's Kepler Space Telescope, which observed thousands of stars from 2009 to 2018. "By simulating the atmospheric models of typical Kepler stars with a magnetic field, and comparing these models to actual observations, we successfully replicated the observed light curves," says Dr. Alexander Shapiro, study coauthor and leader of a European Research Council-funded group at MPS.
The MPS team is also applying their refined models to data from the James Webb Space Telescope, which can dissect starlight into a spectrum to search for molecular signatures in exoplanetary atmospheres. The findings suggest that the influence of stellar magnetic fields varies across different wavelengths, which could have implications for future telescope observations and data analyses.
"As we continue to push the boundaries of space exploration with advanced telescopes like James Webb, refining our theoretical models becomes equally crucial," emphasizes Dr. Shapiro. "Understanding the subtle effects of stellar phenomena like magnetic fields on observational data allows us to extract more precise information about distant worlds."
The researchers plan to extend their studies to stars that differ significantly from the Sun, potentially offering new insights into stellar physics and exoplanet atmospheres. This could also lead to new methods for measuring stellar magnetic fields, which are notoriously difficult to quantify but have significant implications for understanding both stars and the planets that orbit them.
Research Report:Magnetic origin of the discrepancy between stellar limb-darkening models and observations
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