Research on the exoplanet WASP-39b has uncovered the need to include stellar magnetic fields in models to match observations with theoretical predictions, significantly advancing exoplanet study accuracy. (Artist’s concept.) Credit: SciTechDaily.com
From the brightness variations of its host star, an
Stars with low magnetic field strength exhibit a more pronounced limb darkening than those 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 the brightness of a star over a longer period of time. The brightness of a star fluctuates constantly, for example because its luminosity is subject to natural fluctuations. Exoplanets can also leave traces in the light curve. If 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 regularly recurring drop in brightness. Precise evaluations of such curves provide information about the size and orbital period of the planet. Researchers can also obtain information about the composition of the planet’s atmosphere, if the light from the star is split into its different wavelengths or colours.
A close look at a star’s brightness distribution
The limb of a star, the edge of the stellar disk, plays a decisive role in the interpretation of its light curve. Just as in the case of the Sun, the limb appears darker to the observer than the inner area. However, the star does not actually shine less brightly further out. “As the star is a sphere and its surface curved, we look into higher and therefore cooler layers at the limb than in the center,” explains coauthor and MPS-Director Prof. Dr. Laurent Gizon. “This area therefore appears darker to us,” he adds.
It is known that the limb darkening affects the exact shape of the exoplanet signal in the light curve: The dimming determines how steeply the brightness of a star falls during a planetary transit and then rises again. However, it has not been possible to reproduce observational data accurately using conventional models of the stellar atmosphere. The decrease of 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 exoplanets’ signal,” says MPS-Director Prof. Dr. Sami Solanki, coauthor 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 in their interior through enormous flows of hot
The researchers were also able to prove that the discrepancy between observational data and model calculations disappears if the star’s magnetic field is included in the computations. To this end, the team turned to selected data from