Exoplanets true to size

A star's magnetic field must be considered in order to correctly determine the characteristics of their exoplanets from observations by space telescopes such as Kepler, James Webb, or PLATO. This is demonstrated by new model calculations presented today in the journal Nature Astronomy by a research group led by the Max Planck Institute for Solar System Research (MPS) in Germany. The researchers show that the distribution of the star's brightness over its disk depends on the star's level of magnetic activity. This, in turn, affects the signature of an exoplanet in observational data. The new model must be used in order to properly interpret the data from the latest generation of space telescopes pointed at distant worlds outside our Solar System.

700 light years away from Earth in the constellation Virgo, the planet WASP-39b orbits the star WASP-39. The gas giant, which takes little more than four days to complete one orbit, is one of the best-studied exoplanets: Shortly after its commissioning in July 2022, NASA's James Webb Space Telescope turned its high-precision gaze on the distant planet. The data revealed evidence of large quantities of water vapor, of methane and even, for the first time, of carbon dioxide in the atmosphere of WASP-39b. A minor sensation! But there is still one fly in the ointment: researchers have not yet succeeded in reproducing all the crucial details of the observations in model calculations. This stands in the way of an even more precise analyses of the data. In the new study led by the MPS, the authors, including researchers from the Massachusetts Institute of Technology (USA), the Space Telescope Science Institute (USA), Keele University (United Kingdom), and the University of Heidelberg (Germany), show a way to overcome this obstacle.

"The problems arising when interpreting the data from WASP-39b are well known from many other exoplanets -- regardless 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. "As with other stars orbited by exoplanets, the observed light curve of WASP-39 is flatter than previous models can explain," she adds.

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 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 those with a strong magnetic field.

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 NASA's Kepler Space Telescope, which captured the light of thousands and 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" observational data from these calculations. As a comparison with the real data showed, by including the magnetic field, the Kepler data is successfully reproduced.

The team also extended its considerations to data from the James Webb Space Telescope. The telescope is able to split the light of distant stars into its various wavelengths and thus search for the characteristic signs of certain molecules in the atmosphere of the discovered planets. As it turns out, the magnetic field of the parent star influences the stellar limb darkening differently at different wavelengths -- and should therefore be taken into account in future evaluations in order to achieve even more precise results.

From telescopes to models

"In the past decades and years, the way to move forward in exoplanet research was to improve the hardware, the space telescopes designed to search for and characterize new worlds. The James Webb Space Telescope has pushed this development to new limits," says Dr. Alexander Shapiro, coauthor of the current study and head of an ERC-funded research group at the MPS. "The next step is now to improve and refine the models to interpret this excellent data," he adds.

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Nearby star could help explain why our Sun didn’t have sunspots for 70 years

The number of sunspots on our Sun typically ebbs and flows in a predictable 11-year cycle, but one unusual 70-year period when sunspots were incredibly rare has mystified scientists for three hundred years. Now a nearby Sun-like star seems to have paused its own cycles and entered a similar period of rare starspots, according to a team of researchers at Penn State. Continuing to observe this star could help explain what happened to our own Sun during this "Maunder Minimum" as well as lend insight into the Sun's stellar magnetic activity, which can interfere with satellites and global communications and possibly even affect climate on Earth.

The star -- and a catalog of 5 decades of starspot activity of 58 other Sun-like stars -- is described in a new paper that appears online in the Astronomical Journal.

Starspots appear as a dark spot on a star's surface due to temporary lower temperatures in the area resulting from the star's dynamo -- the process that creates its magnetic field. Astronomers have been documenting changes in starspot frequency on our Sun since they were first observed by Galileo and other astronomers in the 1600s, so there is a good record of its 11-year cycle. The exception is the Maunder Minimum, which lasted from the mid 1600s to early 1700s and has perplexed astronomers ever since.

"We don't really know what caused the Maunder Minimum, and we have been looking to other Sun-like stars to see if they can offer some insight," said Anna Baum, an undergraduate at Penn State at the time of the research and first author of the paper. "We have identified a star that we believe has entered a state similar to the Maunder Minimum. It will be really exciting to continue to observe this star during, and hopefully as it comes out of, this minimum, which could be extremely informative about the Sun's activity three hundred years ago."

The research team pulled data from multiple sources to stitch together 50 to 60 years of starspot data for 59 stars. This included data from the Mount Wilson Observatory HK Project -- which was designed to study stellar surface activity and ran from 1966 to 1996 -- and from planet searches at Keck Observatory which include this kind of data as part of their ongoing search for exoplanets from 1996 to 2020. The researchers compiled a database of stars that appeared in both sources and that had other readily available information that might help explain starspot activity. The team also made considerable efforts to standardize measurements from the different telescopes to be able to compare them directly and otherwise clean up the data.

The team identified or confirmed that 29 of these stars have starspot cycles by observing at least two full periods of cycles, which often last more than a decade. Some stars did not appear to have cycles at all, which could be because they are rotating too slowly to have a dynamo and are magnetically 'dead' or because they are near the end of their lives. Several of the stars require further study to confirm whether they have a cycle.

"This continuous, more than 50-year time series allows us to see things that we never would have noticed from the 10-year snapshots that we were doing before," said Jason Wright, professor of astronomy and astrophysics at Penn State and an author of the paper. "Excitingly, Anna has found a promising star that was cycling for decades but appears to have stopped."

According to the researchers, the star -- called HD 166620 -- was estimated to have a cycle of about 17 years but has now entered a period of low activity and has shown no signs of starspots since 2003.

"When we first saw this data, we thought it must have been a mistake, that we pulled together data from two different stars or there was a typo in the catalog or the star was misidentified," said Jacob Luhn, a graduate student at Penn State when the project began who is now at the University of California, Irvine. "But we double and triple checked everything. The times of observation were consistent with the coordinates we expected the star to have. And there aren't that many bright stars in the sky that Mount Wilson observed. No matter how many times we checked, we always come to the conclusion that this star has simply stopped cycling."

The researchers hope to continue studying this star throughout its minimum period and potentially as it comes out of its minimum and begins to cycle once again. This continued observation could provide important information about how the Sun and stars like it generate their magnetic dynamos.

"There's a big debate about what the Maunder Minimum was," said Baum, who is now a doctoral student at Lehigh University studying stellar astronomy and asteroseismology . "Did the Sun's magnetic field basically turn off? Did it lose its dynamo? Or was it still cycling but at a very low level that didn't produce many sunspots? We can't go back in time to take measurements of what it was like, but if we can characterize the magnetic structure and magnetic field strength of this star, we might start to get some answers."

A better understanding of the surface activity and magnetic field of the Sun could have several important implications. For example, strong stellar activity can disable satellites and global communications, and one particularly strong solar storm disabled a power grid in Quebec in 1989. It has also been suggested that sunspot cycles may have a connection to climate on Earth. Additionally, the researchers said that information from this star could impact our search for planets beyond our solar system.

"Starspots and other forms of surface magnetic activity of stars interfere with our ability to detect the planets around them," said Howard Isaacson, a research scientist at the University of California, Berkeley, and an author of the paper. "Improving our understanding of a star's magnetic activity might help us improve our detection efforts."

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