Star spots and planetary transits on distant stars

• NASA is using a new method, StarryStarryProcess, to map star spots. It shows stars with dark, spotty surfaces.
• This method reveals the number, location and brightness of star spots and combines transit data with star rotation for better accuracy.
• This process will help scientists study exoplanets more accurately.

New method to map distant stars

Scientists have devised a new method for mapping the spottiness of distant stars. They are using observations from NASA missions to detect orbiting planets crossing their stars’ faces. The model builds on a technique researchers have used for decades to study star spots.

By improving astronomers’ understanding of spotty stars, the new model – called StarryStarryProcess – can help gather more information. It can study planetary atmospheres and potential habitability using data from telescopes like NASA’s upcoming Pandora mission.

Sabina Sagynbayeva, a graduate student at Stony Brook University in New York, said:

Many of the models researchers use to analyze data from exoplanets, or worlds beyond our solar system, assume that stars are uniformly bright disks. But we know just by looking at our own sun that stars are more complicated than that. Modeling complexity can be difficult, but our approach gives astronomers an idea of how many spots a star might have, where they are located and how bright or dark they are.

A paper describing StarryStarryProcess, led by Sagynbayeva, was published Monday, August 25, 2025, in The Astrophysical Journal.

Planets identified from stellar transits

NASA’s TESS (Transiting Exoplanet Survey Satellite) and now-retired Kepler Space Telescope were designed to identify planets using transits. Any time a planet passes in front of a star, it results in a dip in stellar brightness.

These measurements reveal how the star’s light varies with time during each transit. And astronomers can arrange them in a plot astronomers call a light curve. Typically, a transit light curve traces a smooth sweep down as the planet starts passing in front of the star’s face. It reaches a minimum brightness when the world is fully in front of the star. Then finally, it rises smoothly as the planet exits and the transit ends.

By measuring the time between transits, scientists can determine how far the planet lies from its star and estimate its surface temperature. The amount of missing light from the star during a transit can reveal the planet’s size, which can hint at its composition.

And then there’s star spots

Every now and then, though, a planet’s light curve appears more complicated, with smaller dips and peaks added to the main arc. Scientists think these represent dark surface features akin to sunspots seen on our own sun: star spots.

Our sun’s total number of sunspots varies as it goes through its 11-year solar cycle. Scientists use them to determine and predict the progress of that cycle as well as outbreaks of solar activity that could affect us here on Earth.

Similarly, star spots are cool, dark, temporary patches on a stellar surface whose sizes and numbers change over time. Their variability impacts what astronomers can learn about transiting planets.

Using planetary transits

Scientists have previously analyzed transit light curves from exoplanets and their host stars to look at the smaller dips and peaks. This helps determine the host star’s properties, such as its overall level of spottiness, the inclination angle of the planet’s orbit, the tilt of the star’s spin compared to our line of sight, and other factors. Sagynbayeva’s model uses light curves that include not only transit information, but also the rotation of the star itself. This provides even more detailed information about these stellar properties.

Co-author Brett Morris, a senior software engineer at the Space Telescope Science Institute in Baltimore, said:

Knowing more about the star in turn helps us learn even more about the planet, like a feedback loop. For example, at cool enough temperatures, stars can have water vapor in their atmospheres. If we want to look for water in the atmospheres of planets around those stars – a key indicator of habitability – we better be very sure that we’re not confusing the two.

A transit was observed on a gas giant planet

To test their model, Sagynbayeva and her team looked at transits from a planet called TOI 3884 b. It’s located around 141 light-years away in the constellation Virgo the Maiden.

Discovered by TESS in 2022, astronomers think the planet is a gas giant about five times bigger than Earth and 32 times its mass.

The StarryStarryProcess analysis suggests that the planet’s cool, dim star – called TOI 3384 – has concentrations of spots at its north pole. Its north pole also tips toward Earth so that the planet passes over the pole from our perspective.

Currently, the only available data sets that can be fit by Sagynbayeva’s model are in visible light, which excludes infrared observations taken by NASA’s James Webb Space Telescope. But NASA’s upcoming Pandora mission will benefit from tools like this one. Pandora, a small satellite developed through NASA’s Astrophysics Pioneers Program, will study the atmospheres of exoplanets and the activity of their host stars. It’ll take long-duration multi-wavelength observations. The Pandora mission’s goal is to determine how the properties of a star’s light differs when it passes through a planet’s atmosphere so scientists can better measure those atmospheres using Webb and other missions.

Future studies

Allison Youngblood, TESS project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said:

The TESS satellite has discovered thousands of planets since it launched in 2018. While Pandora will study about 20 worlds, it will advance our ability to pick out which signals come from stars and which come from planets. The more we understand the individual parts of a planetary system, the better we understand the whole system, and our own.

Bottom line: NASA scientists use a new method with TESS and Kepler data to map star spots on distant stars to enhance our understanding of stellar activity and exoplanets.

Via NASA’s Goddard Space Flight Center

Read more: Our 1st 3D weather map from a distant exoplanet