In a groundbreaking new study, researchers from University College London (UCL) have unveiled an intriguing phenomenon that could be distorting our understanding of distant exoplanets. The study, published in The Astrophysical Journal Supplement, suggests that the fluctuating brightness of “temperamental” stars—those with varying hot and cold regions on their surfaces—may be skewing the data we collect about planets beyond our solar system.
The technique most commonly used to study exoplanets is by observing the slight dips in starlight that occur when a planet passes in front of its host star. These events allow astronomers to infer important details about the exoplanet, such as its size, temperature, and the composition of its atmosphere. However, this new research casts doubt on the accuracy of some of these interpretations, revealing that starspot variability could be distorting the signals scientists use to study planets.
The Role of Starspot Variability in Exoplanet Studies
The discovery is significant because the process of detecting and characterizing exoplanets hinges largely on data about their host stars. When a planet transits across the face of a star, it causes a temporary decrease in brightness that scientists use to infer a range of properties about the planet. However, this method assumes that the star’s light is consistent and unchanging.
In reality, stars—especially those that are more active—are far from uniform. Many stars exhibit stellar variability, which occurs when there are regions of a star’s surface that are hotter and colder due to magnetic activity. These temperature differences create patchy surfaces, with some parts of the star being much brighter (hotter) or dimmer (colder) than others. This phenomenon, called stellar faculae (hotter regions) and starspots (cooler regions), can lead to fluctuations in the star’s brightness, which can be easily mistaken for changes caused by a planet’s transit.
How Starspot Variability Affects Planetary Data
The researchers looked at the atmospheres of 20 exoplanets, all of which are roughly the size of Jupiter or Neptune. Their findings indicated that about half of the exoplanet data were affected by stellar variability, leading to potential misinterpretations of key characteristics, such as the planet’s size, temperature, and atmospheric composition.
For instance, if a planet transits across the brightest region of a star, it might appear to block out more of the star’s light than it actually does, leading to an overestimation of the planet’s size. Conversely, if the planet passes in front of a cooler, dimmer part of the star, it may appear smaller than it truly is. Additionally, these variations in brightness could distort temperature readings, making the planet seem hotter or cooler than it is, or even skewing data on the composition of the planet’s atmosphere, such as the presence of water vapor.
According to Dr. Arianna Saba, the lead author of the study and a Ph.D. researcher at UCL Physics & Astronomy, the extent of this stellar contamination was more significant than the team had initially expected. “These results were a surprise—we found more stellar contamination of our data than we were expecting,” she said. “By refining our understanding of how stars’ variability might affect our interpretations of exoplanets, we can improve our models and make smarter use of the much bigger datasets to come from missions including James Webb, Ariel, and Twinkle.”
Challenges in Differentiating Stellar and Planetary Signals
Alexandra Thompson, a co-author and current Ph.D. student at UCL, further elaborated on the challenge of distinguishing between stellar variability and the actual signal from an exoplanet. “We learn about exoplanets from the light of their host stars, and it is sometimes hard to disentangle what is a signal from the star and what is coming from the planet,” Thompson explained. This is especially difficult when a star has a high degree of variability in its surface temperature, as it can create a “patchy” appearance, with regions that are much hotter and brighter than others.
These brighter regions, known as faculae, emit more light, which can skew the interpretation of the planet’s properties. For example, if a planet passes over the hottest part of its star, it could appear to block out more of the star’s light than it would if the planet passed over a cooler region. This would make the planet seem larger than it is. On the other hand, if the planet transits a cooler region of the star, it may appear smaller.
Furthermore, the cool starspots on a star’s surface can mimic the signal of a planet transiting in front of the star, leading researchers to believe there is a planet where none exists. “The reduction in emitted light from a starspot could even mimic the effect of a planet passing in front of a star, leading you to think there might be a planet when there is none,” Thompson said. This underscores the importance of follow-up observations to confirm the existence of exoplanets.
The Importance of Wavelengths in Observing Exoplanets
The researchers also looked at how different wavelengths of light affect the accuracy of their observations. They discovered that variations in the star’s brightness due to stellar activity were most apparent in the optical and near-ultraviolet (UV) wavelengths. At these wavelengths, the impact of stellar variability can more easily distort the planetary data. On the other hand, in the infrared wavelengths, the stellar variations are less pronounced, providing clearer data on the planet itself.
One of the key findings of the study was that the use of a range of wavelengths, particularly in the optical region, can help mitigate the effects of stellar contamination. Dr. Saba explained, “One is to look at the overall shape of the spectrum—that is, the pattern of light at different wavelengths that has passed through the planet from the star—to see if this can be explained by the planet alone or if stellar activity is needed.”
Another useful technique is to compare two observations of the same planet taken at different times. If the observations differ significantly, it is a likely indication that stellar activity is playing a role. “If these observations are very different, the likely explanation is variable stellar activity,” Thompson added.
Methodology of the Study
To conduct this study, the research team analyzed 20 years of data from the Hubble Space Telescope, which had observed these planets using its Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) instruments. The team used a standardized method to process and analyze each planet’s data to ensure consistency and avoid biases that might arise from using different analysis methods.
They compared stellar models that accounted for variability with simpler models that assumed no variability. For six of the 20 planets analyzed, the data was a better fit with models that incorporated the star’s variable activity. Additionally, six other planets might have had minor contamination from their host stars, further highlighting the need for a nuanced approach to exoplanet observations.
The study demonstrated the value of using multiple wavelengths to better understand the complex interplay between starspot variability and exoplanet data. By examining both visible and near-infrared light, the team was able to differentiate between stellar contamination and true planetary signals, which is critical for interpreting the data of distant worlds.
Looking Ahead: Implications for Future Missions
This study offers important insights into the challenges scientists face when studying exoplanets. As the next generation of telescopes, such as the James Webb Space Telescope, Ariel, and Twinkle, prepare to collect even more data on exoplanets, understanding the effects of stellar variability will be crucial to improving the accuracy of planetary models.
Dr. Saba stressed the importance of refining our techniques for studying exoplanets, especially as the amount of data from future space missions grows exponentially. “By improving our models and understanding how stellar variability can distort data, we can ensure that we’re making the best use of the new data we’re getting from these cutting-edge telescopes.”
As scientists continue to unravel the mysteries of distant planets, the ability to account for stellar activity and its impact on observations will be an essential part of advancing our knowledge of the universe beyond our solar system.
Reference: Arianna Saba et al, A Population Analysis of 20 Exoplanets Observed from Optical to Near-infrared Wavelengths with the Hubble Space Telescope: Evidence for Widespread Stellar Contamination, The Astrophysical Journal Supplement Series (2025). DOI: 10.3847/1538-4365/ad8c3c