‘Temperamental’ stars, which vary in brightness over short periods, may be skewing our observations of many distant planets, according to a recent study.
A lot of what we know about exoplanets—planets outside our solar system—comes from observing the drops in starlight when these planets transit in front of their parent stars.
This method can reveal information about a planet’s size (by measuring the amount of starlight blocked) and its atmospheric composition (by analyzing how the planet refracts light as it passes in front of the star).
However, a new study published in The Astrophysical Journal Supplement Series suggests that variations in starlight caused by fluctuating surface temperatures of stars might be altering our understanding of exoplanets more than thought.
The research focused on the atmospheres of 20 planets similar in size to Jupiter and Neptune, revealing that about half of them showed distorted data linked to the variability of their host stars.
If scientists fail to consider these changes, the research team warned, they could misinterpret key characteristics such as the planets’ size, temperature, and atmospheric makeup. They suggested that the risk of misinterpretation can be minimized by examining a range of light wavelengths, particularly in the optical spectrum where stellar contamination is most noticeable.
Lead author Dr. Arianna Saba from UCL Physics & Astronomy said, “These findings were unexpected; we found greater stellar influence on our data than anticipated. Understanding how star variability impacts our exoplanet evaluations is critical. By fine-tuning our approach, we can enhance our models and effectively manage the larger datasets expected from missions like James Webb, Ariel, and Twinkle.”
Second author Alexandra (Alex) Thompson, a PhD student at UCL Physics & Astronomy focusing on exoplanet host stars, stated, “We derive information about exoplanets from the light of their host stars, making it challenging to separate stellar signals from those generated by the planet.”
“Some stars might be considered ‘patchy,’ displaying a mix of colder, darker regions and hotter, brighter ones due to intense magnetic activity.”
“The brighter areas (faculae) emit more light; thus, if a planet crosses in front of a star’s hottest region, it could lead researchers to overestimate its size or infer a higher temperature or denser atmosphere. Conversely, if the planet transits a cooler starspot, it may appear smaller.”
“Additionally, the decrease in light from a starspot could mimic the effect of a planet transit, creating a false indication of a planet’s presence. Therefore, follow-up observations are crucial for confirming exoplanet discoveries.”
“These stellar variations may also skew estimates of water vapor in a planet’s atmosphere, as they can obscure or imitate the signature of water vapor detectable in different wavelengths of light observed by our telescopes.”
In their study, researchers utilized 20 years of Hubble Space Telescope data, combining insights from its Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3).
They processed the data uniformly for each planet to ensure fair comparison, minimizing biases from varying data processing methods.
The team evaluated which combination of planetary and stellar models best fitted their data, contrasting those that considered stellar variability with simpler models that did not. They determined that data from six out of the twenty examined planets matched more closely with models accounting for stellar variability, while six others may have experienced slight contamination from their host star.
The analysis covered visible, near-infrared, and near-ultraviolet wavelengths, recognizing that distortions from stellar activity are significantly clearer in the near-UV and visible light ranges than in the longer infrared wavelengths.
Two methods were suggested to identify if stellar variability is impacting planetary data.
Dr. Saba elaborated: “One approach involves examining the overall spectrum shape—analyzing the light patterns that pass from the star through the planet—to determine if the results can be attributed solely to the planet or if stellar activity needs to be taken into account. The second method involves comparing two observations of the same planet taken at different times, where significant differences suggest variable stellar activity.”
Alex Thompson added, “The risk of misinterpretation can be managed with adequate wavelength coverage. Observations in shorter wavelengths, particularly optical ones used in this research, are beneficial as they highlight stellar contamination effects more clearly.”
