We have the transit method to thank for the large majority of the exoplanets we\u2019ve discovered. When an exoplanet transits its star, the dip in starlight tells astronomers that a planet is present. Analyzing the light can tell them about the planet\u2019s size and atmospheric properties. However, a star\u2019s surface isn\u2019t always uniformly heated. There can be hotter, brighter spots and colder, dimmer spots that change over time.<\/p>\n
New research says these temperamental stars are distorting our understanding of exoplanets. <\/p>\n
<\/p>\n The number of confirmed exoplanets is approaching 6,000. Astronomers want to understand these planets better in all their bewildering variety. The only way to do that is to examine light and how it changes in exquisite detail. When an exoplanet transits in front of its host star, astronomers can \u2018read\u2019 the starlight as it passes through the planet\u2019s atmosphere.<\/p>\n However, new research shows that the stars that host all these planets can pollute the light signal from their orbiting planets, giving us a distorted view of their sizes, temperatures, and atmospheres.<\/p>\n The research is \u201cA Population Analysis of 20 Exoplanets Observed from Optical to Near-infrared Wavelengths with the Hubble Space Telescope: Evidence for Widespread Stellar Contamination,\u201d and it\u2019s published in The Astrophysical Journal Supplement Series. The lead author is Arianna Saba from the Department of Physics and Astronomy at University College London. <\/p>\n A star\u2019s surface is defined in large part by its temperature, which is influenced by the star\u2019s powerful magnetic fields. Magnetic fields can inhibit the heat flow from a star\u2019s interior to its surface, creating a cooler, dimmer region. Conversely, it can channel more heat into other areas, creating brighter regions. <\/p>\n \u201cSome stars might be described as \u2018patchy\u2019 \u2013 they have a greater proportion of colder regions, which are darker, and hotter regions, which are brighter, on their surface. This is due to stronger magnetic activity,\u201d said study co-author Alexandra Thompson. <\/p>\n \u201cHotter, brighter regions (faculae) emit more light, and so, for instance, if a planet passes in front of the hottest part of the star, this might lead researchers to over-estimate how large the planet is, as it will seem to block out more of the star\u2019s light, or they might infer the planet is hotter than it is or has a denser atmosphere,\u201d Thompson explained. \u201cThe reverse is true if the planet passes in front of a cold starspot, making the planet appear \u2018smaller.\u2019<\/p>\n These temperamental stars can also produce false positives.<\/p>\n \u201cOn the other hand, 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. This is why follow up observations are so important to confirm exoplanet detections,\u201d said Thompson.<\/p>\n The question is, how much of our understanding of these exoplanets is polluted by these patchy stars? Is stellar contamination creating a bias in our understanding of the exoplanet population?<\/p>\n To find out, Saba and her co-researchers examined the archival data from 20 exoplanet atmospheres previously observed with the Hubble\u2019s Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) instruments. These workhorse instruments \u201csee\u201d in UV, infrared, and visible light. They wanted to know if observations taken with the same instruments at different times produced different results and if any differences were confined to observations in specific wavelengths.<\/p>\n \u201cTo obtain spectral information from the near-ultraviolet to the near-infrared, we reanalyzed 16 WFC3 and over 50 STIS archival data sets with our dedicated HST pipeline,\u201d the authors write in their paper. \u201cAcross our target sample, we observe significant divergence among multiple observations conducted with the same STIS grating at various epochs, while we do not detect variations in the WFC3 data sets.\u201d<\/p>\n This suggested that stellar contamination is an issue, but the researchers dug deeper to understand how. Using Bayesian tools and other analytic models, they found that stellar activity had contaminated about half of the exoplanet atmospheres in their sample to varying degrees. Six of the exoplanets had pronounced contamination, and six others had lesser degrees of contamination.<\/p>\n \u201cThese results were a surprise \u2013 we found more stellar contamination of our data than we were expecting,\u201d said lead author Saba. \u201cThis is important for us to know. By refining our understanding of how stars\u2019 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.\u201d Twinkle is a low-cost mission that will study exoplanet atmospheres from Low-Earth Orbit. <\/p>\n Stellar contamination of exoplanet observations is no small matter. It can skew results in very pronounced ways. \u201cAccounting for stellar activity can significantly alter planetary atmospheric parameters like molecular abundances (up to 6 orders of magnitude) and temperature (up to 145%), contrasting with the results of analyses that neglect activity,\u201d the authors write in their paper. <\/p>\n According to the researchers, there are two ways to determine if stellar variability is affecting exoplanet data. <\/p>\n \u201cOne is to look at the overall shape of the spectrum \u2013 that is, the pattern of light at different wavelengths that has passed through the planet from the star \u2013 to see if this can be explained by the planet alone or if stellar activity is needed,\u201d said Saba. \u201cThe other is to have two observations of the same planet in the optical region of the spectrum that are taken at different times. If these observations are very different, the likely explanation is variable stellar activity.\u201d<\/p>\n One of the key findings concerns optical and UV observations. Since stellar activity is much more visible in optical and UV, exoplanet observations based on these wavelengths are more likely to reveal the contamination. Conversely, IR observations may overlook the contamination.<\/p>\n \u201cOur results emphasize the importance of considering the effects of stellar contamination in exoplanet transit studies; this issue is particularly true for data sets obtained with facilities that do not cover the optical and\/or UV spectral range where the activity is expected to be more impactful but also more easily detectable,\u201d the paper states. <\/p>\n \u201cThe risk of misinterpretation is manageable with the right wavelength coverage,\u201d said Thompson. \u201cShorter wavelength, optical observations such as those used in this study are particularly helpful, as this is where stellar contamination effects are most apparent.\u201d<\/p>\n This issue clearly needs more investigating, and the researchers say they\u2019ve identified stars that need more follow-up. They also explain that previous exoplanet atmosphere studies should be revisited, especially ones that lacked broad optical or UV coverage. By the same token, future exoplanet atmospheric studies should be multi-wavelength. <\/p>\n According to the authors, the active stars identified in this research should also be studied more thoroughly. This will increase astronomers\u2019 understanding of how they influence observations of exoplanet atmospheres. Better models and analytic tools are also needed.<\/p>\n We\u2019re still in the very early days of examining exoplanet atmospheres, so these results aren\u2019t exactly surprising. The JWST is probing some exoplanet atmospheres, and future missions like the ESA\u2019s ARIEL (Atmospheric Remote-Sensing Infrared Exoplanet Large survey) will do the same. ARIEL will perform the first large-scale survey of the chemistry of exoplanet atmospheres, highlighting the significance of these results.<\/p>\n \u201cOur findings demonstrate the significant role that stellar contamination may have in all exoplanet spectra observations,\u201d the authors write in their conclusion. \u201cTherefore, comprehending, modeling, and correcting for the impact of stellar activity is important for a complete characterization of exoplanet atmospheres.\u201d<\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2025\/02\/activity-02-high-1024x1024.jpg\")
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