Challenges in Interpreting Exoplanet Data

Much of what we understand about exoplanets—planets outside our solar system—comes from observing the dimming of their host star’s light as the planets pass in front of it.

Figure 1. The Cosmic Deception Impacting Exoplanet Discoveries for Years.

By measuring the amount of starlight blocked, scientists can estimate a planet’s size. Additionally, analyzing changes in starlight as it passes through the planet's atmosphere helps reveal clues about its composition. Figure 1 shows The Cosmic Deception Impacting Exoplanet Discoveries for Years.

However, a recent study in suggests that these measurements might be more distorted than previously thought. Variations in a star’s brightness—due to hotter and cooler regions on its surface—could significantly affect the interpretation of exoplanet data.

The Impact of Stellar Variability

The researchers examined the atmospheres of 20 Jupiter- and Neptune-sized planets and found that the variability of the host stars distorted the data for about half of them.

If these variations aren't properly accounted for, the researchers warned, it could lead to misinterpretations of key features, such as the planets’ sizes, temperatures, and atmospheric compositions. However, the risk of misinterpretation could be minimized by considering a range of light wavelengths, particularly in the optical region where stellar contamination effects are most noticeable.

How ‘Patchy’ Stars Distort Observations

Alexandra (Alex) Thompson, the second author of the study and a PhD student at UCL Physics & Astronomy, highlighted the difficulty in separating signals from exoplanets and their host stars. “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.”

Some stars are described as ‘patchy,’ meaning they have a mix of colder, darker regions and hotter, brighter regions due to stronger magnetic activity. The hotter, brighter areas (faculae) emit more light, so when a planet passes in front of the star’s hottest parts, it can make the planet appear larger than it is, since it blocks more of the star’s light. This can lead to the false conclusion that the planet is hotter or has a denser atmosphere. Conversely, if a planet passes in front of a cold starspot, it can make the planet appear smaller.

Mimicking Planetary Signatures and False Positives

Additionally, the reduction in light from a starspot could imitate the effect of a planet passing in front of the star, causing scientists to mistakenly think a planet is present when it’s not. This is why follow-up observations are crucial for confirming exoplanet discoveries.

The star’s variations can also distort atmospheric data, such as estimates of water vapor, as they can either mimic or obscure the signature of water vapor in the light pattern that reaches telescopes.

Hubble’s 20-Year Data Set Sheds Light

The research team utilized 20 years of data from the Hubble Space Telescope, combining information from the Space Telescope Imaging Spectrograph (STIS) and the Wide Field Camera 3 (WFC3). They processed and analyzed the data consistently across the planets to reduce biases from differing data processing methods.

Comparing Models to Account for Stellar Activity

The team compared models that accounted for stellar variability with simpler models that did not, finding that the data for six planets out of the 20 analyzed fit better with models adjusted for stellar variability. Six other planets may have experienced minor contamination from their host star.

They examined light across visible, near-infrared, and near-ultraviolet wavelengths, recognizing that distortions from stellar activity are most prominent in the near-UV and visible (optical) regions compared to longer wavelengths in the infrared.

Two Methods to Detect Stellar Contamination

The team outlined two approaches to detect if stellar variability is affecting the planetary data:

1. Shape of the Spectrum: By analyzing the light pattern at different wavelengths passing through the planet from the star, researchers can determine if stellar activity needs to be considered in the data or if the planet alone can explain the observations.
2. Multiple Observations at Different Times:: If two observations of the same planet in the optical region differ significantly, this suggests stellar variability may be involved.

Thompson emphasized that the risk of misinterpretation is manageable with the right wavelength coverage. “Shorter wavelength, optical observations such as those used in this study are particularly helpful, as this is where stellar contamination effects are most apparent.”

Source: SciTECHDaily

Cite this article:

Priyadharshini S (2025), "The Cosmic Deception That's Been Distorting Exoplanet Discoveries for Years", Anathemas, pp. 232