Star behavior can distort our view of distant planets

For centuries, humanity has gazed at the stars in the night sky, wondering if planets beyond our solar system exist. In the past few decades, technology has given us the ability to find them. Scientists have discovered thousands of exoplanets using a method called the transit technique.

This method involves watching for dips in a star’s brightness as a planet passes in front of it. These dips help scientists determine a planet’s size and atmospheric composition.

However, new research suggests that these measurements may not be as precise as previously thought. Changes in a star’s surface, caused by its own activity, can interfere with how researchers interpret exoplanet data.

A recent study published in The Astrophysical Journal Supplement Series reveals that stellar variability may distort exoplanet observations more than expected. This discovery has major implications for how we study planets beyond our solar system.

The transit method

The transit method has revolutionized the search for exoplanets. As a planet moves across its host star, it blocks a small portion of the starlight.

Scientists measure these changes in brightness to determine the planet’s size. They also study how the star’s light changes as it passes through the planet’s atmosphere. This helps them understand what gases make up the planet’s air.

However, stars are not uniform in brightness. Their surfaces contain regions that are hotter and brighter, as well as cooler and darker spots. These variations can interfere with transit data, leading scientists to misinterpret the characteristics of a planet.

If researchers do not take stellar variability into account, they may draw incorrect conclusions about an exoplanet’s size, temperature, and atmosphere.

How star variability affects exoplanet data

The study examined the atmospheres of 20 exoplanets, focusing on Jupiter- and Neptune-sized worlds. The researchers found that the host stars’ changing brightness distorted data for about half of the planets.

If these variations were not accounted for, researchers could mistakenly believe a planet was larger, hotter, or had a different atmospheric composition than it actually does.

“These results were a surprise – we found more stellar contamination of our data than we were expecting. This is important for us to know,“ said study lead author Dr. Arianna Saba from UCL Physics & Astronomy.

“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.”

Challenge of distinguishing planetary signals

The difficulty in exoplanet research lies in separating the light from the star from the light altered by the planet. If a star has uneven brightness, the planet may appear different depending on which region it passes in front of.

“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,” said Alex Thompson, a PhD student at UCL.

Some stars have stronger magnetic activity, creating patches of darker, cooler regions and brighter, hotter areas. These patches can alter how much light a planet appears to block.

If a planet moves in front of a hot, bright region, it may seem larger or have a denser atmosphere than it actually does. If it crosses a cooler, darker area, it might appear smaller than it really is.

Risk of false planet detections

Stellar variability can sometimes mimic the effect of a planet passing in front of a star.

If a star’s brightness fluctuates, it could create the illusion that a planet exists when there is none. This makes follow-up observations crucial for confirming whether a detected signal truly belongs to a planet.

“These variations from the star can also distort estimates of how much water vapor, for instance, is in a planet’s atmosphere. That is because the variations can mimic or obscure the signature of water vapor in the pattern of light at different wavelengths that reaches our telescopes,” explained Thompson.

This interference means that even basic details about a planet’s atmosphere could be misinterpreted. Scientists must take stellar activity into account when analyzing exoplanet data to avoid drawing incorrect conclusions.

Role of Hubble observations

To investigate the impact of stellar variability, the researchers examined 20 years of data from the Hubble Space Telescope.

The team used two instruments: the Space Telescope Imaging Spectrograph (STIS) and the Wide Field Camera 3 (WFC3). These instruments collect light at different wavelengths, helping scientists identify how much stellar activity affects exoplanet observations.

By analyzing light at visible, near-infrared, and near-ultraviolet wavelengths, the researchers found that stellar activity is more noticeable at shorter wavelengths. This means optical observations are particularly useful in detecting and correcting for stellar contamination.

Correcting star errors in exoplanet analysis

The research team developed two methods to determine whether stellar variability was interfering with planetary data.

“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,“ explained Dr. Saba.

“The 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.”

Using these methods, the researchers found that six of the 20 planets had a better fit with models that accounted for stellar variability. Another six showed minor contamination from their host star.

Importance of wavelength selection

“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,” noted Thompson.

By analyzing exoplanet data at different wavelengths, scientists can separate true planetary signals from distortions caused by stellar activity. This approach helps ensure that their findings are as accurate as possible.

Future studies of stars and exoplanets

This study highlights the complexity of exoplanet research and the importance of refining observation techniques.

Future space missions, such as James Webb, Ariel, and Twinkle, will provide larger and more detailed datasets. These missions will allow scientists to further improve their models and better understand the impact of stellar variability.

As researchers continue to explore new worlds, they must account for the ever-changing nature of stars. By doing so, they will enhance the accuracy of their discoveries and bring us closer to identifying Earth-like planets beyond our solar system.

The study is published in The Astrophysical Journal Supplement Series.

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