Clouds Affect Exoplanet Observations More Than Expected

For about 20 years, researchers have been able to observe the atmospheres of so-called “hot Jupiters”, that is, large, hot gas giants close to their host stars that often have puffy, clear atmospheres. A greater challenge has been studying smaller, cooler rocky planets similar to Earth. The latter have less massive atmospheres and often contain a lot of clouds or haze that can block light.

Scientists have now taken a step toward better understanding these challenges by studying the atmosphere of the puffy exoplanet WASP-107 b. The exoplanet is located about 200 light-years from Earth and is nearly as large as Jupiter, but has a mass of just under one-tenth that of Jupiter.

Its puffy atmosphere makes it easier to study than an Earth-like planet, while also possessing several of the characteristics that make smaller planets difficult to observe. WASP-107 b is certainly warmer than Earth-like planets but cooler than hot gas giants, and it has clouds high up in its atmosphere.

In the new study, a ground-based telescope was used for the first time to observe the planet’s atmosphere at infrared wavelengths. Using the Very Large Telescope (VLT) in Chile, the researchers observed the planet’s atmosphere on two separate occasions. On both occasions, they first measured the light from the star around which the planet orbits. Then, they measured the light that passes through the planet’s atmosphere as the planet passes in front of its star, a phenomenon known as a transit. The measurements were made using transmission spectroscopy with the CRIRES+ spectrograph. The researchers analyzed the spectral lines from the various measurements and identified the chemical composition of the planet’s atmosphere. The results from the ground-based telescope observations were then compared with simulation models.

“Since we are so unaccustomed to studying planets like WASP-107 b with ground-based telescopes, we wanted to combine our actual observations with simulations, both to reflect reality and to determine whether the results were plausible,” says Linn Boldt-Christmas, who led the study during her doctoral studies at the Department of Physics and Astronomy.

Using the VLT observations, along with previous measurements from equipment such as the James Webb Space Telescope, the researchers were able to confirm the presence of carbon monoxide and water in the exoplanet’s atmosphere.

At the same time, the results showed that the exoplanet’s clouds affected the signals much more than expected. The signal reaching the ground-based telescope was weaker than some of the models predicted, illustrating how difficult it can be to study exoplanet atmospheres from the ground.

“The fact that the signal from the VLT was weaker is partly because the planet has unusual clouds in its atmosphere that block much of what we want to see. One of our simulations with clouds produced a signal that was nearly five times weaker than the corresponding simulation without clouds,” says Linn Boldt-Christmas. “We suspected that our methods wouldn’t hold up very well, but we didn’t think it would be this bad!”

The significant differences between the cloud-free simulations and the VLT observations demonstrate how difficult it is to study planets with cloud-rich atmospheres from the ground. The results will help researchers develop more accurate models and improve their methods for future studies of smaller, more Earth-like planets.

“By studying this planet, we’ve been able to test how well our tools work. We’ve learned that we need to develop better methods and models for studying exoplanet atmospheres going forward. The fact that we’re learning this now gives us the opportunity to improve them before we begin studying Earth-like planets and thus take a step closer to finding a planet similar to our own,” says Linn Boldt-Christmas.

Camilla Thulin