A New Method for Detecting Clouds on Exoplanets

Astronomers have confirmed 6,291 exoplanets located in 4,709 distinct star systems. Tens of thousands of additional candidates await verification. With this expanding catalog and advanced instrumentation, the scientific community is undergoing a significant transition. The focus is shifting from the mere detection of new worlds to a detailed characterization of their environments. This new phase of exploration requires a deep understanding of planetary atmospheres. By analyzing starlight that passes through the air above an exoplanet, researchers can identify its specific chemical composition. They are also working to decipher how winds and weather patterns operate on these distant celestial bodies.

An international team of scientists has developed a novel method for studying cloud cycles on faraway planets. They utilized the James Webb Space Telescope, known as JWST, to validate this approach. The team focused their observations on a planet named WASP-94 Ab. This world is classified as a "Hot Jupiter," a type of gas giant that orbits extremely close to its host star. It resides in a binary star system approximately 700 light-years away, situated within the constellation Microscopium. This research represents one of the first successful detections of changing cloud patterns on a Hot Jupiter. It provides scientists with fresh insights into the planet’s formation history and physical composition. Consequently, this new methodology opens unprecedented opportunities for studying exoplanets and searching for worlds that might support life.

The research team was led by Sagnick Mukherjee, a Postdoctoral Fellow at the School of Earth and Space Exploration at Arizona State University. He collaborated with researchers from numerous prestigious institutions, including Johns Hopkins University, the Max Planck Institute for Astronomy, and the Harvard & Smithsonian Center for Astrophysics. Their findings were published in the journal Science on May 21, 2024.

Hot Jupiters are extreme planetary environments. They endure intense heat and radiation from their nearby stars. Because of these harsh conditions, they serve as excellent candidates for studying cloud dynamics. This involves analyzing how clouds form, move, and dissipate. Such studies also help scientists understand the complex physics and chemistry governing these atmospheres. The team used JWST to observe the planet as it moved in front of its star, an event known as a transit. During a transit, starlight filters through the planet’s atmosphere before reaching Earth. This process, called transit spectroscopy, allowed the researchers to examine the light with extreme precision.

JWST’s powerful optical instruments provided the researchers with a unique advantage. They were able to take separate measurements of the planet’s leading edge as it entered the transit. They also measured the trailing edge as the transit concluded. At the leading edge, the team observed air moving from the night side to the day side. At the trailing edge, the wind direction was reversed. These observations revealed that WASP-94 Ab experiences significantly different weather conditions in the morning compared to the evening. In the morning, the skies are obscured by clouds composed of magnesium silicate. In the evening, however, the skies are clear.

By isolating the clouds, the team produced one of the clearest pictures of a planet’s atmosphere to date. David Sing, a professor at Johns Hopkins University, led the observation program. He emphasized that clouds have historically been a major obstacle for astronomers.

“I’ve been looking at exoplanets for 20 years, and general cloudiness has been a thorn in our side. We’ve known for quite a while that clouds are pervasive on Hot Jupiter planets, which is annoying because it’s like trying to look at the planet through a foggy window. Not only have we been able to clear the view, but we can finally pin down what the clouds are made out of and how they’re condensing and evaporating as they move around the planet,” Sing said.

The researchers proposed two potential causes for these distinct weather patterns. One hypothesis suggests that strong winds push clouds upward on the cooler night side. These clouds then plunge onto the hot dayside, sinking deep into the planet’s interior and disappearing before sunset. An alternative possibility is that the atmosphere behaves similarly to morning fog on Earth. On our planet, the sun evaporates fog. On WASP-94 Ab, the sun is far more intense. The clouds form on the night side and boil away when they drift onto the dayside, where temperatures exceed 1,000 °C (1832 °F).

The success of this study relied heavily on JWST’s high sensitivity and resolution. These capabilities allowed the team to examine the trailing edge, where the night skies were clear. Such precision was unattainable with the older Hubble Space Telescope. Harry Baskett, a PhD student at the University of Exeter, explained the significance of this achievement.

“JWST provides us with exquisite observations of hot Jupiters and has recently been able to isolate the signatures of both morning and evening limbs on WASP-94Ab, information which is inherently 3D. For our group in Exeter, this is especially pertinent, as we use and develop a state-of-the-art 3D model, in partnership with our very own Met Office, to simulate the winds and all sort of physical and chemical processes driving the 3D structure of planetary atmospheres. It is really exciting to be able to combine observations and 3D simulations to distinguish weather patterns on exoplanets, infer the presence of clouds and constrain their formation mechanisms. Going forward, I hope that we can continue to combine observations and 3D simulations, to reveal more secrets about hot Jupiters,” Baskett said.

Examining the clear evening sky revealed surprising details about WASP-94 Ab. The planet is more similar to Jupiter than previously believed. Earlier studies suggested the planet possessed significantly higher amounts of oxygen and carbon than Jupiter. This finding was difficult to reconcile with existing models of planetary formation. The new data resolves this discrepancy. The revised measurements indicate that the planet has only five times the amount of oxygen and carbon found in Jupiter. This ratio aligns well with established theories of planet formation.

The team utilized their results as a benchmark to study eight other hot gas giants. They identified the same distinctive cloud cycle in two additional cases: WASP-39 b and WASP-17 b. As a next step, the team plans to utilize data from one of JWST’s ongoing observation programs. They intend to study cloud cycles across a diverse range of exoplanets.

Professor Nathan Mayne, also from the University of Exeter, highlighted the value of combining observations with computer models.

“This exciting project shows the power of combining the exquisite observations from JWST with cutting-edge theoretical and numerical modeling of planetary atmospheres. We have been able to determine what the clouds are made of in the atmosphere of a planet 700 light-years from Earth, which is crazy! This work also helps us to test, develop, and improve our modeling approaches, leading to improvements in Earth weather and climate prediction,” Mayne said.

This research demonstrates how new technology can solve long-standing mysteries in astronomy. By understanding the clouds on distant worlds, scientists can better comprehend the planets themselves. This knowledge also aids in improving our models of weather on Earth. As we continue to explore the cosmos, methods like this will be essential. They allow us to see beyond the fog and understand the true nature of the planets in our galaxy.