A Glimpse into Alien Worlds: James Webb Space Telescope Detects Water Vapor on Ultra-Hot Exoplanet WASP-18 b

The team used NASA's James Webb Space Telescope's NIRISS SOSS 0.85-2.8 um wavelength range to measure the thermal emission spectrum of WASP-18 b, capturing 65% of the total energy emitted by the planet. Due to the extreme heat on the planet's day side, water vapor molecules would disintegrate. However, the Webb Telescope detected water vapor directly on the planet, even in small quantities, demonstrating the observatory's remarkable sensitivity. Image credit: NASA/JPL-Caltech (R. Hurt/IPAC) 

May 31, 2023

In a groundbreaking discovery, NASA's James Webb Space Telescope has revealed new insights about an intriguing exoplanet known as WASP-18 b. Located approximately 400 light-years away, this gas giant exoplanet is a captivating celestial object that has captured the attention of astronomers since its discovery in 2009. WASP-18 b is an ultra-hot gas giant, ten times more massive than Jupiter, and boasts an astonishingly short orbital period of just 23 hours around its star, which is slightly larger than our Sun. Now, with the aid of the James Webb Space Telescope, researchers have obtained detailed observations of WASP-18 b, uncovering fascinating information about its atmosphere and temperature distribution.

The Discovery of Water Vapor

Through careful examination of WASP-18 b's atmospheric properties, scientists have identified the presence of water vapor, marking a significant breakthrough in our understanding of exoplanetary atmospheres. By studying the exoplanet during its secondary eclipse, a phenomenon where it moves behind its star and reappears, researchers were able to measure the combined light from the star and the planet, as well as refine measurements obtained solely from the star. This unique approach provided valuable insights into the composition and characteristics of WASP-18 b's atmosphere.

Temperature Variations and Brightness Mapping

One striking feature of WASP-18 b is its tidally-locked nature, where the same side of the planet always faces its star, similar to the relationship between the Moon and Earth. Utilizing the James Webb Space Telescope, scientists created a temperature map that reveals dramatic temperature variations across the planet's day side. From the hottest point directly facing the star to the terminator, the boundary between the illuminated and dark regions, there is a staggering temperature difference of up to 1,000 degrees. This stark contrast showcases the extreme nature of this exoplanet and its unique climate.

The Role of the James Webb Space Telescope

The James Webb Space Telescope has played a pivotal role in unraveling the mysteries of WASP-18 b. Its enhanced sensitivity has allowed researchers to generate detailed maps of hot giant exoplanets like never before. This groundbreaking study marks the first time a planet has been extensively mapped using the capabilities of the James Webb Space Telescope. The observations have confirmed several predictions made by existing models, such as the sharp temperature drop away from the point directly facing the star. The obtained data has greatly contributed to our understanding of exoplanetary atmospheres and their complex dynamics.

Implications and Possible Explanations

The temperature gradients observed across the day side of WASP-18 b have raised intriguing questions regarding the factors that influence atmospheric circulation on this exoplanet. Researchers have noticed a lack of east-west winds in the brightness map, which aligns with models incorporating atmospheric drag. One possible explanation for this phenomenon is the presence of a strong magnetic field on WASP-18 b. If confirmed, this discovery would be highly significant and add to our knowledge of exoplanetary magnetic fields.

Water Vapor Detection and Atmospheric Composition

Despite the scorching temperatures on WASP-18 b, the James Webb Space Telescope's observations have revealed the presence of water vapor in the planet's atmosphere. This finding is remarkable considering that the intense heat would typically break apart most water molecules. The sensitivity of the James Webb Space Telescope in detecting even minute traces of water is a testament to its advanced capabilities. The recorded amounts of water vapor at different elevations indicate its presence throughout various layers of WASP-18 b's atmosphere.

Insights into Planet Formation

The composition analysis of WASP-18 b's spectrum provides crucial insights not only into the atmospheric constituents but also into the planet's formation process. Comparisons with the composition of its host star suggest that WASP-18 b likely formed from the remnant gas present during the star's early stages. Understanding the formation mechanisms of enigmatic exoplanets like WASP-18 b, which do not have counterparts within our solar system, is crucial for developing a comprehensive understanding of the universe.

Contributions and Future Research

The discovery of water vapor and the detailed temperature mapping of WASP-18 b are the result of collaborative efforts involving over 100 scientists worldwide. These scientists, such as Louis-Philippe Coulombe, Ryan Challener, Anjali Piette, and Megan Mansfield, have been instrumental in pushing the boundaries of exoplanetary research. Their work is part of the Transiting Exoplanet Community Early Release Science Program, which aims to unravel the mysteries of exoplanets through the James Webb Space Telescope. The findings from this study set the stage for future research, where similar measurements and investigations will be conducted on a broader range of exoplanets.

The James Webb Space Telescope's discovery of water vapor and the temperature extremes on the exoplanet WASP-18 b represents a significant milestone in exoplanetary research. This gas giant, with its unique orbital characteristics and extreme climate, has provided valuable insights into the diversity of exoplanetary systems. By mapping its temperature distribution and identifying water vapor in its atmosphere, scientists have expanded our understanding of exoplanetary atmospheres, atmospheric circulation, and the formation of peculiar celestial objects. The data obtained through the James Webb Space Telescope opens up new avenues of research and paves the way for future discoveries in the fascinating field of exoplanetary science.

Using the James Webb Space Telescope, scientists conducted observations of the exoplanet WASP-18 b and its star throughout the eclipse event. By analyzing the variations in light as the planet moved behind the star, researchers were able to uncover the planet's brightness. These measurements enabled the creation of a temperature map showcasing the day side of the planet, with temperatures ranging from 2,800 to 4,800 degrees Fahrenheit (1,500 to 2,600 degrees Celsius). Image credit: NASA/JPL-Caltech (R. Hurt/IPAC) 

Source - NASA

FAQs related to James Webb Space Telescope's observation of WASP-18b

The James Webb Space Telescope (JWST) played a crucial role in the discovery of water vapor on exoplanet WASP-18 b. By using its advanced instruments and sensitive detectors, the JWST observed the planet as it passed behind its star, a phenomenon known as a secondary eclipse. Scientists measured the change in combined light from the star and the planet, allowing them to refine the measurements from just the star as the planet moved behind it. These observations provided valuable data that revealed the presence of water vapor in the planet's atmosphere.

WASP-18 b is an incredibly fascinating exoplanet for several reasons. First, its orbital period around its star is only 23 hours, making it significantly faster than any planet in our solar system. Additionally, it is an ultra-hot gas giant, approximately 10 times more massive than Jupiter. The fact that it is so different from the planets in our solar system makes it an intriguing target for scientists. Its proximity to us, at a distance of 400 light-years, allows for detailed observations and further study of its atmosphere and composition.


The temperature map of WASP-18 b, generated through observations made by the James Webb Space Telescope, offers valuable insights into the atmospheric conditions of the planet. By measuring the variations in temperature across the planet's day side, scientists can understand the temperature distribution and changes from the hottest point facing the star to the terminator, where the day and night sides meet. This map provides critical information about the planet's climate, heat redistribution, and atmospheric dynamics, allowing scientists to better understand the complex processes occurring on WASP-18 b.


The James Webb Space Telescope plays a pivotal role in the study of exoplanets and their atmospheres. With its advanced instruments and capabilities, the JWST enables scientists to observe exoplanets in greater detail than ever before. By analyzing the light emitted or absorbed by exoplanets as they pass in front of or behind their stars, the JWST can provide valuable data about their compositions, temperatures, and atmospheric properties. This information contributes to our understanding of exoplanet formation, evolution, and the potential for habitability.


Scientists use a technique called transit spectroscopy to measure the change in light when an exoplanet like WASP-18 b passes behind its star. They observe the combined light from the star and the planet, and then compare it to the measurements from just the star as the planet moves behind it. By analyzing the variations in light intensity and the specific wavelengths absorbed or emitted by the planet's atmosphere, scientists can deduce valuable information about the composition, temperature, and other atmospheric properties of the exoplanet.


The presence of water vapor on WASP-18 b, despite its extreme temperatures, demonstrates the remarkable capabilities of the James Webb Space Telescope. The JWST's sensitive detectors and advanced instruments allowed scientists to detect and study water vapor on a distant exoplanet with unprecedented precision. This discovery showcases the telescope's ability to detect and analyze subtle atmospheric features, even under challenging conditions, paving the way for future observations of exoplanets and expanding our understanding of their compositions and potential for habitability.


The findings from the James Webb Space Telescope observations of WASP-18 b significantly contribute to our understanding of planetary formation and evolution. By studying the planet's composition and its similarity to its host star, scientists can infer that WASP-18 b likely formed from the leftover gas present after the star's birth. This information provides insights into the mechanisms by which strange and unique planets like WASP-18 b come into existence. The observations help refine our models and theories about planetary formation processes, adding to the overall understanding of how planets form and evolve.


While the detection of water vapor on WASP-18 b is exciting, it is important to note that the planet itself is an ultra-hot gas giant and not considered habitable. However, the discovery does highlight the capabilities of the James Webb Space Telescope in detecting and studying the atmospheric compositions of exoplanets. These capabilities can be crucial in future studies to identify and analyze the atmospheres of potentially habitable exoplanets, where the presence of water vapor and other molecules could indicate the potential for life-sustaining conditions.


Astronomers face several challenges and limitations when studying exoplanets. One major challenge is the vast distances and faintness of exoplanets, which makes their observations challenging. Additionally, exoplanets are often much smaller and dimmer than their host stars, making it difficult to separate their signals from the overwhelming starlight. The James Webb Space Telescope addresses these challenges by employing advanced technologies, such as sensitive detectors and precise spectrographs, which enable the telescope to detect and analyze the faint signals emitted or absorbed by exoplanets. Its large primary mirror and optimized observational techniques allow for enhanced sensitivity, making it a powerful tool for exoplanet research.


The detection of water vapor on WASP-18 b adds valuable information to the growing body of knowledge about exoplanets and their potential for supporting life. While WASP-18 b itself is not habitable due to its extreme temperatures, the presence of water vapor indicates the possibility of water or other volatile compounds on exoplanets in extreme environments. Understanding the distribution and behavior of water vapor in different exoplanetary atmospheres helps scientists refine their understanding of atmospheric processes, which can be relevant to the search for potentially habitable exoplanets in the future.