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Webb maps and finds traces of water in the atmosphere of the ultra-hot gas giant WASP-18 b.


WASP-18 b, located 400 light-years away, has been a topic of intense study for astronomers since its discovery in 2009. This ultra-hot gas giant is 10 times more massive than Jupiter and orbits its star (slightly larger than our Sun) in just 23 hours, making it unique compared to our solar system. Not only have observatories on the ground studied WASP-18 b, but the Hubble, Chandra, TESS, and Spitzer space telescopes have also collected data on the exoplanet. Recently, the James Webb Space Telescope has gotten involved as well, and the firsts keep coming as astronomers continue to uncover new and exciting information about this intriguing world.

Scientists have identified water vapor in the atmosphere of WASP-18 b and have made a temperature map of the planet by using a secondary eclipse. This event occurs when the planet slips behind its star and reappears from it. By studying the combined light from the star and planet, scientists can refine their measurements from just the star as it moves behind it. This technique helps them determine the temperature of the planet and provides an understanding of its atmosphere. Additionally, they can use this to gather more information about the exoplanet and its environment.

WASP-18 b is a tidally-locked planet, which means that the same side of the planet always faces the star it orbits. This same side is known as the dayside, and due to the tidal locking, it experiences extreme temperatures, with the hottest point facing the star up to 1,000 degrees hotter than the terminator, where day and night sides of the planet meet in permanent twilight. The temperature map reveals this drastic temperature difference due to its unique orbital geometry, similar to that of our Moon, where one side of it always faces our planet Earth.

The James Webb Space Telescope (JWST) has given scientists the sensitivity to map hot giant planets like WASP-18 b in far greater detail than ever before. This is a groundbreaking achievement, as it is the first time a planet has been mapped using JWST. Megan Mansfield, a Sagan Fellow at the University of Arizona and one of the paper’s authors, expressed her excitement for the findings and how they match up with the models they created. According to Mansfield, a sharp temperature drop away from the point of the planet directly facing the star was observed in the data. This indicates that JWST is able to provide invaluable insight into the planetary systems of our universe, and assists researchers in furthering their understanding of them.

The team mapped temperature gradients across the day side of the planet and discovered that the temperatures were significantly cooler at the terminator, suggesting something is hindering winds from efficiently redistributing heat to the night side. The lack of east-west winds was matched by models with atmospheric drag, leading co-author Ryan Challener of the University of Michigan to theorize a strong magnetic field could be the cause. If true, this would be an exciting discovery as such a strong magnetic field on an exoplanet would be unprecedented. Further investigations are needed to confirm or disprove Challener’s hypothesis and unlock the mystery of what is affecting the winds on WASP-18 b.

Researchers studying the gas giant planet, WASP-18 b, were able to record temperature changes at different elevations in its atmosphere. To their amazement, they observed temperatures that increased with elevation, varying by hundreds of degrees. What was even more amazing was that despite the extreme temperatures of almost 5,000 degrees Fahrenheit (2,700 C), the spectrum of the planet’s atmosphere still clearly showed small but precisely measured water features. This indicates that the Webb telescope’s extraordinary sensitivity was able to detect the remaining water vapor present at various elevations in the atmosphere of WASP-18 b, despite being so hot that it would normally tear most water molecules apart.

Water is detected in the JWST Atmospheric Spectrum of WASP-18 b.

The James Webb Space Telescope’s NIRISS SOSS 0.85-2.8 um wavelength range allowed scientists to obtain the thermal emission spectrum of WASP-18 b, capturing 65% of the total energy emitted by the planet. This incredibly hot tidally locked planet, with its same side always facing its star, is so hot that water vapor molecules would break apart on the day side. Surprisingly,Webb Telescope directly observed even relatively small amounts of water vapor on WASP-18 b, indicating the amazing sensitivity of this observatory.

Louis-Philippe Coulombe, a graduate student at the University of Montreal and lead author of the WASP-18 b paper, remarked on his experience in seeing the subtle but precisely measured signature of water in the planet’s JWST spectrum. He expressed his excitement for this discovery, which will enable the detection of a wide range of molecules in the years to come. While this was a great feeling for him, it was also an important milestone in the field of exoplanetary exploration. For example, this discovery has enabled scientists to gain a better understanding of the atmospheres of these distant planets and how they compare to those in our own Solar System.

The research team was able to observe the water features on WASP-18 b for about six hours with the Near-Infrared Imager and Slitless Spectrograph (NIRISS) provided by the Canadian Space Agency, and they were thrilled to finally be able to identify these elusive features. Anjali Piette, a postdoctoral fellow at the Carnegie Institution for Science and one of the authors of the research paper, said that it was really exciting to finally be able to see these water features using JWST observations, since they had been so difficult to identify in previous observations. This new research will surely help us better understand the makeup and composition of WASP-18 b, as well as gain insights into exoplanet atmospheres in general.

More than 100 scientists from around the globe are working on the early science from Webb, through the Transiting Exoplanet Community Early Release Science Program led by Natalie Batalha, an astronomer from the University of California, Santa Cruz.This program is helping to facilitate the research of groundbreaking work and is providing early career scientists like Coulombe, Challener, Piette, and Mansfield with the opportunity to further their research. A key target for these scientists has been WASP-18 b, a planet with a notably large mass and close proximity to both its star and us; as such, it has been an ideal target. Analysis of WASP-18b’s spectrum has provided new insight into the formation of the planet. According to Coulombe, its composition is similar to that of its star, which suggests that it was formed from the gas left behind after its star formed. This information is invaluable in understanding more about the creation of strange planets such as WASP-18b, for which there is no comparison in our own solar system.

Webb maps and finds traces of water in the atmosphere of the ultra-hot gas giant WASP-18 b.

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