James Webb Space Telescope finds water in super-hot exoplanet's atmosphere

An artist's impression of the super hot exoplanet WASP-18 b
The exoplanet WASP-18 b orbits so close to its parents star that temperatures in its atmosphere reach up to 5,000 degrees Fahrenheit. (Image credit: NASA/JPL-Caltech (K. Miller/IPAC))

The James Webb Space Telescope has found traces of water vapor in the atmosphere of a super-hot gas giant exoplanet that orbits its star in less than one Earth day. 

The exoplanet in question, WASP-18 b, is a gas giant 10 times more massive than the solar system's largest planet, Jupiter. The planet is quite extreme, as it orbits the sun-like star WASP-18, which is located some 400 light-years away from Earth, at an average distance of just 1.9 million miles (3.1 million kilometers). For comparison, the solar system's innermost planet, Mercury, circles the sun at a distance of 39.4 million miles (63.4 million km). 

Due to such close proximity to the parent star, the temperatures in WASP-18 b's atmosphere are so high that most water molecules break apart, NASA said in a statement. The fact that Webb managed to resolve signatures of the residual water is a testament to the telescope's observing powers. 

Related: Exoplanets, dark matter and more: Big discoveries coming from James Webb Space Telescope, astronomers say

"The spectrum of the planet’s atmosphere clearly shows multiple small but precisely measured water features, present despite the extreme temperatures of almost 5,000 degrees Fahrenheit (2,700 degrees Celsius)," NASA wrote in the statement. "It’s so hot that it would tear most water molecules apart, so still seeing its presence speaks to Webb’s extraordinary sensitivity to detect remaining water."

WASP-18 b, discovered in 2008, has been studied by other telescopes, including the Hubble Space Telescope, NASA's X-ray space telescope Chandra, the exoplanet hunter TESS and the now-retired infrared Spitzer Space Telescope. None of these space telescopes, however, was sensitive enough to see the signatures of water in the planet's atmosphere.

"Because the water features in this spectrum are so subtle, they were difficult to identify in previous observations," Anjali Piette, a postdoctoral fellow at the Carnegie Institution for Science and one of the authors of the new research, said in the statement. "That made it really exciting to finally see water features with these JWST observations."

In addition to being so massive, hot and close to its parent star, WASP-18 b is also tidally locked. That means one side of the planet constantly faces the star, just like the moon's near side always faces Earth. As a result of this tidal locking, considerable differences in temperature exist across the planet's surface. The Webb measurements, for the first time, enabled scientists to map these differences in detail. 

The James Webb Space Telescope detected traces of water in the super hot atmosphere of exoplanet WASP-18 b

The signature of water detected in the super hot atmosphere of exoplanet WASP-18 b by the James Webb Space Telescope. (Image credit: NASA/JPL-Caltech (R. Hurt/IPAC))

The measurements found that the most intensely illuminated parts of the planet can be up to 2,000 degrees F (1,100 degrees C) hotter than those in the twilight zone. The scientists didn't expect such significant temperature differences and now think that there must be some not yet understood mechanism in action that prevents the distribution of heat around the planet's globe. 

"The brightness map of WASP-18 b shows a lack of east-west winds that is best matched by models with atmospheric drag," co-author Ryan Challener, of the University of Michigan, said in the statement. "One possible explanation is that this planet has a strong magnetic field, which would be an exciting discovery!" 

To create the temperature map, the researchers calculated the planet's infrared glow by measuring the difference in the glow of the parent star during the time the planet transited in front of the star's disk and then when it disappeared behind it. 

"JWST is giving us the sensitivity to make much more detailed maps of hot giant planets like WASP-18 b than ever before," Megan Mansfield, a Sagan Fellow at the University of Arizona and one of the authors of the paper describing the results. said in the statement. "This is the first time a planet has been mapped with JWST, and it’s really exciting to see that some of what our models predicted, such as a sharp drop in temperature away from the point on the planet directly facing the star, is actually seen in the data."

The new study was published online Wednesday (May 31) in the journal Nature.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Tereza Pultarova
Senior Writer

Tereza is a London-based science and technology journalist, aspiring fiction writer and amateur gymnast. Originally from Prague, the Czech Republic, she spent the first seven years of her career working as a reporter, script-writer and presenter for various TV programmes of the Czech Public Service Television. She later took a career break to pursue further education and added a Master's in Science from the International Space University, France, to her Bachelor's in Journalism and Master's in Cultural Anthropology from Prague's Charles University. She worked as a reporter at the Engineering and Technology magazine, freelanced for a range of publications including Live Science, Space.com, Professional Engineering, Via Satellite and Space News and served as a maternity cover science editor at the European Space Agency.

  • billslugg
    At 2200°C, 3% of water molecules are disassociated.
    At 3000°C, 50% are.
    This planet is about 40% disassociated.
    Plenty of water molecules to show up in the spectrometer.
  • rod
    This is a hot exoplanet :) ref - A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b, https://www.nature.com/articles/s41586-023-06230-1, 31-May-2023. "Abstract Close-in giant exoplanets with temperatures greater than 2,000 K (“ultra-hot Jupiters”) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble and Spitzer Space Telescopes1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis3–12. .."

    WASP-18 b was difficult to measure in past reports on the atmosphere, now better with JWST.

    Exploring the Ability of HST WFC3 G141 to Uncover Trends in Populations of Exoplanet Atmospheres Through a Homogeneous Transmission Survey of 70 Gaseous Planets, https://arxiv.org/abs/2211.00649
    My observation. 61-page PDF report attached. The report studies atmosphers of 70 gaseous exoplanets. WASP-18 b is referenced, http://exoplanet.eu/catalog/wasp-18_b/ From the paper, "5. DISCUSSION AND CONCLUSIONS The Hubble Space Telescope has been at the forefront of exoplanet atmospheric characterisation over the last two decades. While many different instruments on this facility have been used, WFC3 has perhaps been the mostly widely utilised due to its sensitivity to water. In this work we have presented a population study of atmospheres, each studied with the WFC3 G141 grism as the planet transits its host star. Of the 70 planets studied, we found strong evidence (>3 sigma) for atmospheric features on 37 of them, with some evidence (2-3 sigma) for spectral modulation on an additional 14 planets. We note that for several planets (e.g. WASP-18 b), the derived spectrum has error bars that are several scale heights in size, meaning no atmospheric constraints could be expected..."

    JWST seems to do better :)
  • rod
    WASP-18 b has no counterpart in our solar system. It orbits much closer than Mercury to our Sun and very massive, more than 10 Jupiter masses.