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Water Traces on WASP-18-b

Webb Maps and Finds Traces Water on WASP-18 b

There’s a fascinating exoplanet 400 light-years away that’s so intriguing that astronomers have been researching it since its discovery in 2009. WASP-18 b takes only 23 hours to complete one orbit around its star (which is slightly larger than our Sun). Nothing like it exists in our solar system. WASP-18 b, an extrasolar planet ten times the mass of Jupiter, has been detected by NASA’s Hubble, Chandra, TESS, and Spitzer satellite telescopes as well as ground-based observatories. Astronomers are already looking via NASA’s James Webb Space Telescope, and the ”firsts” keep arriving.

The discovery: Scientists discovered water vapor in extrasolar planet’s (WASP-18 b) atmosphere and created a temperature map of the planet as it disappeared behind and resurfaced from its star. This is referred to as a secondary eclipse. Scientists can read the combined light from the star and the planet, then refine the measurements from the star alone when the planet travels behind it.

The same side of WASP-18 b, known as the dayside, always faces the star, just as the same side of the Moon always faces Earth. The temperature, or brightness, map depicts a large temperature shift – up to 1,000 degrees – from the warmest point facing the star to the terminator, where the day and night sides of the tidally-locked planet meet in perpetual twilight.

‘‘JWST is giving us the sensitivity to make much more detailed maps of extrasolar planets like WASP-18 b than ever before. 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!’’ said Megan Mansfield, a Sagan Fellow at the University of Arizona, and one of the authors of the paper describing the results.

Temperature gradients were mapped over the planet’s dayside by the researchers. Given how much cooler the globe is at the terminator, something is most certainly preventing winds from efficiently dispersing heat to the night side. But what is influencing the winds remains a mystery.

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

According to one interpretation of the eclipse map, magnetic factors cause winds to blow from the planet’s equator up over the North pole and down over the South pole, rather than east-west as we would expect.

Temperature fluctuations were measured at various elevations of the gas giant planet’s atmospheric layers. Temperatures rose with altitude and varied by hundreds of degrees.

Despite tremendous temperatures of over 5,000 degrees Fahrenheit (2,700 degrees Celsius), the spectrum of the planet’s atmosphere indicates many small but accurately measured water structures. It’s so hot that most water molecules would be ripped apart, thus recognizing its presence speaks to Webb’s amazing sensitivity to detect leftover water. The levels of water vapor detected in the atmosphere of WASP-18 b show that it exists at varied heights.

thermal emission spectrum of WASP-18 b
Credit: NASA/JPL-Caltech (R. Hurt/IPAC)

‘‘It was a great feeling to look at WASP-18 b’s JWST spectrum for the first time and see the subtle but precisely measured signature of water,’’ said Louis-Philippe Coulombe, a graduate student at the University of Montreal and lead author of the WASP-18 b paper. ‘‘Using such measurements, we will be able to detect such molecules for a wide range of planets in the years to come!’’

Extrasolar planet WASP-18 b was observed for around six hours by researchers using one of Webb’s instruments, the Near-Infrared Imager and Slitless Spectrograph (NIRISS), which was donated by the Canadian Space Agency.

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

The discoverers are as follows: Through the Transiting Exoplanet Community Early Release Science Program, which is coordinated by Natalie Batalha, an astronomer at the University of California, Santa Cruz, who helped coordinate the new research, more than 100 scientists from across the world are working on early science from Webb. Young scientists like Coulombe, Challener, Piette, and Mansfield are doing a lot of innovative work.

exoplanet WASP-18 b
Scientists used the James Webb Space Telescope to observe the exoplanet WASP-18 b and its star before, during and after the planet was eclipsed. By measuring the change in light when the planet travels behind the star, the planet’s brightness is revealed. From these measurements, scientists were able to make a temperature map of the planet’s day side. Displayed temperature range: 2,800 to 4,800 degrees Fahrenheit (1,500 to 2,600 degrees Celsius). Credit: NASA/JPL-Caltech (R. Hurt/IPAC)

WASP-18 b’s proximity to its star and us, as well as its massive mass, contributed to its appeal to scientists. WASP-18 b is one of the largest worlds whose atmospheres we can study. We’d like to know how such planets arise and end up where they are. This, too, has some early Webb responses.

‘‘By analyzing WASP-18b’s spectrum, we not only learn about the various molecules that can be found in its atmosphere but also about the way it formed. We find from our observations that WASP-18 b’s composition is very similar to that of its star, meaning it most likely formed from the leftover gas that was present just after the star was born,’’ Coulombe said. ‘‘Those results are very valuable to get a clear picture of how strange planets like WASP-18 b, which have no counterpart in our solar system, come to exist.’’

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