There’s a fascinating exoplanet 400 light-years away that’s so intriguing that astronomers have been studying it since its discovery in 2009. WASP-18 b’s orbit around its star, which is slightly larger than our sun, takes just 23 hours. There is no comparable planet in our solar system. A new study led by Louis-Philippe Coulombe, a graduate student at the University of Montreal, on this exoplanet, an ultra-hot gas giant 10 times more massive than Jupiter, based on new data from Canada’s NIRISS instrument on the James Webb Space Telescope (JWST) holds many surprises in store for us!
Mapping an exoplanet
An international team of astronomers has detected water vapor in the atmosphere of exoplanet WASP-18b and created a temperature map of the planet as it disappeared and reappeared behind its star. This event is known assecondary solar eclipse. Scientists can observe the combined light from the star and planet, and then use the light from the star alone to refine their measurements as the planet moves behind it.
The same side, called the day side, of WASP-18 b always faces its star, just as the same side of the moon always faces Earth. This is called synchronous rotationat which the gravity lock. The exoplanet’s temperature, or luminosity, map shows a huge temperature difference — as much as 1,000 degrees — between the hottest point opposite the star and the terminator, where the day and night sides of the trapped planet meet in perpetual twilight.
“The JWST gives us the sensitivity to create much more detailed maps of hot giant planets like WASP-18b than ever before. This is the first time a planet has been mapped with the JWST, and it’s really exciting to see that some of the predictions from our models, such as a sharp temperature drop at the part of the planet that faces the star directly, are actually visible are in the data,” said Megan Mansfield, a Sagan Fellow at the University of Arizona and one of the authors of the article describing the findings.
The team mapped temperature gradients on the planet’s day side. Since the planet is much cooler at the Terminator, it’s likely that something is preventing the winds from efficiently redistributing heat to the nightside. But what affects the winds remains a mystery.
“The WASP-18b luminosity map shows the absence of east-west winds, which would best fit atmospheric drag models. 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.
One interpretation of the eclipse map says that magnetic effects are causing winds to blow from the planet’s equator up over the North Pole and down over the South Pole, rather than from east to west as one would otherwise expect.
The researchers recorded the temperature changes at different altitudes in the atmosphere layers of the gas giant planet. They found that temperatures increased with altitude and varied by several hundred degrees.
water vapor sign
The spectrum of the planet’s atmosphere clearly shows numerous traces of water vapor, small but accurately measured and present despite extreme temperatures of almost 2,700 degrees Celsius. The heat is so great that it would destroy most of the water molecules. The presence of this water therefore testifies to the exceptional sensitivity of the Webb telescope in detecting the remains of water. The amounts measured in the atmosphere of WASP-18b indicate that water vapor is present at different altitudes.
The team obtained the thermal emission spectrum of WASP-18b by measuring the amount of light it emits in the wavelengths of the Webb Telescope’s NIRISS instrument in SOSS mode ranging from 0.85 to 2.8 microns, yielding 65% of the total energy emitted by the planet has been recorded. WASP-18 b is so hot on the day side of this tidal planet that water molecules would vaporize. Webb observed water vapor directly on the planet, even in relatively small amounts, indicating the observatory’s sensitivity.
“It was a great feeling to look at the JWST spectrum of WASP-18b for the first time and see the subtle but precisely measured signature of water,” said Louis-Philippe Coulombe, a PhD student at the University of Montreal. , member of the Trottier Institute for Research on Exoplanets (iREx) and lead author of the article on WASP-18 b. “Thanks to this type of measurements, we will be able to detect such molecules for a large number of planets in the coming years!””. Björn Benneke, UdeM professor, iREx member and co-author of the article, is Coulombe’s research director and has led the global effort to study WASP-18b since 2016.
The work of the NIRISS instrument and young scientists
The team of astronomers observed WASP-18b for about six hours using one of Webb’s instruments, the Near Infrared Slitless Imager and Spectrograph (NIRISS), provided by the Canadian Space Agency and several partners, including the University of Montreal and iREx.
“Since water features are very subtle in this spectrum, they have been difficult to identify in previous observations. So it’s very exciting to finally see water features with these JWST observations,” said Anjali Piette, a postdoctoral researcher at the Carnegie Institution for Science and one of the authors of the new study.
Observations of WASP-18b were collected as part of the Transiting Exoplanet Community Early Release science program led by Natalie Batalha, an astronomer at the University of California, Santa Cruz, who is helping to coordinate the new study and more than a hundred researchers at the teams helped. Much of this pioneering work is being done by young scientists such as Coulombe, Challener, Piette and Mansfield.
The proximity to both its star and us helped make WASP-18b such an intriguing target for these scientists, as did its large mass. WASP-18 b is one of the most massive planets whose atmosphere we can study. Astronomers want to understand how such planets form and end up where they are in their systems. Again, Webb provides some answers.
“By analyzing the spectrum of WASP-18b, we not only learn about the different molecules in its atmosphere, but also how it formed. “Our observations show that the composition of WASP-18 b is very similar to that of its star, meaning it likely formed from the remnants of gas present shortly after the star’s birth,” Coulombe said. “These results are very useful in getting a clear picture of how strange planets like WASP-18b, which have no equivalent in our solar system, formed.”
About this study
The article “A broadband Thermal Emission Spectrum of the Ultra-Hot Jupiter WASP-18b” authored by the JWST Transiting Exoplanet Community Early Release Science Team was published in the journal Nature on May 31, 2023.
Scientific contacts
Louis Philippe Coulombe
graduate student
Trottier Institute for Exoplanet Research, University of Montreal
[email protected]
Bjorn Benneke
Teacher
Trottier Institute for Exoplanet Research, University of Montreal
[email protected]
514-578-2716
media contact
Nathalie Ouellette
Associate Director and Scientific Outreach Project Webb
Trottier Institute for Exoplanet Research, University of Montreal
[email protected]
613-531-1762
Additional Links
NASA press release
University of Montreal press release
natural item
Scientific article about arXiv
Animation of the secondary solar eclipse of WASP-18b