The James Webb Space Telescope has just achieved an unprecedented feat by mapping for the first time the atmosphere of a distant planet in three dimensions. This breakthrough opens a new window into understanding extraterrestrial worlds.
Using a technique called spectroscopic eclipse mapping, astronomers were able to reconstruct the thermal structure of WASP-18b, a gas giant located 400 light-years from Earth. This planet, described as an 'ultra-hot Jupiter', has temperatures approaching 2,760 degrees Celsius (5,000 degrees Fahrenheit).
The study published in
Nature Astronomy reveals how tiny variations in stellar brightness, when the planet passes behind its star, allow scientists to determine temperatures at different altitudes and geographic positions.
Artist's representation of WASP-18b showing regions where water vapor dissociates under the effect of extreme heat.
Credit: NASA/GSFC The multi-wavelength analysis performed with Webb's NIRISS instrument allowed scientists to distinguish several atmospheric layers. Each wavelength probes a specific altitude, from the upper layers rich in water vapor to the deeper regions where this molecule is absent. This approach revealed a central hot spot surrounded by a cooler ring, confirming theoretical models about the thermal distribution of planetary atmospheres.
The 3D mapping revealed a remarkable phenomenon: the dissociation of water vapor in the hottest areas. While the peripheral regions retain their water molecules intact, the central point experiences such high temperatures that chemical bonds break. This direct observation validates for the first time theoretical predictions about water behavior under extreme conditions.
This method opens exciting prospects for studying other exoplanets. Researchers plan to apply this technique to hundreds of already identified 'hot Jupiters', and perhaps one day to rocky planets similar to Earth. The accuracy of the maps could further improve with new observations, offering an increasingly detailed understanding of extraterrestrial climates.
Tidal locking phenomenon
Tidal locking occurs when a planet always shows the same face to its star, as is the case for WASP-18b. This rotation-revolution synchronization results from gravitational forces that gradually slow down planetary rotation.
On these tidally locked worlds, the daytime hemisphere experiences permanent sunlight while the nighttime hemisphere remains in eternal darkness. This configuration creates extreme thermal contrasts that profoundly influence atmospheric circulation and the planet's chemistry.
The illuminated side of WASP-18b reaches temperatures so high that rocks could melt, while the dark side remains much cooler. These thermal differences generate violent winds that redistribute heat around the planet, constantly changing its atmospheric structure.
This phenomenon is not rare in the Universe: our Moon is tidally locked with Earth, as are many exoplanets close to their star. Understanding these mechanisms helps astronomers interpret observations of distant worlds.