Astronomers, leveraging the unprecedented sensitivity of the James Webb Space Telescope (JWST), have achieved a major milestone: creating the first true three-dimensional map of a planet orbiting a distant star. The target of this groundbreaking research, the exoplanet WASP-18b, is an "ultra-hot Jupiter" so scorching that its heat actively rips apart water molecules in its atmosphere.
The Subject: WASP-18b, an Ultra-Hot Jupiter
WASP-18b is a gas giant located approximately 400 light-years from Earth. It possesses a mass roughly 10 times that of Jupiter and is in an incredibly tight, fast orbit around its host star, completing one full revolution in just 23 hours.
Tidally Locked: Due to its extreme proximity to its star, WASP-18b is tidally locked, meaning one side—the "dayside"—always faces the star, resulting in a permanent, scorching hot region.
Extreme Temperatures: Temperatures on the dayside of WASP-18b can reach nearly 5,000 degrees Fahrenheit (about 2,760 degrees Celsius), making it one of the hottest exoplanets known.
🗺️ The New Technique: 3D Spectroscopic Eclipse Mapping
The creation of the 3D map was made possible by a novel technique called 3D eclipse mapping, or spectroscopic eclipse mapping.
Observing the Eclipse: The method focuses on the moments when the exoplanet passes behind its star, an event known as a secondary eclipse. Before the planet is completely obscured, and as it begins to reappear, the light detected by the JWST subtly changes.
Mapping Brightness: Scientists can link these minute changes in light to specific regions—latitude and longitude—on the exoplanet's surface, creating a brightness map.
Achieving 3D: Crucially, the JWST's instruments—specifically the Near-Infrared Imager and Slitless Spectrograph (NIRISS)—allowed the researchers to analyze this light across many different wavelengths (colors). Each wavelength of light penetrates the atmosphere to a different altitude, and molecules like water absorb light at specific wavelengths.
By analyzing the data across this spectrum, scientists could piece together layers of different temperatures and chemical compositions, ultimately converting the 2D brightness map into a full three-dimensional temperature map that includes altitude.
🔥 Key Findings: The Tearing Apart of Water
The 3D map unveiled an atmosphere with distinct thermal zones on the planet's permanent dayside, confirming theoretical predictions and offering a new window into extreme atmospheric physics:
Circular Hotspot: The map confirms the existence of a circular hotspot directly facing the star where the most intense radiation lands. This is the hottest part of the planet.
Winds Fail: Surrounding the hotspot is a comparatively cooler "ring" nearer the planet's edges, suggesting that the planet's atmospheric winds are not strong enough to efficiently redistribute the extreme heat across the entire dayside or to the permanently dark night side.
Water Dissociation: Most remarkably, the hotspot region showed lower levels of water vapor compared to the overall atmospheric average. This is interpreted as direct evidence that the planet is so intensely hot in this region that it is breaking down the water molecules ($H_2O$) into their constituent atoms (hydrogen and oxygen) through a process called thermal dissociation. While water vapor was detected in the cooler regions, its depletion in the hotspot confirms that the planet's environment is chemically destructive.
This observation is a major validation of high-temperature atmospheric chemistry models.
💡 The Future of Exoplanet Research
This study marks a significant technological and scientific leap forward. Previous methods either provided only one-dimensional atmospheric averages or two-dimensional maps. The ability to create a 3D thermal map allows astronomers to:
Characterize Atmospheres: Study atmospheric circulation, climate zones, and chemistry on other worlds with a level of detail previously reserved for planets within our own solar system.
Study Exoplanet Populations: The technique can be applied to many other "hot Jupiters," which constitute hundreds of the thousands of confirmed exoplanets, enabling comparative studies of alien atmospheres.
This achievement opens the door for a much deeper understanding of the complex, exotic, and often violent atmospheric conditions on planets far beyond our sun.